diff --git a/src/de/from_primitive.rs b/src/de/from_primitive.rs
new file mode 100644
index 0000000..544f3f1
--- /dev/null
+++ b/src/de/from_primitive.rs
@@ -0,0 +1,260 @@
+use lib::*;
+
+macro_rules! int_to_int {
+    ($dst:ident, $n:ident) => {
+        if $dst::min_value() as i64 <= $n as i64 && $n as i64 <= $dst::max_value() as i64 {
+            Some($n as $dst)
+        } else {
+            None
+        }
+    };
+}
+
+macro_rules! int_to_uint {
+    ($dst:ident, $n:ident) => {
+        if 0 <= $n && $n as u64 <= $dst::max_value() as u64 {
+            Some($n as $dst)
+        } else {
+            None
+        }
+    };
+}
+
+macro_rules! uint_to {
+    ($dst:ident, $n:ident) => {
+        if $n as u64 <= $dst::max_value() as u64 {
+            Some($n as $dst)
+        } else {
+            None
+        }
+    };
+}
+
+pub trait FromPrimitive: Sized {
+    fn from_i8(n: i8) -> Option<Self>;
+    fn from_i16(n: i16) -> Option<Self>;
+    fn from_i32(n: i32) -> Option<Self>;
+    fn from_i64(n: i64) -> Option<Self>;
+    fn from_u8(n: u8) -> Option<Self>;
+    fn from_u16(n: u16) -> Option<Self>;
+    fn from_u32(n: u32) -> Option<Self>;
+    fn from_u64(n: u64) -> Option<Self>;
+}
+
+macro_rules! impl_from_primitive_for_int {
+    ($t:ident) => {
+        impl FromPrimitive for $t {
+            #[inline]
+            fn from_i8(n: i8) -> Option<Self> {
+                int_to_int!($t, n)
+            }
+            #[inline]
+            fn from_i16(n: i16) -> Option<Self> {
+                int_to_int!($t, n)
+            }
+            #[inline]
+            fn from_i32(n: i32) -> Option<Self> {
+                int_to_int!($t, n)
+            }
+            #[inline]
+            fn from_i64(n: i64) -> Option<Self> {
+                int_to_int!($t, n)
+            }
+            #[inline]
+            fn from_u8(n: u8) -> Option<Self> {
+                uint_to!($t, n)
+            }
+            #[inline]
+            fn from_u16(n: u16) -> Option<Self> {
+                uint_to!($t, n)
+            }
+            #[inline]
+            fn from_u32(n: u32) -> Option<Self> {
+                uint_to!($t, n)
+            }
+            #[inline]
+            fn from_u64(n: u64) -> Option<Self> {
+                uint_to!($t, n)
+            }
+        }
+    };
+}
+
+macro_rules! impl_from_primitive_for_uint {
+    ($t:ident) => {
+        impl FromPrimitive for $t {
+            #[inline]
+            fn from_i8(n: i8) -> Option<Self> {
+                int_to_uint!($t, n)
+            }
+            #[inline]
+            fn from_i16(n: i16) -> Option<Self> {
+                int_to_uint!($t, n)
+            }
+            #[inline]
+            fn from_i32(n: i32) -> Option<Self> {
+                int_to_uint!($t, n)
+            }
+            #[inline]
+            fn from_i64(n: i64) -> Option<Self> {
+                int_to_uint!($t, n)
+            }
+            #[inline]
+            fn from_u8(n: u8) -> Option<Self> {
+                uint_to!($t, n)
+            }
+            #[inline]
+            fn from_u16(n: u16) -> Option<Self> {
+                uint_to!($t, n)
+            }
+            #[inline]
+            fn from_u32(n: u32) -> Option<Self> {
+                uint_to!($t, n)
+            }
+            #[inline]
+            fn from_u64(n: u64) -> Option<Self> {
+                uint_to!($t, n)
+            }
+        }
+    };
+}
+
+macro_rules! impl_from_primitive_for_float {
+    ($t:ident) => {
+        impl FromPrimitive for $t {
+            #[inline]
+            fn from_i8(n: i8) -> Option<Self> {
+                Some(n as Self)
+            }
+            #[inline]
+            fn from_i16(n: i16) -> Option<Self> {
+                Some(n as Self)
+            }
+            #[inline]
+            fn from_i32(n: i32) -> Option<Self> {
+                Some(n as Self)
+            }
+            #[inline]
+            fn from_i64(n: i64) -> Option<Self> {
+                Some(n as Self)
+            }
+            #[inline]
+            fn from_u8(n: u8) -> Option<Self> {
+                Some(n as Self)
+            }
+            #[inline]
+            fn from_u16(n: u16) -> Option<Self> {
+                Some(n as Self)
+            }
+            #[inline]
+            fn from_u32(n: u32) -> Option<Self> {
+                Some(n as Self)
+            }
+            #[inline]
+            fn from_u64(n: u64) -> Option<Self> {
+                Some(n as Self)
+            }
+        }
+    };
+}
+
+impl_from_primitive_for_int!(isize);
+impl_from_primitive_for_int!(i8);
+impl_from_primitive_for_int!(i16);
+impl_from_primitive_for_int!(i32);
+impl_from_primitive_for_int!(i64);
+impl_from_primitive_for_uint!(usize);
+impl_from_primitive_for_uint!(u8);
+impl_from_primitive_for_uint!(u16);
+impl_from_primitive_for_uint!(u32);
+impl_from_primitive_for_uint!(u64);
+impl_from_primitive_for_float!(f32);
+impl_from_primitive_for_float!(f64);
+
+serde_if_integer128! {
+    impl FromPrimitive for i128 {
+        #[inline]
+        fn from_i8(n: i8) -> Option<Self> {
+            Some(n as i128)
+        }
+        #[inline]
+        fn from_i16(n: i16) -> Option<Self> {
+            Some(n as i128)
+        }
+        #[inline]
+        fn from_i32(n: i32) -> Option<Self> {
+            Some(n as i128)
+        }
+        #[inline]
+        fn from_i64(n: i64) -> Option<Self> {
+            Some(n as i128)
+        }
+        #[inline]
+        fn from_u8(n: u8) -> Option<Self> {
+            Some(n as i128)
+        }
+        #[inline]
+        fn from_u16(n: u16) -> Option<Self> {
+            Some(n as i128)
+        }
+        #[inline]
+        fn from_u32(n: u32) -> Option<Self> {
+            Some(n as i128)
+        }
+        #[inline]
+        fn from_u64(n: u64) -> Option<Self> {
+            Some(n as i128)
+        }
+    }
+
+    impl FromPrimitive for u128 {
+        #[inline]
+        fn from_i8(n: i8) -> Option<Self> {
+            if n >= 0 {
+                Some(n as u128)
+            } else {
+                None
+            }
+        }
+        #[inline]
+        fn from_i16(n: i16) -> Option<Self> {
+            if n >= 0 {
+                Some(n as u128)
+            } else {
+                None
+            }
+        }
+        #[inline]
+        fn from_i32(n: i32) -> Option<Self> {
+            if n >= 0 {
+                Some(n as u128)
+            } else {
+                None
+            }
+        }
+        #[inline]
+        fn from_i64(n: i64) -> Option<Self> {
+            if n >= 0 {
+                Some(n as u128)
+            } else {
+                None
+            }
+        }
+        #[inline]
+        fn from_u8(n: u8) -> Option<Self> {
+            Some(n as u128)
+        }
+        #[inline]
+        fn from_u16(n: u16) -> Option<Self> {
+            Some(n as u128)
+        }
+        #[inline]
+        fn from_u32(n: u32) -> Option<Self> {
+            Some(n as u128)
+        }
+        #[inline]
+        fn from_u64(n: u64) -> Option<Self> {
+            Some(n as u128)
+        }
+    }
+}
diff --git a/src/de/ignored_any.rs b/src/de/ignored_any.rs
new file mode 100644
index 0000000..68a644e
--- /dev/null
+++ b/src/de/ignored_any.rs
@@ -0,0 +1,227 @@
+use lib::*;
+
+use de::{
+    Deserialize, Deserializer, EnumAccess, Error, MapAccess, SeqAccess, VariantAccess, Visitor,
+};
+
+/// An efficient way of discarding data from a deserializer.
+///
+/// Think of this like `serde_json::Value` in that it can be deserialized from
+/// any type, except that it does not store any information about the data that
+/// gets deserialized.
+///
+/// ```edition2018
+/// use std::fmt;
+/// use std::marker::PhantomData;
+///
+/// use serde::de::{
+///     self, Deserialize, DeserializeSeed, Deserializer, IgnoredAny, SeqAccess, Visitor,
+/// };
+///
+/// /// A seed that can be used to deserialize only the `n`th element of a sequence
+/// /// while efficiently discarding elements of any type before or after index `n`.
+/// ///
+/// /// For example to deserialize only the element at index 3:
+/// ///
+/// /// ```
+/// /// NthElement::new(3).deserialize(deserializer)
+/// /// ```
+/// pub struct NthElement<T> {
+///     n: usize,
+///     marker: PhantomData<T>,
+/// }
+///
+/// impl<T> NthElement<T> {
+///     pub fn new(n: usize) -> Self {
+///         NthElement {
+///             n: n,
+///             marker: PhantomData,
+///         }
+///     }
+/// }
+///
+/// impl<'de, T> Visitor<'de> for NthElement<T>
+/// where
+///     T: Deserialize<'de>,
+/// {
+///     type Value = T;
+///
+///     fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+///         write!(
+///             formatter,
+///             "a sequence in which we care about element {}",
+///             self.n
+///         )
+///     }
+///
+///     fn visit_seq<A>(self, mut seq: A) -> Result<Self::Value, A::Error>
+///     where
+///         A: SeqAccess<'de>,
+///     {
+///         // Skip over the first `n` elements.
+///         for i in 0..self.n {
+///             // It is an error if the sequence ends before we get to element `n`.
+///             if seq.next_element::<IgnoredAny>()?.is_none() {
+///                 return Err(de::Error::invalid_length(i, &self));
+///             }
+///         }
+///
+///         // Deserialize the one we care about.
+///         let nth = match seq.next_element()? {
+///             Some(nth) => nth,
+///             None => {
+///                 return Err(de::Error::invalid_length(self.n, &self));
+///             }
+///         };
+///
+///         // Skip over any remaining elements in the sequence after `n`.
+///         while let Some(IgnoredAny) = seq.next_element()? {
+///             // ignore
+///         }
+///
+///         Ok(nth)
+///     }
+/// }
+///
+/// impl<'de, T> DeserializeSeed<'de> for NthElement<T>
+/// where
+///     T: Deserialize<'de>,
+/// {
+///     type Value = T;
+///
+///     fn deserialize<D>(self, deserializer: D) -> Result<Self::Value, D::Error>
+///     where
+///         D: Deserializer<'de>,
+///     {
+///         deserializer.deserialize_seq(self)
+///     }
+/// }
+///
+/// # fn example<'de, D>(deserializer: D) -> Result<(), D::Error>
+/// # where
+/// #     D: Deserializer<'de>,
+/// # {
+/// // Deserialize only the sequence element at index 3 from this deserializer.
+/// // The element at index 3 is required to be a string. Elements before and
+/// // after index 3 are allowed to be of any type.
+/// let s: String = NthElement::new(3).deserialize(deserializer)?;
+/// #     Ok(())
+/// # }
+/// ```
+#[derive(Copy, Clone, Debug, Default)]
+pub struct IgnoredAny;
+
+impl<'de> Visitor<'de> for IgnoredAny {
+    type Value = IgnoredAny;
+
+    fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        formatter.write_str("anything at all")
+    }
+
+    #[inline]
+    fn visit_bool<E>(self, x: bool) -> Result<Self::Value, E> {
+        let _ = x;
+        Ok(IgnoredAny)
+    }
+
+    #[inline]
+    fn visit_i64<E>(self, x: i64) -> Result<Self::Value, E> {
+        let _ = x;
+        Ok(IgnoredAny)
+    }
+
+    #[inline]
+    fn visit_u64<E>(self, x: u64) -> Result<Self::Value, E> {
+        let _ = x;
+        Ok(IgnoredAny)
+    }
+
+    #[inline]
+    fn visit_f64<E>(self, x: f64) -> Result<Self::Value, E> {
+        let _ = x;
+        Ok(IgnoredAny)
+    }
+
+    #[inline]
+    fn visit_str<E>(self, s: &str) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        let _ = s;
+        Ok(IgnoredAny)
+    }
+
+    #[inline]
+    fn visit_none<E>(self) -> Result<Self::Value, E> {
+        Ok(IgnoredAny)
+    }
+
+    #[inline]
+    fn visit_some<D>(self, deserializer: D) -> Result<Self::Value, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        IgnoredAny::deserialize(deserializer)
+    }
+
+    #[inline]
+    fn visit_newtype_struct<D>(self, deserializer: D) -> Result<Self::Value, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        IgnoredAny::deserialize(deserializer)
+    }
+
+    #[inline]
+    fn visit_unit<E>(self) -> Result<Self::Value, E> {
+        Ok(IgnoredAny)
+    }
+
+    #[inline]
+    fn visit_seq<A>(self, mut seq: A) -> Result<Self::Value, A::Error>
+    where
+        A: SeqAccess<'de>,
+    {
+        while let Some(IgnoredAny) = try!(seq.next_element()) {
+            // Gobble
+        }
+        Ok(IgnoredAny)
+    }
+
+    #[inline]
+    fn visit_map<A>(self, mut map: A) -> Result<Self::Value, A::Error>
+    where
+        A: MapAccess<'de>,
+    {
+        while let Some((IgnoredAny, IgnoredAny)) = try!(map.next_entry()) {
+            // Gobble
+        }
+        Ok(IgnoredAny)
+    }
+
+    #[inline]
+    fn visit_bytes<E>(self, bytes: &[u8]) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        let _ = bytes;
+        Ok(IgnoredAny)
+    }
+
+    fn visit_enum<A>(self, data: A) -> Result<Self::Value, A::Error>
+    where
+        A: EnumAccess<'de>,
+    {
+        data.variant::<IgnoredAny>()?.1.newtype_variant()
+    }
+}
+
+impl<'de> Deserialize<'de> for IgnoredAny {
+    #[inline]
+    fn deserialize<D>(deserializer: D) -> Result<IgnoredAny, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        deserializer.deserialize_ignored_any(IgnoredAny)
+    }
+}
diff --git a/src/de/impls.rs b/src/de/impls.rs
new file mode 100644
index 0000000..e460a40
--- /dev/null
+++ b/src/de/impls.rs
@@ -0,0 +1,2612 @@
+use lib::*;
+
+use de::{
+    Deserialize, Deserializer, EnumAccess, Error, SeqAccess, Unexpected, VariantAccess, Visitor,
+};
+
+#[cfg(any(core_duration, feature = "std", feature = "alloc"))]
+use de::MapAccess;
+
+use de::from_primitive::FromPrimitive;
+use private::de::InPlaceSeed;
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+use private::de::size_hint;
+
+////////////////////////////////////////////////////////////////////////////////
+
+struct UnitVisitor;
+
+impl<'de> Visitor<'de> for UnitVisitor {
+    type Value = ();
+
+    fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        formatter.write_str("unit")
+    }
+
+    fn visit_unit<E>(self) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        Ok(())
+    }
+}
+
+impl<'de> Deserialize<'de> for () {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        deserializer.deserialize_unit(UnitVisitor)
+    }
+}
+
+#[cfg(feature = "unstable")]
+impl<'de> Deserialize<'de> for ! {
+    fn deserialize<D>(_deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        Err(Error::custom("cannot deserialize `!`"))
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+struct BoolVisitor;
+
+impl<'de> Visitor<'de> for BoolVisitor {
+    type Value = bool;
+
+    fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        formatter.write_str("a boolean")
+    }
+
+    fn visit_bool<E>(self, v: bool) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        Ok(v)
+    }
+}
+
+impl<'de> Deserialize<'de> for bool {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        deserializer.deserialize_bool(BoolVisitor)
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+macro_rules! visit_integer_method {
+    ($src_ty:ident, $method:ident, $from_method:ident, $group:ident, $group_ty:ident) => {
+        #[inline]
+        fn $method<E>(self, v: $src_ty) -> Result<Self::Value, E>
+        where
+            E: Error,
+        {
+            match FromPrimitive::$from_method(v) {
+                Some(v) => Ok(v),
+                None => Err(Error::invalid_value(
+                    Unexpected::$group(v as $group_ty),
+                    &self,
+                )),
+            }
+        }
+    };
+}
+
+macro_rules! visit_float_method {
+    ($src_ty:ident, $method:ident) => {
+        #[inline]
+        fn $method<E>(self, v: $src_ty) -> Result<Self::Value, E>
+        where
+            E: Error,
+        {
+            Ok(v as Self::Value)
+        }
+    };
+}
+
+macro_rules! impl_deserialize_num {
+    ($ty:ident, $method:ident, $($visit:ident),*) => {
+        impl<'de> Deserialize<'de> for $ty {
+            #[inline]
+            fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+            where
+                D: Deserializer<'de>,
+            {
+                struct PrimitiveVisitor;
+
+                impl<'de> Visitor<'de> for PrimitiveVisitor {
+                    type Value = $ty;
+
+                    fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+                        formatter.write_str(stringify!($ty))
+                    }
+
+                    $(
+                        impl_deserialize_num!($visit $ty);
+                    )*
+                }
+
+                deserializer.$method(PrimitiveVisitor)
+            }
+        }
+    };
+
+    (integer $ty:ident) => {
+        visit_integer_method!(i8, visit_i8, from_i8, Signed, i64);
+        visit_integer_method!(i16, visit_i16, from_i16, Signed, i64);
+        visit_integer_method!(i32, visit_i32, from_i32, Signed, i64);
+        visit_integer_method!(i64, visit_i64, from_i64, Signed, i64);
+
+        visit_integer_method!(u8, visit_u8, from_u8, Unsigned, u64);
+        visit_integer_method!(u16, visit_u16, from_u16, Unsigned, u64);
+        visit_integer_method!(u32, visit_u32, from_u32, Unsigned, u64);
+        visit_integer_method!(u64, visit_u64, from_u64, Unsigned, u64);
+    };
+
+    (float $ty:ident) => {
+        visit_float_method!(f32, visit_f32);
+        visit_float_method!(f64, visit_f64);
+    };
+}
+
+impl_deserialize_num!(i8, deserialize_i8, integer);
+impl_deserialize_num!(i16, deserialize_i16, integer);
+impl_deserialize_num!(i32, deserialize_i32, integer);
+impl_deserialize_num!(i64, deserialize_i64, integer);
+impl_deserialize_num!(isize, deserialize_i64, integer);
+
+impl_deserialize_num!(u8, deserialize_u8, integer);
+impl_deserialize_num!(u16, deserialize_u16, integer);
+impl_deserialize_num!(u32, deserialize_u32, integer);
+impl_deserialize_num!(u64, deserialize_u64, integer);
+impl_deserialize_num!(usize, deserialize_u64, integer);
+
+impl_deserialize_num!(f32, deserialize_f32, integer, float);
+impl_deserialize_num!(f64, deserialize_f64, integer, float);
+
+serde_if_integer128! {
+    impl<'de> Deserialize<'de> for i128 {
+        #[inline]
+        fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+        where
+            D: Deserializer<'de>,
+        {
+            struct PrimitiveVisitor;
+
+            impl<'de> Visitor<'de> for PrimitiveVisitor {
+                type Value = i128;
+
+                fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+                    formatter.write_str("i128")
+                }
+
+                impl_deserialize_num!(integer i128);
+
+                #[inline]
+                fn visit_i128<E>(self, v: i128) -> Result<Self::Value, E>
+                where
+                    E: Error,
+                {
+                    Ok(v)
+                }
+
+                #[inline]
+                fn visit_u128<E>(self, v: u128) -> Result<Self::Value, E>
+                where
+                    E: Error,
+                {
+                    if v <= i128::max_value() as u128 {
+                        Ok(v as i128)
+                    } else {
+                        Err(Error::invalid_value(Unexpected::Other("u128"), &self))
+                    }
+                }
+            }
+
+            deserializer.deserialize_i128(PrimitiveVisitor)
+        }
+    }
+
+    impl<'de> Deserialize<'de> for u128 {
+        #[inline]
+        fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+        where
+            D: Deserializer<'de>,
+        {
+            struct PrimitiveVisitor;
+
+            impl<'de> Visitor<'de> for PrimitiveVisitor {
+                type Value = u128;
+
+                fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+                    formatter.write_str("u128")
+                }
+
+                impl_deserialize_num!(integer u128);
+
+                #[inline]
+                fn visit_i128<E>(self, v: i128) -> Result<Self::Value, E>
+                where
+                    E: Error,
+                {
+                    if v >= 0 {
+                        Ok(v as u128)
+                    } else {
+                        Err(Error::invalid_value(Unexpected::Other("i128"), &self))
+                    }
+                }
+
+                #[inline]
+                fn visit_u128<E>(self, v: u128) -> Result<Self::Value, E>
+                where
+                    E: Error,
+                {
+                    Ok(v)
+                }
+            }
+
+            deserializer.deserialize_u128(PrimitiveVisitor)
+        }
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+struct CharVisitor;
+
+impl<'de> Visitor<'de> for CharVisitor {
+    type Value = char;
+
+    fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        formatter.write_str("a character")
+    }
+
+    #[inline]
+    fn visit_char<E>(self, v: char) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        Ok(v)
+    }
+
+    #[inline]
+    fn visit_str<E>(self, v: &str) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        let mut iter = v.chars();
+        match (iter.next(), iter.next()) {
+            (Some(c), None) => Ok(c),
+            _ => Err(Error::invalid_value(Unexpected::Str(v), &self)),
+        }
+    }
+}
+
+impl<'de> Deserialize<'de> for char {
+    #[inline]
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        deserializer.deserialize_char(CharVisitor)
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+struct StringVisitor;
+#[cfg(any(feature = "std", feature = "alloc"))]
+struct StringInPlaceVisitor<'a>(&'a mut String);
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+impl<'de> Visitor<'de> for StringVisitor {
+    type Value = String;
+
+    fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        formatter.write_str("a string")
+    }
+
+    fn visit_str<E>(self, v: &str) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        Ok(v.to_owned())
+    }
+
+    fn visit_string<E>(self, v: String) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        Ok(v)
+    }
+
+    fn visit_bytes<E>(self, v: &[u8]) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        match str::from_utf8(v) {
+            Ok(s) => Ok(s.to_owned()),
+            Err(_) => Err(Error::invalid_value(Unexpected::Bytes(v), &self)),
+        }
+    }
+
+    fn visit_byte_buf<E>(self, v: Vec<u8>) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        match String::from_utf8(v) {
+            Ok(s) => Ok(s),
+            Err(e) => Err(Error::invalid_value(
+                Unexpected::Bytes(&e.into_bytes()),
+                &self,
+            )),
+        }
+    }
+}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+impl<'a, 'de> Visitor<'de> for StringInPlaceVisitor<'a> {
+    type Value = ();
+
+    fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        formatter.write_str("a string")
+    }
+
+    fn visit_str<E>(self, v: &str) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        self.0.clear();
+        self.0.push_str(v);
+        Ok(())
+    }
+
+    fn visit_string<E>(self, v: String) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        *self.0 = v;
+        Ok(())
+    }
+
+    fn visit_bytes<E>(self, v: &[u8]) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        match str::from_utf8(v) {
+            Ok(s) => {
+                self.0.clear();
+                self.0.push_str(s);
+                Ok(())
+            }
+            Err(_) => Err(Error::invalid_value(Unexpected::Bytes(v), &self)),
+        }
+    }
+
+    fn visit_byte_buf<E>(self, v: Vec<u8>) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        match String::from_utf8(v) {
+            Ok(s) => {
+                *self.0 = s;
+                Ok(())
+            }
+            Err(e) => Err(Error::invalid_value(
+                Unexpected::Bytes(&e.into_bytes()),
+                &self,
+            )),
+        }
+    }
+}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+impl<'de> Deserialize<'de> for String {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        deserializer.deserialize_string(StringVisitor)
+    }
+
+    fn deserialize_in_place<D>(deserializer: D, place: &mut Self) -> Result<(), D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        deserializer.deserialize_string(StringInPlaceVisitor(place))
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+struct StrVisitor;
+
+impl<'a> Visitor<'a> for StrVisitor {
+    type Value = &'a str;
+
+    fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        formatter.write_str("a borrowed string")
+    }
+
+    fn visit_borrowed_str<E>(self, v: &'a str) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        Ok(v) // so easy
+    }
+
+    fn visit_borrowed_bytes<E>(self, v: &'a [u8]) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        str::from_utf8(v).map_err(|_| Error::invalid_value(Unexpected::Bytes(v), &self))
+    }
+}
+
+impl<'de: 'a, 'a> Deserialize<'de> for &'a str {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        deserializer.deserialize_str(StrVisitor)
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+struct BytesVisitor;
+
+impl<'a> Visitor<'a> for BytesVisitor {
+    type Value = &'a [u8];
+
+    fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        formatter.write_str("a borrowed byte array")
+    }
+
+    fn visit_borrowed_bytes<E>(self, v: &'a [u8]) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        Ok(v)
+    }
+
+    fn visit_borrowed_str<E>(self, v: &'a str) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        Ok(v.as_bytes())
+    }
+}
+
+impl<'de: 'a, 'a> Deserialize<'de> for &'a [u8] {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        deserializer.deserialize_bytes(BytesVisitor)
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+#[cfg(feature = "std")]
+struct CStringVisitor;
+
+#[cfg(feature = "std")]
+impl<'de> Visitor<'de> for CStringVisitor {
+    type Value = CString;
+
+    fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        formatter.write_str("byte array")
+    }
+
+    fn visit_seq<A>(self, mut seq: A) -> Result<Self::Value, A::Error>
+    where
+        A: SeqAccess<'de>,
+    {
+        let len = size_hint::cautious(seq.size_hint());
+        let mut values = Vec::with_capacity(len);
+
+        while let Some(value) = try!(seq.next_element()) {
+            values.push(value);
+        }
+
+        CString::new(values).map_err(Error::custom)
+    }
+
+    fn visit_bytes<E>(self, v: &[u8]) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        CString::new(v).map_err(Error::custom)
+    }
+
+    fn visit_byte_buf<E>(self, v: Vec<u8>) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        CString::new(v).map_err(Error::custom)
+    }
+
+    fn visit_str<E>(self, v: &str) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        CString::new(v).map_err(Error::custom)
+    }
+
+    fn visit_string<E>(self, v: String) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        CString::new(v).map_err(Error::custom)
+    }
+}
+
+#[cfg(feature = "std")]
+impl<'de> Deserialize<'de> for CString {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        deserializer.deserialize_byte_buf(CStringVisitor)
+    }
+}
+
+macro_rules! forwarded_impl {
+    (
+        $(#[doc = $doc:tt])*
+        ( $($id: ident),* ), $ty: ty, $func: expr
+    ) => {
+        $(#[doc = $doc])*
+        impl<'de $(, $id : Deserialize<'de>,)*> Deserialize<'de> for $ty {
+            fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+            where
+                D: Deserializer<'de>,
+            {
+                Deserialize::deserialize(deserializer).map($func)
+            }
+        }
+    }
+}
+
+#[cfg(all(feature = "std", de_boxed_c_str))]
+forwarded_impl!((), Box<CStr>, CString::into_boxed_c_str);
+
+#[cfg(core_reverse)]
+forwarded_impl!((T), Reverse<T>, Reverse);
+
+////////////////////////////////////////////////////////////////////////////////
+
+struct OptionVisitor<T> {
+    marker: PhantomData<T>,
+}
+
+impl<'de, T> Visitor<'de> for OptionVisitor<T>
+where
+    T: Deserialize<'de>,
+{
+    type Value = Option<T>;
+
+    fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        formatter.write_str("option")
+    }
+
+    #[inline]
+    fn visit_unit<E>(self) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        Ok(None)
+    }
+
+    #[inline]
+    fn visit_none<E>(self) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        Ok(None)
+    }
+
+    #[inline]
+    fn visit_some<D>(self, deserializer: D) -> Result<Self::Value, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        T::deserialize(deserializer).map(Some)
+    }
+
+    #[doc(hidden)]
+    fn __private_visit_untagged_option<D>(self, deserializer: D) -> Result<Self::Value, ()>
+    where
+        D: Deserializer<'de>,
+    {
+        Ok(T::deserialize(deserializer).ok())
+    }
+}
+
+impl<'de, T> Deserialize<'de> for Option<T>
+where
+    T: Deserialize<'de>,
+{
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        deserializer.deserialize_option(OptionVisitor {
+            marker: PhantomData,
+        })
+    }
+
+    // The Some variant's repr is opaque, so we can't play cute tricks with its
+    // tag to have deserialize_in_place build the content in place unconditionally.
+    //
+    // FIXME: investigate whether branching on the old value being Some to
+    // deserialize_in_place the value is profitable (probably data-dependent?)
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+struct PhantomDataVisitor<T: ?Sized> {
+    marker: PhantomData<T>,
+}
+
+impl<'de, T: ?Sized> Visitor<'de> for PhantomDataVisitor<T> {
+    type Value = PhantomData<T>;
+
+    fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        formatter.write_str("unit")
+    }
+
+    #[inline]
+    fn visit_unit<E>(self) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        Ok(PhantomData)
+    }
+}
+
+impl<'de, T: ?Sized> Deserialize<'de> for PhantomData<T> {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        let visitor = PhantomDataVisitor {
+            marker: PhantomData,
+        };
+        deserializer.deserialize_unit_struct("PhantomData", visitor)
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+macro_rules! seq_impl {
+    (
+        $ty:ident < T $(: $tbound1:ident $(+ $tbound2:ident)*)* $(, $typaram:ident : $bound1:ident $(+ $bound2:ident)*)* >,
+        $access:ident,
+        $clear:expr,
+        $with_capacity:expr,
+        $reserve:expr,
+        $insert:expr
+    ) => {
+        impl<'de, T $(, $typaram)*> Deserialize<'de> for $ty<T $(, $typaram)*>
+        where
+            T: Deserialize<'de> $(+ $tbound1 $(+ $tbound2)*)*,
+            $($typaram: $bound1 $(+ $bound2)*,)*
+        {
+            fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+            where
+                D: Deserializer<'de>,
+            {
+                struct SeqVisitor<T $(, $typaram)*> {
+                    marker: PhantomData<$ty<T $(, $typaram)*>>,
+                }
+
+                impl<'de, T $(, $typaram)*> Visitor<'de> for SeqVisitor<T $(, $typaram)*>
+                where
+                    T: Deserialize<'de> $(+ $tbound1 $(+ $tbound2)*)*,
+                    $($typaram: $bound1 $(+ $bound2)*,)*
+                {
+                    type Value = $ty<T $(, $typaram)*>;
+
+                    fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+                        formatter.write_str("a sequence")
+                    }
+
+                    #[inline]
+                    fn visit_seq<A>(self, mut $access: A) -> Result<Self::Value, A::Error>
+                    where
+                        A: SeqAccess<'de>,
+                    {
+                        let mut values = $with_capacity;
+
+                        while let Some(value) = try!($access.next_element()) {
+                            $insert(&mut values, value);
+                        }
+
+                        Ok(values)
+                    }
+                }
+
+                let visitor = SeqVisitor { marker: PhantomData };
+                deserializer.deserialize_seq(visitor)
+            }
+
+            fn deserialize_in_place<D>(deserializer: D, place: &mut Self) -> Result<(), D::Error>
+            where
+                D: Deserializer<'de>,
+            {
+                struct SeqInPlaceVisitor<'a, T: 'a $(, $typaram: 'a)*>(&'a mut $ty<T $(, $typaram)*>);
+
+                impl<'a, 'de, T $(, $typaram)*> Visitor<'de> for SeqInPlaceVisitor<'a, T $(, $typaram)*>
+                where
+                    T: Deserialize<'de> $(+ $tbound1 $(+ $tbound2)*)*,
+                    $($typaram: $bound1 $(+ $bound2)*,)*
+                {
+                    type Value = ();
+
+                    fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+                        formatter.write_str("a sequence")
+                    }
+
+                    #[inline]
+                    fn visit_seq<A>(mut self, mut $access: A) -> Result<Self::Value, A::Error>
+                    where
+                        A: SeqAccess<'de>,
+                    {
+                        $clear(&mut self.0);
+                        $reserve(&mut self.0, size_hint::cautious($access.size_hint()));
+
+                        // FIXME: try to overwrite old values here? (Vec, VecDeque, LinkedList)
+                        while let Some(value) = try!($access.next_element()) {
+                            $insert(&mut self.0, value);
+                        }
+
+                        Ok(())
+                    }
+                }
+
+                deserializer.deserialize_seq(SeqInPlaceVisitor(place))
+            }
+        }
+    }
+}
+
+// Dummy impl of reserve
+#[cfg(any(feature = "std", feature = "alloc"))]
+fn nop_reserve<T>(_seq: T, _n: usize) {}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+seq_impl!(
+    BinaryHeap<T: Ord>,
+    seq,
+    BinaryHeap::clear,
+    BinaryHeap::with_capacity(size_hint::cautious(seq.size_hint())),
+    BinaryHeap::reserve,
+    BinaryHeap::push
+);
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+seq_impl!(
+    BTreeSet<T: Eq + Ord>,
+    seq,
+    BTreeSet::clear,
+    BTreeSet::new(),
+    nop_reserve,
+    BTreeSet::insert
+);
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+seq_impl!(
+    LinkedList<T>,
+    seq,
+    LinkedList::clear,
+    LinkedList::new(),
+    nop_reserve,
+    LinkedList::push_back
+);
+
+#[cfg(feature = "std")]
+seq_impl!(
+    HashSet<T: Eq + Hash, S: BuildHasher + Default>,
+    seq,
+    HashSet::clear,
+    HashSet::with_capacity_and_hasher(size_hint::cautious(seq.size_hint()), S::default()),
+    HashSet::reserve,
+    HashSet::insert);
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+seq_impl!(
+    VecDeque<T>,
+    seq,
+    VecDeque::clear,
+    VecDeque::with_capacity(size_hint::cautious(seq.size_hint())),
+    VecDeque::reserve,
+    VecDeque::push_back
+);
+
+////////////////////////////////////////////////////////////////////////////////
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+impl<'de, T> Deserialize<'de> for Vec<T>
+where
+    T: Deserialize<'de>,
+{
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        struct VecVisitor<T> {
+            marker: PhantomData<T>,
+        }
+
+        impl<'de, T> Visitor<'de> for VecVisitor<T>
+        where
+            T: Deserialize<'de>,
+        {
+            type Value = Vec<T>;
+
+            fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+                formatter.write_str("a sequence")
+            }
+
+            fn visit_seq<A>(self, mut seq: A) -> Result<Self::Value, A::Error>
+            where
+                A: SeqAccess<'de>,
+            {
+                let mut values = Vec::with_capacity(size_hint::cautious(seq.size_hint()));
+
+                while let Some(value) = try!(seq.next_element()) {
+                    values.push(value);
+                }
+
+                Ok(values)
+            }
+        }
+
+        let visitor = VecVisitor {
+            marker: PhantomData,
+        };
+        deserializer.deserialize_seq(visitor)
+    }
+
+    fn deserialize_in_place<D>(deserializer: D, place: &mut Self) -> Result<(), D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        struct VecInPlaceVisitor<'a, T: 'a>(&'a mut Vec<T>);
+
+        impl<'a, 'de, T> Visitor<'de> for VecInPlaceVisitor<'a, T>
+        where
+            T: Deserialize<'de>,
+        {
+            type Value = ();
+
+            fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+                formatter.write_str("a sequence")
+            }
+
+            fn visit_seq<A>(self, mut seq: A) -> Result<Self::Value, A::Error>
+            where
+                A: SeqAccess<'de>,
+            {
+                let hint = size_hint::cautious(seq.size_hint());
+                if let Some(additional) = hint.checked_sub(self.0.len()) {
+                    self.0.reserve(additional);
+                }
+
+                for i in 0..self.0.len() {
+                    let next = {
+                        let next_place = InPlaceSeed(&mut self.0[i]);
+                        try!(seq.next_element_seed(next_place))
+                    };
+                    if next.is_none() {
+                        self.0.truncate(i);
+                        return Ok(());
+                    }
+                }
+
+                while let Some(value) = try!(seq.next_element()) {
+                    self.0.push(value);
+                }
+
+                Ok(())
+            }
+        }
+
+        deserializer.deserialize_seq(VecInPlaceVisitor(place))
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+struct ArrayVisitor<A> {
+    marker: PhantomData<A>,
+}
+struct ArrayInPlaceVisitor<'a, A: 'a>(&'a mut A);
+
+impl<A> ArrayVisitor<A> {
+    fn new() -> Self {
+        ArrayVisitor {
+            marker: PhantomData,
+        }
+    }
+}
+
+impl<'de, T> Visitor<'de> for ArrayVisitor<[T; 0]> {
+    type Value = [T; 0];
+
+    fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        formatter.write_str("an empty array")
+    }
+
+    #[inline]
+    fn visit_seq<A>(self, _: A) -> Result<Self::Value, A::Error>
+    where
+        A: SeqAccess<'de>,
+    {
+        Ok([])
+    }
+}
+
+// Does not require T: Deserialize<'de>.
+impl<'de, T> Deserialize<'de> for [T; 0] {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        deserializer.deserialize_tuple(0, ArrayVisitor::<[T; 0]>::new())
+    }
+}
+
+macro_rules! array_impls {
+    ($($len:expr => ($($n:tt)+))+) => {
+        $(
+            impl<'de, T> Visitor<'de> for ArrayVisitor<[T; $len]>
+            where
+                T: Deserialize<'de>,
+            {
+                type Value = [T; $len];
+
+                fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+                    formatter.write_str(concat!("an array of length ", $len))
+                }
+
+                #[inline]
+                fn visit_seq<A>(self, mut seq: A) -> Result<Self::Value, A::Error>
+                where
+                    A: SeqAccess<'de>,
+                {
+                    Ok([$(
+                        match try!(seq.next_element()) {
+                            Some(val) => val,
+                            None => return Err(Error::invalid_length($n, &self)),
+                        }
+                    ),+])
+                }
+            }
+
+            impl<'a, 'de, T> Visitor<'de> for ArrayInPlaceVisitor<'a, [T; $len]>
+            where
+                T: Deserialize<'de>,
+            {
+                type Value = ();
+
+                fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+                    formatter.write_str(concat!("an array of length ", $len))
+                }
+
+                #[inline]
+                fn visit_seq<A>(self, mut seq: A) -> Result<Self::Value, A::Error>
+                where
+                    A: SeqAccess<'de>,
+                {
+                    let mut fail_idx = None;
+                    for (idx, dest) in self.0[..].iter_mut().enumerate() {
+                        if try!(seq.next_element_seed(InPlaceSeed(dest))).is_none() {
+                            fail_idx = Some(idx);
+                            break;
+                        }
+                    }
+                    if let Some(idx) = fail_idx {
+                        return Err(Error::invalid_length(idx, &self));
+                    }
+                    Ok(())
+                }
+            }
+
+            impl<'de, T> Deserialize<'de> for [T; $len]
+            where
+                T: Deserialize<'de>,
+            {
+                fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+                where
+                    D: Deserializer<'de>,
+                {
+                    deserializer.deserialize_tuple($len, ArrayVisitor::<[T; $len]>::new())
+                }
+
+                fn deserialize_in_place<D>(deserializer: D, place: &mut Self) -> Result<(), D::Error>
+                where
+                    D: Deserializer<'de>,
+                {
+                    deserializer.deserialize_tuple($len, ArrayInPlaceVisitor(place))
+                }
+            }
+        )+
+    }
+}
+
+array_impls! {
+    1 => (0)
+    2 => (0 1)
+    3 => (0 1 2)
+    4 => (0 1 2 3)
+    5 => (0 1 2 3 4)
+    6 => (0 1 2 3 4 5)
+    7 => (0 1 2 3 4 5 6)
+    8 => (0 1 2 3 4 5 6 7)
+    9 => (0 1 2 3 4 5 6 7 8)
+    10 => (0 1 2 3 4 5 6 7 8 9)
+    11 => (0 1 2 3 4 5 6 7 8 9 10)
+    12 => (0 1 2 3 4 5 6 7 8 9 10 11)
+    13 => (0 1 2 3 4 5 6 7 8 9 10 11 12)
+    14 => (0 1 2 3 4 5 6 7 8 9 10 11 12 13)
+    15 => (0 1 2 3 4 5 6 7 8 9 10 11 12 13 14)
+    16 => (0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15)
+    17 => (0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16)
+    18 => (0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17)
+    19 => (0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18)
+    20 => (0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19)
+    21 => (0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20)
+    22 => (0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21)
+    23 => (0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22)
+    24 => (0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23)
+    25 => (0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24)
+    26 => (0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25)
+    27 => (0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26)
+    28 => (0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27)
+    29 => (0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28)
+    30 => (0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29)
+    31 => (0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30)
+    32 => (0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31)
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+macro_rules! tuple_impls {
+    ($($len:tt => ($($n:tt $name:ident)+))+) => {
+        $(
+            impl<'de, $($name: Deserialize<'de>),+> Deserialize<'de> for ($($name,)+) {
+                #[inline]
+                fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+                where
+                    D: Deserializer<'de>,
+                {
+                    struct TupleVisitor<$($name,)+> {
+                        marker: PhantomData<($($name,)+)>,
+                    }
+
+                    impl<'de, $($name: Deserialize<'de>),+> Visitor<'de> for TupleVisitor<$($name,)+> {
+                        type Value = ($($name,)+);
+
+                        fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+                            formatter.write_str(concat!("a tuple of size ", $len))
+                        }
+
+                        #[inline]
+                        #[allow(non_snake_case)]
+                        fn visit_seq<A>(self, mut seq: A) -> Result<Self::Value, A::Error>
+                        where
+                            A: SeqAccess<'de>,
+                        {
+                            $(
+                                let $name = match try!(seq.next_element()) {
+                                    Some(value) => value,
+                                    None => return Err(Error::invalid_length($n, &self)),
+                                };
+                            )+
+
+                            Ok(($($name,)+))
+                        }
+                    }
+
+                    deserializer.deserialize_tuple($len, TupleVisitor { marker: PhantomData })
+                }
+
+                #[inline]
+                fn deserialize_in_place<D>(deserializer: D, place: &mut Self) -> Result<(), D::Error>
+                where
+                    D: Deserializer<'de>,
+                {
+                    struct TupleInPlaceVisitor<'a, $($name: 'a,)+>(&'a mut ($($name,)+));
+
+                    impl<'a, 'de, $($name: Deserialize<'de>),+> Visitor<'de> for TupleInPlaceVisitor<'a, $($name,)+> {
+                        type Value = ();
+
+                        fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+                            formatter.write_str(concat!("a tuple of size ", $len))
+                        }
+
+                        #[inline]
+                        #[allow(non_snake_case)]
+                        fn visit_seq<A>(self, mut seq: A) -> Result<Self::Value, A::Error>
+                        where
+                            A: SeqAccess<'de>,
+                        {
+                            $(
+                                if try!(seq.next_element_seed(InPlaceSeed(&mut (self.0).$n))).is_none() {
+                                    return Err(Error::invalid_length($n, &self));
+                                }
+                            )+
+
+                            Ok(())
+                        }
+                    }
+
+                    deserializer.deserialize_tuple($len, TupleInPlaceVisitor(place))
+                }
+            }
+        )+
+    }
+}
+
+tuple_impls! {
+    1  => (0 T0)
+    2  => (0 T0 1 T1)
+    3  => (0 T0 1 T1 2 T2)
+    4  => (0 T0 1 T1 2 T2 3 T3)
+    5  => (0 T0 1 T1 2 T2 3 T3 4 T4)
+    6  => (0 T0 1 T1 2 T2 3 T3 4 T4 5 T5)
+    7  => (0 T0 1 T1 2 T2 3 T3 4 T4 5 T5 6 T6)
+    8  => (0 T0 1 T1 2 T2 3 T3 4 T4 5 T5 6 T6 7 T7)
+    9  => (0 T0 1 T1 2 T2 3 T3 4 T4 5 T5 6 T6 7 T7 8 T8)
+    10 => (0 T0 1 T1 2 T2 3 T3 4 T4 5 T5 6 T6 7 T7 8 T8 9 T9)
+    11 => (0 T0 1 T1 2 T2 3 T3 4 T4 5 T5 6 T6 7 T7 8 T8 9 T9 10 T10)
+    12 => (0 T0 1 T1 2 T2 3 T3 4 T4 5 T5 6 T6 7 T7 8 T8 9 T9 10 T10 11 T11)
+    13 => (0 T0 1 T1 2 T2 3 T3 4 T4 5 T5 6 T6 7 T7 8 T8 9 T9 10 T10 11 T11 12 T12)
+    14 => (0 T0 1 T1 2 T2 3 T3 4 T4 5 T5 6 T6 7 T7 8 T8 9 T9 10 T10 11 T11 12 T12 13 T13)
+    15 => (0 T0 1 T1 2 T2 3 T3 4 T4 5 T5 6 T6 7 T7 8 T8 9 T9 10 T10 11 T11 12 T12 13 T13 14 T14)
+    16 => (0 T0 1 T1 2 T2 3 T3 4 T4 5 T5 6 T6 7 T7 8 T8 9 T9 10 T10 11 T11 12 T12 13 T13 14 T14 15 T15)
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+macro_rules! map_impl {
+    (
+        $ty:ident < K $(: $kbound1:ident $(+ $kbound2:ident)*)*, V $(, $typaram:ident : $bound1:ident $(+ $bound2:ident)*)* >,
+        $access:ident,
+        $with_capacity:expr
+    ) => {
+        impl<'de, K, V $(, $typaram)*> Deserialize<'de> for $ty<K, V $(, $typaram)*>
+        where
+            K: Deserialize<'de> $(+ $kbound1 $(+ $kbound2)*)*,
+            V: Deserialize<'de>,
+            $($typaram: $bound1 $(+ $bound2)*),*
+        {
+            fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+            where
+                D: Deserializer<'de>,
+            {
+                struct MapVisitor<K, V $(, $typaram)*> {
+                    marker: PhantomData<$ty<K, V $(, $typaram)*>>,
+                }
+
+                impl<'de, K, V $(, $typaram)*> Visitor<'de> for MapVisitor<K, V $(, $typaram)*>
+                where
+                    K: Deserialize<'de> $(+ $kbound1 $(+ $kbound2)*)*,
+                    V: Deserialize<'de>,
+                    $($typaram: $bound1 $(+ $bound2)*),*
+                {
+                    type Value = $ty<K, V $(, $typaram)*>;
+
+                    fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+                        formatter.write_str("a map")
+                    }
+
+                    #[inline]
+                    fn visit_map<A>(self, mut $access: A) -> Result<Self::Value, A::Error>
+                    where
+                        A: MapAccess<'de>,
+                    {
+                        let mut values = $with_capacity;
+
+                        while let Some((key, value)) = try!($access.next_entry()) {
+                            values.insert(key, value);
+                        }
+
+                        Ok(values)
+                    }
+                }
+
+                let visitor = MapVisitor { marker: PhantomData };
+                deserializer.deserialize_map(visitor)
+            }
+        }
+    }
+}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+map_impl!(
+    BTreeMap<K: Ord, V>,
+    map,
+    BTreeMap::new());
+
+#[cfg(feature = "std")]
+map_impl!(
+    HashMap<K: Eq + Hash, V, S: BuildHasher + Default>,
+    map,
+    HashMap::with_capacity_and_hasher(size_hint::cautious(map.size_hint()), S::default()));
+
+////////////////////////////////////////////////////////////////////////////////
+
+#[cfg(feature = "std")]
+macro_rules! parse_ip_impl {
+    ($expecting:tt $ty:ty; $size:tt) => {
+        impl<'de> Deserialize<'de> for $ty {
+            fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+            where
+                D: Deserializer<'de>,
+            {
+                if deserializer.is_human_readable() {
+                    struct IpAddrVisitor;
+
+                    impl<'de> Visitor<'de> for IpAddrVisitor {
+                        type Value = $ty;
+
+                        fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+                            formatter.write_str($expecting)
+                        }
+
+                        fn visit_str<E>(self, s: &str) -> Result<Self::Value, E>
+                        where
+                            E: Error,
+                        {
+                            s.parse().map_err(Error::custom)
+                        }
+                    }
+
+                    deserializer.deserialize_str(IpAddrVisitor)
+                } else {
+                    <[u8; $size]>::deserialize(deserializer).map(<$ty>::from)
+                }
+            }
+        }
+    };
+}
+
+#[cfg(feature = "std")]
+macro_rules! variant_identifier {
+    (
+        $name_kind: ident ( $($variant: ident; $bytes: expr; $index: expr),* )
+        $expecting_message: expr,
+        $variants_name: ident
+    ) => {
+        enum $name_kind {
+            $( $variant ),*
+        }
+
+        static $variants_name: &'static [&'static str] = &[ $( stringify!($variant) ),*];
+
+        impl<'de> Deserialize<'de> for $name_kind {
+            fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+            where
+                D: Deserializer<'de>,
+            {
+                struct KindVisitor;
+
+                impl<'de> Visitor<'de> for KindVisitor {
+                    type Value = $name_kind;
+
+                    fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+                        formatter.write_str($expecting_message)
+                    }
+
+                    fn visit_u32<E>(self, value: u32) -> Result<Self::Value, E>
+                    where
+                        E: Error,
+                    {
+                        match value {
+                            $(
+                                $index => Ok($name_kind :: $variant),
+                            )*
+                            _ => Err(Error::invalid_value(Unexpected::Unsigned(value as u64), &self),),
+                        }
+                    }
+
+                    fn visit_str<E>(self, value: &str) -> Result<Self::Value, E>
+                    where
+                        E: Error,
+                    {
+                        match value {
+                            $(
+                                stringify!($variant) => Ok($name_kind :: $variant),
+                            )*
+                            _ => Err(Error::unknown_variant(value, $variants_name)),
+                        }
+                    }
+
+                    fn visit_bytes<E>(self, value: &[u8]) -> Result<Self::Value, E>
+                    where
+                        E: Error,
+                    {
+                        match value {
+                            $(
+                                $bytes => Ok($name_kind :: $variant),
+                            )*
+                            _ => {
+                                match str::from_utf8(value) {
+                                    Ok(value) => Err(Error::unknown_variant(value, $variants_name)),
+                                    Err(_) => Err(Error::invalid_value(Unexpected::Bytes(value), &self)),
+                                }
+                            }
+                        }
+                    }
+                }
+
+                deserializer.deserialize_identifier(KindVisitor)
+            }
+        }
+    }
+}
+
+#[cfg(feature = "std")]
+macro_rules! deserialize_enum {
+    (
+        $name: ident $name_kind: ident ( $($variant: ident; $bytes: expr; $index: expr),* )
+        $expecting_message: expr,
+        $deserializer: expr
+    ) => {
+        variant_identifier!{
+            $name_kind ( $($variant; $bytes; $index),* )
+            $expecting_message,
+            VARIANTS
+        }
+
+        struct EnumVisitor;
+        impl<'de> Visitor<'de> for EnumVisitor {
+            type Value = $name;
+
+            fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+                formatter.write_str(concat!("a ", stringify!($name)))
+            }
+
+
+            fn visit_enum<A>(self, data: A) -> Result<Self::Value, A::Error>
+            where
+                A: EnumAccess<'de>,
+            {
+                match try!(data.variant()) {
+                    $(
+                        ($name_kind :: $variant, v) => v.newtype_variant().map($name :: $variant),
+                    )*
+                }
+            }
+        }
+        $deserializer.deserialize_enum(stringify!($name), VARIANTS, EnumVisitor)
+    }
+}
+
+#[cfg(feature = "std")]
+impl<'de> Deserialize<'de> for net::IpAddr {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        if deserializer.is_human_readable() {
+            struct IpAddrVisitor;
+
+            impl<'de> Visitor<'de> for IpAddrVisitor {
+                type Value = net::IpAddr;
+
+                fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+                    formatter.write_str("IP address")
+                }
+
+                fn visit_str<E>(self, s: &str) -> Result<Self::Value, E>
+                where
+                    E: Error,
+                {
+                    s.parse().map_err(Error::custom)
+                }
+            }
+
+            deserializer.deserialize_str(IpAddrVisitor)
+        } else {
+            use lib::net::IpAddr;
+            deserialize_enum! {
+                IpAddr IpAddrKind (V4; b"V4"; 0, V6; b"V6"; 1)
+                "`V4` or `V6`",
+                deserializer
+            }
+        }
+    }
+}
+
+#[cfg(feature = "std")]
+parse_ip_impl!("IPv4 address" net::Ipv4Addr; 4);
+
+#[cfg(feature = "std")]
+parse_ip_impl!("IPv6 address" net::Ipv6Addr; 16);
+
+#[cfg(feature = "std")]
+macro_rules! parse_socket_impl {
+    ($expecting:tt $ty:ty, $new:expr) => {
+        impl<'de> Deserialize<'de> for $ty {
+            fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+            where
+                D: Deserializer<'de>,
+            {
+                if deserializer.is_human_readable() {
+                    struct SocketAddrVisitor;
+
+                    impl<'de> Visitor<'de> for SocketAddrVisitor {
+                        type Value = $ty;
+
+                        fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+                            formatter.write_str($expecting)
+                        }
+
+                        fn visit_str<E>(self, s: &str) -> Result<Self::Value, E>
+                        where
+                            E: Error,
+                        {
+                            s.parse().map_err(Error::custom)
+                        }
+                    }
+
+                    deserializer.deserialize_str(SocketAddrVisitor)
+                } else {
+                    <(_, u16)>::deserialize(deserializer).map(|(ip, port)| $new(ip, port))
+                }
+            }
+        }
+    };
+}
+
+#[cfg(feature = "std")]
+impl<'de> Deserialize<'de> for net::SocketAddr {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        if deserializer.is_human_readable() {
+            struct SocketAddrVisitor;
+
+            impl<'de> Visitor<'de> for SocketAddrVisitor {
+                type Value = net::SocketAddr;
+
+                fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+                    formatter.write_str("socket address")
+                }
+
+                fn visit_str<E>(self, s: &str) -> Result<Self::Value, E>
+                where
+                    E: Error,
+                {
+                    s.parse().map_err(Error::custom)
+                }
+            }
+
+            deserializer.deserialize_str(SocketAddrVisitor)
+        } else {
+            use lib::net::SocketAddr;
+            deserialize_enum! {
+                SocketAddr SocketAddrKind (V4; b"V4"; 0, V6; b"V6"; 1)
+                "`V4` or `V6`",
+                deserializer
+            }
+        }
+    }
+}
+
+#[cfg(feature = "std")]
+parse_socket_impl!("IPv4 socket address" net::SocketAddrV4, net::SocketAddrV4::new);
+
+#[cfg(feature = "std")]
+parse_socket_impl!("IPv6 socket address" net::SocketAddrV6, |ip, port| net::SocketAddrV6::new(
+    ip, port, 0, 0
+));
+
+////////////////////////////////////////////////////////////////////////////////
+
+#[cfg(feature = "std")]
+struct PathVisitor;
+
+#[cfg(feature = "std")]
+impl<'a> Visitor<'a> for PathVisitor {
+    type Value = &'a Path;
+
+    fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        formatter.write_str("a borrowed path")
+    }
+
+    fn visit_borrowed_str<E>(self, v: &'a str) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        Ok(v.as_ref())
+    }
+
+    fn visit_borrowed_bytes<E>(self, v: &'a [u8]) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        str::from_utf8(v)
+            .map(AsRef::as_ref)
+            .map_err(|_| Error::invalid_value(Unexpected::Bytes(v), &self))
+    }
+}
+
+#[cfg(feature = "std")]
+impl<'de: 'a, 'a> Deserialize<'de> for &'a Path {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        deserializer.deserialize_str(PathVisitor)
+    }
+}
+
+#[cfg(feature = "std")]
+struct PathBufVisitor;
+
+#[cfg(feature = "std")]
+impl<'de> Visitor<'de> for PathBufVisitor {
+    type Value = PathBuf;
+
+    fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        formatter.write_str("path string")
+    }
+
+    fn visit_str<E>(self, v: &str) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        Ok(From::from(v))
+    }
+
+    fn visit_string<E>(self, v: String) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        Ok(From::from(v))
+    }
+
+    fn visit_bytes<E>(self, v: &[u8]) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        str::from_utf8(v)
+            .map(From::from)
+            .map_err(|_| Error::invalid_value(Unexpected::Bytes(v), &self))
+    }
+
+    fn visit_byte_buf<E>(self, v: Vec<u8>) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        String::from_utf8(v)
+            .map(From::from)
+            .map_err(|e| Error::invalid_value(Unexpected::Bytes(&e.into_bytes()), &self))
+    }
+}
+
+#[cfg(feature = "std")]
+impl<'de> Deserialize<'de> for PathBuf {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        deserializer.deserialize_string(PathBufVisitor)
+    }
+}
+
+#[cfg(all(feature = "std", de_boxed_path))]
+forwarded_impl!((), Box<Path>, PathBuf::into_boxed_path);
+
+////////////////////////////////////////////////////////////////////////////////
+
+// If this were outside of the serde crate, it would just use:
+//
+//    #[derive(Deserialize)]
+//    #[serde(variant_identifier)]
+#[cfg(all(feature = "std", any(unix, windows)))]
+variant_identifier! {
+    OsStringKind (Unix; b"Unix"; 0, Windows; b"Windows"; 1)
+    "`Unix` or `Windows`",
+    OSSTR_VARIANTS
+}
+
+#[cfg(all(feature = "std", any(unix, windows)))]
+struct OsStringVisitor;
+
+#[cfg(all(feature = "std", any(unix, windows)))]
+impl<'de> Visitor<'de> for OsStringVisitor {
+    type Value = OsString;
+
+    fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        formatter.write_str("os string")
+    }
+
+    #[cfg(unix)]
+    fn visit_enum<A>(self, data: A) -> Result<Self::Value, A::Error>
+    where
+        A: EnumAccess<'de>,
+    {
+        use std::os::unix::ffi::OsStringExt;
+
+        match try!(data.variant()) {
+            (OsStringKind::Unix, v) => v.newtype_variant().map(OsString::from_vec),
+            (OsStringKind::Windows, _) => Err(Error::custom(
+                "cannot deserialize Windows OS string on Unix",
+            )),
+        }
+    }
+
+    #[cfg(windows)]
+    fn visit_enum<A>(self, data: A) -> Result<Self::Value, A::Error>
+    where
+        A: EnumAccess<'de>,
+    {
+        use std::os::windows::ffi::OsStringExt;
+
+        match try!(data.variant()) {
+            (OsStringKind::Windows, v) => v
+                .newtype_variant::<Vec<u16>>()
+                .map(|vec| OsString::from_wide(&vec)),
+            (OsStringKind::Unix, _) => Err(Error::custom(
+                "cannot deserialize Unix OS string on Windows",
+            )),
+        }
+    }
+}
+
+#[cfg(all(feature = "std", any(unix, windows)))]
+impl<'de> Deserialize<'de> for OsString {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        deserializer.deserialize_enum("OsString", OSSTR_VARIANTS, OsStringVisitor)
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+forwarded_impl!((T), Box<T>, Box::new);
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+forwarded_impl!((T), Box<[T]>, Vec::into_boxed_slice);
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+forwarded_impl!((), Box<str>, String::into_boxed_str);
+
+#[cfg(all(
+    not(de_rc_dst),
+    feature = "rc",
+    any(feature = "std", feature = "alloc")
+))]
+forwarded_impl! {
+    /// This impl requires the [`"rc"`] Cargo feature of Serde.
+    ///
+    /// Deserializing a data structure containing `Arc` will not attempt to
+    /// deduplicate `Arc` references to the same data. Every deserialized `Arc`
+    /// will end up with a strong count of 1.
+    ///
+    /// [`"rc"`]: https://serde.rs/feature-flags.html#-features-rc
+    (T), Arc<T>, Arc::new
+}
+
+#[cfg(all(
+    not(de_rc_dst),
+    feature = "rc",
+    any(feature = "std", feature = "alloc")
+))]
+forwarded_impl! {
+    /// This impl requires the [`"rc"`] Cargo feature of Serde.
+    ///
+    /// Deserializing a data structure containing `Rc` will not attempt to
+    /// deduplicate `Rc` references to the same data. Every deserialized `Rc`
+    /// will end up with a strong count of 1.
+    ///
+    /// [`"rc"`]: https://serde.rs/feature-flags.html#-features-rc
+    (T), Rc<T>, Rc::new
+}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+impl<'de, 'a, T: ?Sized> Deserialize<'de> for Cow<'a, T>
+where
+    T: ToOwned,
+    T::Owned: Deserialize<'de>,
+{
+    #[inline]
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        T::Owned::deserialize(deserializer).map(Cow::Owned)
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+/// This impl requires the [`"rc"`] Cargo feature of Serde. The resulting
+/// `Weak<T>` has a reference count of 0 and cannot be upgraded.
+///
+/// [`"rc"`]: https://serde.rs/feature-flags.html#-features-rc
+#[cfg(all(feature = "rc", any(feature = "std", feature = "alloc")))]
+impl<'de, T: ?Sized> Deserialize<'de> for RcWeak<T>
+where
+    T: Deserialize<'de>,
+{
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        try!(Option::<T>::deserialize(deserializer));
+        Ok(RcWeak::new())
+    }
+}
+
+/// This impl requires the [`"rc"`] Cargo feature of Serde. The resulting
+/// `Weak<T>` has a reference count of 0 and cannot be upgraded.
+///
+/// [`"rc"`]: https://serde.rs/feature-flags.html#-features-rc
+#[cfg(all(feature = "rc", any(feature = "std", feature = "alloc")))]
+impl<'de, T: ?Sized> Deserialize<'de> for ArcWeak<T>
+where
+    T: Deserialize<'de>,
+{
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        try!(Option::<T>::deserialize(deserializer));
+        Ok(ArcWeak::new())
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+#[cfg(all(de_rc_dst, feature = "rc", any(feature = "std", feature = "alloc")))]
+macro_rules! box_forwarded_impl {
+    (
+        $(#[doc = $doc:tt])*
+        $t:ident
+    ) => {
+        $(#[doc = $doc])*
+        impl<'de, T: ?Sized> Deserialize<'de> for $t<T>
+        where
+            Box<T>: Deserialize<'de>,
+        {
+            fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+            where
+                D: Deserializer<'de>,
+            {
+                Box::deserialize(deserializer).map(Into::into)
+            }
+        }
+    };
+}
+
+#[cfg(all(de_rc_dst, feature = "rc", any(feature = "std", feature = "alloc")))]
+box_forwarded_impl! {
+    /// This impl requires the [`"rc"`] Cargo feature of Serde.
+    ///
+    /// Deserializing a data structure containing `Rc` will not attempt to
+    /// deduplicate `Rc` references to the same data. Every deserialized `Rc`
+    /// will end up with a strong count of 1.
+    ///
+    /// [`"rc"`]: https://serde.rs/feature-flags.html#-features-rc
+    Rc
+}
+
+#[cfg(all(de_rc_dst, feature = "rc", any(feature = "std", feature = "alloc")))]
+box_forwarded_impl! {
+    /// This impl requires the [`"rc"`] Cargo feature of Serde.
+    ///
+    /// Deserializing a data structure containing `Arc` will not attempt to
+    /// deduplicate `Arc` references to the same data. Every deserialized `Arc`
+    /// will end up with a strong count of 1.
+    ///
+    /// [`"rc"`]: https://serde.rs/feature-flags.html#-features-rc
+    Arc
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+impl<'de, T> Deserialize<'de> for Cell<T>
+where
+    T: Deserialize<'de> + Copy,
+{
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        T::deserialize(deserializer).map(Cell::new)
+    }
+}
+
+forwarded_impl!((T), RefCell<T>, RefCell::new);
+
+#[cfg(feature = "std")]
+forwarded_impl!((T), Mutex<T>, Mutex::new);
+
+#[cfg(feature = "std")]
+forwarded_impl!((T), RwLock<T>, RwLock::new);
+
+////////////////////////////////////////////////////////////////////////////////
+
+// This is a cleaned-up version of the impl generated by:
+//
+//     #[derive(Deserialize)]
+//     #[serde(deny_unknown_fields)]
+//     struct Duration {
+//         secs: u64,
+//         nanos: u32,
+//     }
+#[cfg(any(core_duration, feature = "std"))]
+impl<'de> Deserialize<'de> for Duration {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        // If this were outside of the serde crate, it would just use:
+        //
+        //    #[derive(Deserialize)]
+        //    #[serde(field_identifier, rename_all = "lowercase")]
+        enum Field {
+            Secs,
+            Nanos,
+        };
+
+        impl<'de> Deserialize<'de> for Field {
+            fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+            where
+                D: Deserializer<'de>,
+            {
+                struct FieldVisitor;
+
+                impl<'de> Visitor<'de> for FieldVisitor {
+                    type Value = Field;
+
+                    fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+                        formatter.write_str("`secs` or `nanos`")
+                    }
+
+                    fn visit_str<E>(self, value: &str) -> Result<Self::Value, E>
+                    where
+                        E: Error,
+                    {
+                        match value {
+                            "secs" => Ok(Field::Secs),
+                            "nanos" => Ok(Field::Nanos),
+                            _ => Err(Error::unknown_field(value, FIELDS)),
+                        }
+                    }
+
+                    fn visit_bytes<E>(self, value: &[u8]) -> Result<Self::Value, E>
+                    where
+                        E: Error,
+                    {
+                        match value {
+                            b"secs" => Ok(Field::Secs),
+                            b"nanos" => Ok(Field::Nanos),
+                            _ => {
+                                let value = ::export::from_utf8_lossy(value);
+                                Err(Error::unknown_field(&value, FIELDS))
+                            }
+                        }
+                    }
+                }
+
+                deserializer.deserialize_identifier(FieldVisitor)
+            }
+        }
+
+        struct DurationVisitor;
+
+        impl<'de> Visitor<'de> for DurationVisitor {
+            type Value = Duration;
+
+            fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+                formatter.write_str("struct Duration")
+            }
+
+            fn visit_seq<A>(self, mut seq: A) -> Result<Self::Value, A::Error>
+            where
+                A: SeqAccess<'de>,
+            {
+                let secs: u64 = match try!(seq.next_element()) {
+                    Some(value) => value,
+                    None => {
+                        return Err(Error::invalid_length(0, &self));
+                    }
+                };
+                let nanos: u32 = match try!(seq.next_element()) {
+                    Some(value) => value,
+                    None => {
+                        return Err(Error::invalid_length(1, &self));
+                    }
+                };
+                Ok(Duration::new(secs, nanos))
+            }
+
+            fn visit_map<A>(self, mut map: A) -> Result<Self::Value, A::Error>
+            where
+                A: MapAccess<'de>,
+            {
+                let mut secs: Option<u64> = None;
+                let mut nanos: Option<u32> = None;
+                while let Some(key) = try!(map.next_key()) {
+                    match key {
+                        Field::Secs => {
+                            if secs.is_some() {
+                                return Err(<A::Error as Error>::duplicate_field("secs"));
+                            }
+                            secs = Some(try!(map.next_value()));
+                        }
+                        Field::Nanos => {
+                            if nanos.is_some() {
+                                return Err(<A::Error as Error>::duplicate_field("nanos"));
+                            }
+                            nanos = Some(try!(map.next_value()));
+                        }
+                    }
+                }
+                let secs = match secs {
+                    Some(secs) => secs,
+                    None => return Err(<A::Error as Error>::missing_field("secs")),
+                };
+                let nanos = match nanos {
+                    Some(nanos) => nanos,
+                    None => return Err(<A::Error as Error>::missing_field("nanos")),
+                };
+                Ok(Duration::new(secs, nanos))
+            }
+        }
+
+        const FIELDS: &'static [&'static str] = &["secs", "nanos"];
+        deserializer.deserialize_struct("Duration", FIELDS, DurationVisitor)
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+#[cfg(feature = "std")]
+impl<'de> Deserialize<'de> for SystemTime {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        // Reuse duration
+        enum Field {
+            Secs,
+            Nanos,
+        };
+
+        impl<'de> Deserialize<'de> for Field {
+            fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+            where
+                D: Deserializer<'de>,
+            {
+                struct FieldVisitor;
+
+                impl<'de> Visitor<'de> for FieldVisitor {
+                    type Value = Field;
+
+                    fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+                        formatter.write_str("`secs_since_epoch` or `nanos_since_epoch`")
+                    }
+
+                    fn visit_str<E>(self, value: &str) -> Result<Self::Value, E>
+                    where
+                        E: Error,
+                    {
+                        match value {
+                            "secs_since_epoch" => Ok(Field::Secs),
+                            "nanos_since_epoch" => Ok(Field::Nanos),
+                            _ => Err(Error::unknown_field(value, FIELDS)),
+                        }
+                    }
+
+                    fn visit_bytes<E>(self, value: &[u8]) -> Result<Self::Value, E>
+                    where
+                        E: Error,
+                    {
+                        match value {
+                            b"secs_since_epoch" => Ok(Field::Secs),
+                            b"nanos_since_epoch" => Ok(Field::Nanos),
+                            _ => {
+                                let value = String::from_utf8_lossy(value);
+                                Err(Error::unknown_field(&value, FIELDS))
+                            }
+                        }
+                    }
+                }
+
+                deserializer.deserialize_identifier(FieldVisitor)
+            }
+        }
+
+        struct DurationVisitor;
+
+        impl<'de> Visitor<'de> for DurationVisitor {
+            type Value = Duration;
+
+            fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+                formatter.write_str("struct SystemTime")
+            }
+
+            fn visit_seq<A>(self, mut seq: A) -> Result<Self::Value, A::Error>
+            where
+                A: SeqAccess<'de>,
+            {
+                let secs: u64 = match try!(seq.next_element()) {
+                    Some(value) => value,
+                    None => {
+                        return Err(Error::invalid_length(0, &self));
+                    }
+                };
+                let nanos: u32 = match try!(seq.next_element()) {
+                    Some(value) => value,
+                    None => {
+                        return Err(Error::invalid_length(1, &self));
+                    }
+                };
+                Ok(Duration::new(secs, nanos))
+            }
+
+            fn visit_map<A>(self, mut map: A) -> Result<Self::Value, A::Error>
+            where
+                A: MapAccess<'de>,
+            {
+                let mut secs: Option<u64> = None;
+                let mut nanos: Option<u32> = None;
+                while let Some(key) = try!(map.next_key()) {
+                    match key {
+                        Field::Secs => {
+                            if secs.is_some() {
+                                return Err(<A::Error as Error>::duplicate_field(
+                                    "secs_since_epoch",
+                                ));
+                            }
+                            secs = Some(try!(map.next_value()));
+                        }
+                        Field::Nanos => {
+                            if nanos.is_some() {
+                                return Err(<A::Error as Error>::duplicate_field(
+                                    "nanos_since_epoch",
+                                ));
+                            }
+                            nanos = Some(try!(map.next_value()));
+                        }
+                    }
+                }
+                let secs = match secs {
+                    Some(secs) => secs,
+                    None => return Err(<A::Error as Error>::missing_field("secs_since_epoch")),
+                };
+                let nanos = match nanos {
+                    Some(nanos) => nanos,
+                    None => return Err(<A::Error as Error>::missing_field("nanos_since_epoch")),
+                };
+                Ok(Duration::new(secs, nanos))
+            }
+        }
+
+        const FIELDS: &'static [&'static str] = &["secs_since_epoch", "nanos_since_epoch"];
+        let duration = try!(deserializer.deserialize_struct("SystemTime", FIELDS, DurationVisitor));
+        Ok(UNIX_EPOCH + duration)
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+// Similar to:
+//
+//     #[derive(Deserialize)]
+//     #[serde(deny_unknown_fields)]
+//     struct Range {
+//         start: u64,
+//         end: u32,
+//     }
+impl<'de, Idx> Deserialize<'de> for Range<Idx>
+where
+    Idx: Deserialize<'de>,
+{
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        let (start, end) = deserializer.deserialize_struct(
+            "Range",
+            range::FIELDS,
+            range::RangeVisitor {
+                expecting: "struct Range",
+                phantom: PhantomData,
+            },
+        )?;
+        Ok(start..end)
+    }
+}
+
+#[cfg(range_inclusive)]
+impl<'de, Idx> Deserialize<'de> for RangeInclusive<Idx>
+where
+    Idx: Deserialize<'de>,
+{
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        let (start, end) = deserializer.deserialize_struct(
+            "RangeInclusive",
+            range::FIELDS,
+            range::RangeVisitor {
+                expecting: "struct RangeInclusive",
+                phantom: PhantomData,
+            },
+        )?;
+        Ok(RangeInclusive::new(start, end))
+    }
+}
+
+mod range {
+    use lib::*;
+
+    use de::{Deserialize, Deserializer, Error, MapAccess, SeqAccess, Visitor};
+
+    pub const FIELDS: &'static [&'static str] = &["start", "end"];
+
+    // If this were outside of the serde crate, it would just use:
+    //
+    //    #[derive(Deserialize)]
+    //    #[serde(field_identifier, rename_all = "lowercase")]
+    enum Field {
+        Start,
+        End,
+    }
+
+    impl<'de> Deserialize<'de> for Field {
+        fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+        where
+            D: Deserializer<'de>,
+        {
+            struct FieldVisitor;
+
+            impl<'de> Visitor<'de> for FieldVisitor {
+                type Value = Field;
+
+                fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+                    formatter.write_str("`start` or `end`")
+                }
+
+                fn visit_str<E>(self, value: &str) -> Result<Self::Value, E>
+                where
+                    E: Error,
+                {
+                    match value {
+                        "start" => Ok(Field::Start),
+                        "end" => Ok(Field::End),
+                        _ => Err(Error::unknown_field(value, FIELDS)),
+                    }
+                }
+
+                fn visit_bytes<E>(self, value: &[u8]) -> Result<Self::Value, E>
+                where
+                    E: Error,
+                {
+                    match value {
+                        b"start" => Ok(Field::Start),
+                        b"end" => Ok(Field::End),
+                        _ => {
+                            let value = ::export::from_utf8_lossy(value);
+                            Err(Error::unknown_field(&value, FIELDS))
+                        }
+                    }
+                }
+            }
+
+            deserializer.deserialize_identifier(FieldVisitor)
+        }
+    }
+
+    pub struct RangeVisitor<Idx> {
+        pub expecting: &'static str,
+        pub phantom: PhantomData<Idx>,
+    }
+
+    impl<'de, Idx> Visitor<'de> for RangeVisitor<Idx>
+    where
+        Idx: Deserialize<'de>,
+    {
+        type Value = (Idx, Idx);
+
+        fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+            formatter.write_str(self.expecting)
+        }
+
+        fn visit_seq<A>(self, mut seq: A) -> Result<Self::Value, A::Error>
+        where
+            A: SeqAccess<'de>,
+        {
+            let start: Idx = match try!(seq.next_element()) {
+                Some(value) => value,
+                None => {
+                    return Err(Error::invalid_length(0, &self));
+                }
+            };
+            let end: Idx = match try!(seq.next_element()) {
+                Some(value) => value,
+                None => {
+                    return Err(Error::invalid_length(1, &self));
+                }
+            };
+            Ok((start, end))
+        }
+
+        fn visit_map<A>(self, mut map: A) -> Result<Self::Value, A::Error>
+        where
+            A: MapAccess<'de>,
+        {
+            let mut start: Option<Idx> = None;
+            let mut end: Option<Idx> = None;
+            while let Some(key) = try!(map.next_key()) {
+                match key {
+                    Field::Start => {
+                        if start.is_some() {
+                            return Err(<A::Error as Error>::duplicate_field("start"));
+                        }
+                        start = Some(try!(map.next_value()));
+                    }
+                    Field::End => {
+                        if end.is_some() {
+                            return Err(<A::Error as Error>::duplicate_field("end"));
+                        }
+                        end = Some(try!(map.next_value()));
+                    }
+                }
+            }
+            let start = match start {
+                Some(start) => start,
+                None => return Err(<A::Error as Error>::missing_field("start")),
+            };
+            let end = match end {
+                Some(end) => end,
+                None => return Err(<A::Error as Error>::missing_field("end")),
+            };
+            Ok((start, end))
+        }
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+#[cfg(any(ops_bound, collections_bound))]
+impl<'de, T> Deserialize<'de> for Bound<T>
+where
+    T: Deserialize<'de>,
+{
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        enum Field {
+            Unbounded,
+            Included,
+            Excluded,
+        }
+
+        impl<'de> Deserialize<'de> for Field {
+            #[inline]
+            fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+            where
+                D: Deserializer<'de>,
+            {
+                struct FieldVisitor;
+
+                impl<'de> Visitor<'de> for FieldVisitor {
+                    type Value = Field;
+
+                    fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+                        formatter.write_str("`Unbounded`, `Included` or `Excluded`")
+                    }
+
+                    fn visit_u32<E>(self, value: u32) -> Result<Self::Value, E>
+                    where
+                        E: Error,
+                    {
+                        match value {
+                            0 => Ok(Field::Unbounded),
+                            1 => Ok(Field::Included),
+                            2 => Ok(Field::Excluded),
+                            _ => Err(Error::invalid_value(
+                                Unexpected::Unsigned(value as u64),
+                                &self,
+                            )),
+                        }
+                    }
+
+                    fn visit_str<E>(self, value: &str) -> Result<Self::Value, E>
+                    where
+                        E: Error,
+                    {
+                        match value {
+                            "Unbounded" => Ok(Field::Unbounded),
+                            "Included" => Ok(Field::Included),
+                            "Excluded" => Ok(Field::Excluded),
+                            _ => Err(Error::unknown_variant(value, VARIANTS)),
+                        }
+                    }
+
+                    fn visit_bytes<E>(self, value: &[u8]) -> Result<Self::Value, E>
+                    where
+                        E: Error,
+                    {
+                        match value {
+                            b"Unbounded" => Ok(Field::Unbounded),
+                            b"Included" => Ok(Field::Included),
+                            b"Excluded" => Ok(Field::Excluded),
+                            _ => match str::from_utf8(value) {
+                                Ok(value) => Err(Error::unknown_variant(value, VARIANTS)),
+                                Err(_) => {
+                                    Err(Error::invalid_value(Unexpected::Bytes(value), &self))
+                                }
+                            },
+                        }
+                    }
+                }
+
+                deserializer.deserialize_identifier(FieldVisitor)
+            }
+        }
+
+        struct BoundVisitor<T>(PhantomData<Bound<T>>);
+
+        impl<'de, T> Visitor<'de> for BoundVisitor<T>
+        where
+            T: Deserialize<'de>,
+        {
+            type Value = Bound<T>;
+
+            fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+                formatter.write_str("enum Bound")
+            }
+
+            fn visit_enum<A>(self, data: A) -> Result<Self::Value, A::Error>
+            where
+                A: EnumAccess<'de>,
+            {
+                match try!(data.variant()) {
+                    (Field::Unbounded, v) => v.unit_variant().map(|()| Bound::Unbounded),
+                    (Field::Included, v) => v.newtype_variant().map(Bound::Included),
+                    (Field::Excluded, v) => v.newtype_variant().map(Bound::Excluded),
+                }
+            }
+        }
+
+        const VARIANTS: &'static [&'static str] = &["Unbounded", "Included", "Excluded"];
+
+        deserializer.deserialize_enum("Bound", VARIANTS, BoundVisitor(PhantomData))
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+macro_rules! nonzero_integers {
+    ( $( $T: ident, )+ ) => {
+        $(
+            #[cfg(num_nonzero)]
+            impl<'de> Deserialize<'de> for num::$T {
+                fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+                where
+                    D: Deserializer<'de>,
+                {
+                    let value = try!(Deserialize::deserialize(deserializer));
+                    match <num::$T>::new(value) {
+                        Some(nonzero) => Ok(nonzero),
+                        None => Err(Error::custom("expected a non-zero value")),
+                    }
+                }
+            }
+        )+
+    };
+}
+
+nonzero_integers! {
+    NonZeroU8,
+    NonZeroU16,
+    NonZeroU32,
+    NonZeroU64,
+    NonZeroUsize,
+}
+
+#[cfg(num_nonzero_signed)]
+nonzero_integers! {
+    NonZeroI8,
+    NonZeroI16,
+    NonZeroI32,
+    NonZeroI64,
+    NonZeroIsize,
+}
+
+// Currently 128-bit integers do not work on Emscripten targets so we need an
+// additional `#[cfg]`
+serde_if_integer128! {
+    nonzero_integers! {
+        NonZeroU128,
+    }
+
+    #[cfg(num_nonzero_signed)]
+    nonzero_integers! {
+        NonZeroI128,
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+impl<'de, T, E> Deserialize<'de> for Result<T, E>
+where
+    T: Deserialize<'de>,
+    E: Deserialize<'de>,
+{
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        // If this were outside of the serde crate, it would just use:
+        //
+        //    #[derive(Deserialize)]
+        //    #[serde(variant_identifier)]
+        enum Field {
+            Ok,
+            Err,
+        }
+
+        impl<'de> Deserialize<'de> for Field {
+            #[inline]
+            fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+            where
+                D: Deserializer<'de>,
+            {
+                struct FieldVisitor;
+
+                impl<'de> Visitor<'de> for FieldVisitor {
+                    type Value = Field;
+
+                    fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+                        formatter.write_str("`Ok` or `Err`")
+                    }
+
+                    fn visit_u32<E>(self, value: u32) -> Result<Self::Value, E>
+                    where
+                        E: Error,
+                    {
+                        match value {
+                            0 => Ok(Field::Ok),
+                            1 => Ok(Field::Err),
+                            _ => Err(Error::invalid_value(
+                                Unexpected::Unsigned(value as u64),
+                                &self,
+                            )),
+                        }
+                    }
+
+                    fn visit_str<E>(self, value: &str) -> Result<Self::Value, E>
+                    where
+                        E: Error,
+                    {
+                        match value {
+                            "Ok" => Ok(Field::Ok),
+                            "Err" => Ok(Field::Err),
+                            _ => Err(Error::unknown_variant(value, VARIANTS)),
+                        }
+                    }
+
+                    fn visit_bytes<E>(self, value: &[u8]) -> Result<Self::Value, E>
+                    where
+                        E: Error,
+                    {
+                        match value {
+                            b"Ok" => Ok(Field::Ok),
+                            b"Err" => Ok(Field::Err),
+                            _ => match str::from_utf8(value) {
+                                Ok(value) => Err(Error::unknown_variant(value, VARIANTS)),
+                                Err(_) => {
+                                    Err(Error::invalid_value(Unexpected::Bytes(value), &self))
+                                }
+                            },
+                        }
+                    }
+                }
+
+                deserializer.deserialize_identifier(FieldVisitor)
+            }
+        }
+
+        struct ResultVisitor<T, E>(PhantomData<Result<T, E>>);
+
+        impl<'de, T, E> Visitor<'de> for ResultVisitor<T, E>
+        where
+            T: Deserialize<'de>,
+            E: Deserialize<'de>,
+        {
+            type Value = Result<T, E>;
+
+            fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+                formatter.write_str("enum Result")
+            }
+
+            fn visit_enum<A>(self, data: A) -> Result<Self::Value, A::Error>
+            where
+                A: EnumAccess<'de>,
+            {
+                match try!(data.variant()) {
+                    (Field::Ok, v) => v.newtype_variant().map(Ok),
+                    (Field::Err, v) => v.newtype_variant().map(Err),
+                }
+            }
+        }
+
+        const VARIANTS: &'static [&'static str] = &["Ok", "Err"];
+
+        deserializer.deserialize_enum("Result", VARIANTS, ResultVisitor(PhantomData))
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+#[cfg(feature = "std")]
+impl<'de, T> Deserialize<'de> for Wrapping<T>
+where
+    T: Deserialize<'de>,
+{
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        Deserialize::deserialize(deserializer).map(Wrapping)
+    }
+}
+
+#[cfg(all(feature = "std", std_atomic))]
+macro_rules! atomic_impl {
+    ($($ty:ident)*) => {
+        $(
+            impl<'de> Deserialize<'de> for $ty {
+                fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+                where
+                    D: Deserializer<'de>,
+                {
+                    Deserialize::deserialize(deserializer).map(Self::new)
+                }
+            }
+        )*
+    };
+}
+
+#[cfg(all(feature = "std", std_atomic))]
+atomic_impl! {
+    AtomicBool
+    AtomicI8 AtomicI16 AtomicI32 AtomicIsize
+    AtomicU8 AtomicU16 AtomicU32 AtomicUsize
+}
+
+#[cfg(all(feature = "std", std_atomic64))]
+atomic_impl! {
+    AtomicI64 AtomicU64
+}
diff --git a/src/de/mod.rs b/src/de/mod.rs
new file mode 100644
index 0000000..f1b8b31
--- /dev/null
+++ b/src/de/mod.rs
@@ -0,0 +1,2276 @@
+//! Generic data structure deserialization framework.
+//!
+//! The two most important traits in this module are [`Deserialize`] and
+//! [`Deserializer`].
+//!
+//!  - **A type that implements `Deserialize` is a data structure** that can be
+//!    deserialized from any data format supported by Serde, and conversely
+//!  - **A type that implements `Deserializer` is a data format** that can
+//!    deserialize any data structure supported by Serde.
+//!
+//! # The Deserialize trait
+//!
+//! Serde provides [`Deserialize`] implementations for many Rust primitive and
+//! standard library types. The complete list is below. All of these can be
+//! deserialized using Serde out of the box.
+//!
+//! Additionally, Serde provides a procedural macro called [`serde_derive`] to
+//! automatically generate [`Deserialize`] implementations for structs and enums
+//! in your program. See the [derive section of the manual] for how to use this.
+//!
+//! In rare cases it may be necessary to implement [`Deserialize`] manually for
+//! some type in your program. See the [Implementing `Deserialize`] section of
+//! the manual for more about this.
+//!
+//! Third-party crates may provide [`Deserialize`] implementations for types
+//! that they expose. For example the [`linked-hash-map`] crate provides a
+//! [`LinkedHashMap<K, V>`] type that is deserializable by Serde because the
+//! crate provides an implementation of [`Deserialize`] for it.
+//!
+//! # The Deserializer trait
+//!
+//! [`Deserializer`] implementations are provided by third-party crates, for
+//! example [`serde_json`], [`serde_yaml`] and [`bincode`].
+//!
+//! A partial list of well-maintained formats is given on the [Serde
+//! website][data formats].
+//!
+//! # Implementations of Deserialize provided by Serde
+//!
+//! This is a slightly different set of types than what is supported for
+//! serialization. Some types can be serialized by Serde but not deserialized.
+//! One example is `OsStr`.
+//!
+//!  - **Primitive types**:
+//!    - bool
+//!    - i8, i16, i32, i64, i128, isize
+//!    - u8, u16, u32, u64, u128, usize
+//!    - f32, f64
+//!    - char
+//!  - **Compound types**:
+//!    - \[T; 0\] through \[T; 32\]
+//!    - tuples up to size 16
+//!  - **Common standard library types**:
+//!    - String
+//!    - Option\<T\>
+//!    - Result\<T, E\>
+//!    - PhantomData\<T\>
+//!  - **Wrapper types**:
+//!    - Box\<T\>
+//!    - Box\<\[T\]\>
+//!    - Box\<str\>
+//!    - Cow\<'a, T\>
+//!    - Cell\<T\>
+//!    - RefCell\<T\>
+//!    - Mutex\<T\>
+//!    - RwLock\<T\>
+//!    - Rc\<T\>&emsp;*(if* features = ["rc"] *is enabled)*
+//!    - Arc\<T\>&emsp;*(if* features = ["rc"] *is enabled)*
+//!  - **Collection types**:
+//!    - BTreeMap\<K, V\>
+//!    - BTreeSet\<T\>
+//!    - BinaryHeap\<T\>
+//!    - HashMap\<K, V, H\>
+//!    - HashSet\<T, H\>
+//!    - LinkedList\<T\>
+//!    - VecDeque\<T\>
+//!    - Vec\<T\>
+//!  - **Zero-copy types**:
+//!    - &str
+//!    - &\[u8\]
+//!  - **FFI types**:
+//!    - CString
+//!    - Box\<CStr\>
+//!    - OsString
+//!  - **Miscellaneous standard library types**:
+//!    - Duration
+//!    - SystemTime
+//!    - Path
+//!    - PathBuf
+//!    - Range\<T\>
+//!    - RangeInclusive\<T\>
+//!    - Bound\<T\>
+//!    - num::NonZero*
+//!    - `!` *(unstable)*
+//!  - **Net types**:
+//!    - IpAddr
+//!    - Ipv4Addr
+//!    - Ipv6Addr
+//!    - SocketAddr
+//!    - SocketAddrV4
+//!    - SocketAddrV6
+//!
+//! [Implementing `Deserialize`]: https://serde.rs/impl-deserialize.html
+//! [`Deserialize`]: ../trait.Deserialize.html
+//! [`Deserializer`]: ../trait.Deserializer.html
+//! [`LinkedHashMap<K, V>`]: https://docs.rs/linked-hash-map/*/linked_hash_map/struct.LinkedHashMap.html
+//! [`bincode`]: https://github.com/TyOverby/bincode
+//! [`linked-hash-map`]: https://crates.io/crates/linked-hash-map
+//! [`serde_derive`]: https://crates.io/crates/serde_derive
+//! [`serde_json`]: https://github.com/serde-rs/json
+//! [`serde_yaml`]: https://github.com/dtolnay/serde-yaml
+//! [derive section of the manual]: https://serde.rs/derive.html
+//! [data formats]: https://serde.rs/#data-formats
+
+use lib::*;
+
+////////////////////////////////////////////////////////////////////////////////
+
+pub mod value;
+
+mod from_primitive;
+mod ignored_any;
+mod impls;
+mod utf8;
+
+pub use self::ignored_any::IgnoredAny;
+
+#[cfg(feature = "std")]
+#[doc(no_inline)]
+pub use std::error::Error as StdError;
+#[cfg(not(feature = "std"))]
+#[doc(no_inline)]
+pub use std_error::Error as StdError;
+
+////////////////////////////////////////////////////////////////////////////////
+
+macro_rules! declare_error_trait {
+    (Error: Sized $(+ $($supertrait:ident)::+)*) => {
+        /// The `Error` trait allows `Deserialize` implementations to create descriptive
+        /// error messages belonging to the `Deserializer` against which they are
+        /// currently running.
+        ///
+        /// Every `Deserializer` declares an `Error` type that encompasses both
+        /// general-purpose deserialization errors as well as errors specific to the
+        /// particular deserialization format. For example the `Error` type of
+        /// `serde_json` can represent errors like an invalid JSON escape sequence or an
+        /// unterminated string literal, in addition to the error cases that are part of
+        /// this trait.
+        ///
+        /// Most deserializers should only need to provide the `Error::custom` method
+        /// and inherit the default behavior for the other methods.
+        ///
+        /// # Example implementation
+        ///
+        /// The [example data format] presented on the website shows an error
+        /// type appropriate for a basic JSON data format.
+        ///
+        /// [example data format]: https://serde.rs/data-format.html
+        pub trait Error: Sized $(+ $($supertrait)::+)* {
+            /// Raised when there is general error when deserializing a type.
+            ///
+            /// The message should not be capitalized and should not end with a period.
+            ///
+            /// ```edition2018
+            /// # use std::str::FromStr;
+            /// #
+            /// # struct IpAddr;
+            /// #
+            /// # impl FromStr for IpAddr {
+            /// #     type Err = String;
+            /// #
+            /// #     fn from_str(_: &str) -> Result<Self, String> {
+            /// #         unimplemented!()
+            /// #     }
+            /// # }
+            /// #
+            /// use serde::de::{self, Deserialize, Deserializer};
+            ///
+            /// impl<'de> Deserialize<'de> for IpAddr {
+            ///     fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+            ///     where
+            ///         D: Deserializer<'de>,
+            ///     {
+            ///         let s = String::deserialize(deserializer)?;
+            ///         s.parse().map_err(de::Error::custom)
+            ///     }
+            /// }
+            /// ```
+            fn custom<T>(msg: T) -> Self
+            where
+                T: Display;
+
+            /// Raised when a `Deserialize` receives a type different from what it was
+            /// expecting.
+            ///
+            /// The `unexp` argument provides information about what type was received.
+            /// This is the type that was present in the input file or other source data
+            /// of the Deserializer.
+            ///
+            /// The `exp` argument provides information about what type was being
+            /// expected. This is the type that is written in the program.
+            ///
+            /// For example if we try to deserialize a String out of a JSON file
+            /// containing an integer, the unexpected type is the integer and the
+            /// expected type is the string.
+            #[cold]
+            fn invalid_type(unexp: Unexpected, exp: &Expected) -> Self {
+                Error::custom(format_args!("invalid type: {}, expected {}", unexp, exp))
+            }
+
+            /// Raised when a `Deserialize` receives a value of the right type but that
+            /// is wrong for some other reason.
+            ///
+            /// The `unexp` argument provides information about what value was received.
+            /// This is the value that was present in the input file or other source
+            /// data of the Deserializer.
+            ///
+            /// The `exp` argument provides information about what value was being
+            /// expected. This is the type that is written in the program.
+            ///
+            /// For example if we try to deserialize a String out of some binary data
+            /// that is not valid UTF-8, the unexpected value is the bytes and the
+            /// expected value is a string.
+            #[cold]
+            fn invalid_value(unexp: Unexpected, exp: &Expected) -> Self {
+                Error::custom(format_args!("invalid value: {}, expected {}", unexp, exp))
+            }
+
+            /// Raised when deserializing a sequence or map and the input data contains
+            /// too many or too few elements.
+            ///
+            /// The `len` argument is the number of elements encountered. The sequence
+            /// or map may have expected more arguments or fewer arguments.
+            ///
+            /// The `exp` argument provides information about what data was being
+            /// expected. For example `exp` might say that a tuple of size 6 was
+            /// expected.
+            #[cold]
+            fn invalid_length(len: usize, exp: &Expected) -> Self {
+                Error::custom(format_args!("invalid length {}, expected {}", len, exp))
+            }
+
+            /// Raised when a `Deserialize` enum type received a variant with an
+            /// unrecognized name.
+            #[cold]
+            fn unknown_variant(variant: &str, expected: &'static [&'static str]) -> Self {
+                if expected.is_empty() {
+                    Error::custom(format_args!(
+                        "unknown variant `{}`, there are no variants",
+                        variant
+                    ))
+                } else {
+                    Error::custom(format_args!(
+                        "unknown variant `{}`, expected {}",
+                        variant,
+                        OneOf { names: expected }
+                    ))
+                }
+            }
+
+            /// Raised when a `Deserialize` struct type received a field with an
+            /// unrecognized name.
+            #[cold]
+            fn unknown_field(field: &str, expected: &'static [&'static str]) -> Self {
+                if expected.is_empty() {
+                    Error::custom(format_args!(
+                        "unknown field `{}`, there are no fields",
+                        field
+                    ))
+                } else {
+                    Error::custom(format_args!(
+                        "unknown field `{}`, expected {}",
+                        field,
+                        OneOf { names: expected }
+                    ))
+                }
+            }
+
+            /// Raised when a `Deserialize` struct type expected to receive a required
+            /// field with a particular name but that field was not present in the
+            /// input.
+            #[cold]
+            fn missing_field(field: &'static str) -> Self {
+                Error::custom(format_args!("missing field `{}`", field))
+            }
+
+            /// Raised when a `Deserialize` struct type received more than one of the
+            /// same field.
+            #[cold]
+            fn duplicate_field(field: &'static str) -> Self {
+                Error::custom(format_args!("duplicate field `{}`", field))
+            }
+        }
+    }
+}
+
+#[cfg(feature = "std")]
+declare_error_trait!(Error: Sized + StdError);
+
+#[cfg(not(feature = "std"))]
+declare_error_trait!(Error: Sized + Debug + Display);
+
+/// `Unexpected` represents an unexpected invocation of any one of the `Visitor`
+/// trait methods.
+///
+/// This is used as an argument to the `invalid_type`, `invalid_value`, and
+/// `invalid_length` methods of the `Error` trait to build error messages.
+///
+/// ```edition2018
+/// # use std::fmt;
+/// #
+/// # use serde::de::{self, Unexpected, Visitor};
+/// #
+/// # struct Example;
+/// #
+/// # impl<'de> Visitor<'de> for Example {
+/// #     type Value = ();
+/// #
+/// #     fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+/// #         write!(formatter, "definitely not a boolean")
+/// #     }
+/// #
+/// fn visit_bool<E>(self, v: bool) -> Result<Self::Value, E>
+/// where
+///     E: de::Error,
+/// {
+///     Err(de::Error::invalid_type(Unexpected::Bool(v), &self))
+/// }
+/// # }
+/// ```
+#[derive(Copy, Clone, PartialEq, Debug)]
+pub enum Unexpected<'a> {
+    /// The input contained a boolean value that was not expected.
+    Bool(bool),
+
+    /// The input contained an unsigned integer `u8`, `u16`, `u32` or `u64` that
+    /// was not expected.
+    Unsigned(u64),
+
+    /// The input contained a signed integer `i8`, `i16`, `i32` or `i64` that
+    /// was not expected.
+    Signed(i64),
+
+    /// The input contained a floating point `f32` or `f64` that was not
+    /// expected.
+    Float(f64),
+
+    /// The input contained a `char` that was not expected.
+    Char(char),
+
+    /// The input contained a `&str` or `String` that was not expected.
+    Str(&'a str),
+
+    /// The input contained a `&[u8]` or `Vec<u8>` that was not expected.
+    Bytes(&'a [u8]),
+
+    /// The input contained a unit `()` that was not expected.
+    Unit,
+
+    /// The input contained an `Option<T>` that was not expected.
+    Option,
+
+    /// The input contained a newtype struct that was not expected.
+    NewtypeStruct,
+
+    /// The input contained a sequence that was not expected.
+    Seq,
+
+    /// The input contained a map that was not expected.
+    Map,
+
+    /// The input contained an enum that was not expected.
+    Enum,
+
+    /// The input contained a unit variant that was not expected.
+    UnitVariant,
+
+    /// The input contained a newtype variant that was not expected.
+    NewtypeVariant,
+
+    /// The input contained a tuple variant that was not expected.
+    TupleVariant,
+
+    /// The input contained a struct variant that was not expected.
+    StructVariant,
+
+    /// A message stating what uncategorized thing the input contained that was
+    /// not expected.
+    ///
+    /// The message should be a noun or noun phrase, not capitalized and without
+    /// a period. An example message is "unoriginal superhero".
+    Other(&'a str),
+}
+
+impl<'a> fmt::Display for Unexpected<'a> {
+    fn fmt(&self, formatter: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+        use self::Unexpected::*;
+        match *self {
+            Bool(b) => write!(formatter, "boolean `{}`", b),
+            Unsigned(i) => write!(formatter, "integer `{}`", i),
+            Signed(i) => write!(formatter, "integer `{}`", i),
+            Float(f) => write!(formatter, "floating point `{}`", f),
+            Char(c) => write!(formatter, "character `{}`", c),
+            Str(s) => write!(formatter, "string {:?}", s),
+            Bytes(_) => write!(formatter, "byte array"),
+            Unit => write!(formatter, "unit value"),
+            Option => write!(formatter, "Option value"),
+            NewtypeStruct => write!(formatter, "newtype struct"),
+            Seq => write!(formatter, "sequence"),
+            Map => write!(formatter, "map"),
+            Enum => write!(formatter, "enum"),
+            UnitVariant => write!(formatter, "unit variant"),
+            NewtypeVariant => write!(formatter, "newtype variant"),
+            TupleVariant => write!(formatter, "tuple variant"),
+            StructVariant => write!(formatter, "struct variant"),
+            Other(other) => formatter.write_str(other),
+        }
+    }
+}
+
+/// `Expected` represents an explanation of what data a `Visitor` was expecting
+/// to receive.
+///
+/// This is used as an argument to the `invalid_type`, `invalid_value`, and
+/// `invalid_length` methods of the `Error` trait to build error messages. The
+/// message should be a noun or noun phrase that completes the sentence "This
+/// Visitor expects to receive ...", for example the message could be "an
+/// integer between 0 and 64". The message should not be capitalized and should
+/// not end with a period.
+///
+/// Within the context of a `Visitor` implementation, the `Visitor` itself
+/// (`&self`) is an implementation of this trait.
+///
+/// ```edition2018
+/// # use std::fmt;
+/// #
+/// # use serde::de::{self, Unexpected, Visitor};
+/// #
+/// # struct Example;
+/// #
+/// # impl<'de> Visitor<'de> for Example {
+/// #     type Value = ();
+/// #
+/// #     fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+/// #         write!(formatter, "definitely not a boolean")
+/// #     }
+/// #
+/// fn visit_bool<E>(self, v: bool) -> Result<Self::Value, E>
+/// where
+///     E: de::Error,
+/// {
+///     Err(de::Error::invalid_type(Unexpected::Bool(v), &self))
+/// }
+/// # }
+/// ```
+///
+/// Outside of a `Visitor`, `&"..."` can be used.
+///
+/// ```edition2018
+/// # use serde::de::{self, Unexpected};
+/// #
+/// # fn example<E>() -> Result<(), E>
+/// # where
+/// #     E: de::Error,
+/// # {
+/// #     let v = true;
+/// return Err(de::Error::invalid_type(Unexpected::Bool(v), &"a negative integer"));
+/// # }
+/// ```
+pub trait Expected {
+    /// Format an explanation of what data was being expected. Same signature as
+    /// the `Display` and `Debug` traits.
+    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result;
+}
+
+impl<'de, T> Expected for T
+where
+    T: Visitor<'de>,
+{
+    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        self.expecting(formatter)
+    }
+}
+
+impl<'a> Expected for &'a str {
+    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        formatter.write_str(self)
+    }
+}
+
+impl<'a> Display for Expected + 'a {
+    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        Expected::fmt(self, formatter)
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+/// A **data structure** that can be deserialized from any data format supported
+/// by Serde.
+///
+/// Serde provides `Deserialize` implementations for many Rust primitive and
+/// standard library types. The complete list is [here][de]. All of these can
+/// be deserialized using Serde out of the box.
+///
+/// Additionally, Serde provides a procedural macro called `serde_derive` to
+/// automatically generate `Deserialize` implementations for structs and enums
+/// in your program. See the [derive section of the manual][derive] for how to
+/// use this.
+///
+/// In rare cases it may be necessary to implement `Deserialize` manually for
+/// some type in your program. See the [Implementing
+/// `Deserialize`][impl-deserialize] section of the manual for more about this.
+///
+/// Third-party crates may provide `Deserialize` implementations for types that
+/// they expose. For example the `linked-hash-map` crate provides a
+/// `LinkedHashMap<K, V>` type that is deserializable by Serde because the crate
+/// provides an implementation of `Deserialize` for it.
+///
+/// [de]: https://docs.serde.rs/serde/de/index.html
+/// [derive]: https://serde.rs/derive.html
+/// [impl-deserialize]: https://serde.rs/impl-deserialize.html
+///
+/// # Lifetime
+///
+/// The `'de` lifetime of this trait is the lifetime of data that may be
+/// borrowed by `Self` when deserialized. See the page [Understanding
+/// deserializer lifetimes] for a more detailed explanation of these lifetimes.
+///
+/// [Understanding deserializer lifetimes]: https://serde.rs/lifetimes.html
+pub trait Deserialize<'de>: Sized {
+    /// Deserialize this value from the given Serde deserializer.
+    ///
+    /// See the [Implementing `Deserialize`][impl-deserialize] section of the
+    /// manual for more information about how to implement this method.
+    ///
+    /// [impl-deserialize]: https://serde.rs/impl-deserialize.html
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>;
+
+    /// Deserializes a value into `self` from the given Deserializer.
+    ///
+    /// The purpose of this method is to allow the deserializer to reuse
+    /// resources and avoid copies. As such, if this method returns an error,
+    /// `self` will be in an indeterminate state where some parts of the struct
+    /// have been overwritten. Although whatever state that is will be
+    /// memory-safe.
+    ///
+    /// This is generally useful when repeatedly deserializing values that
+    /// are processed one at a time, where the value of `self` doesn't matter
+    /// when the next deserialization occurs.
+    ///
+    /// If you manually implement this, your recursive deserializations should
+    /// use `deserialize_in_place`.
+    ///
+    /// This method is stable and an official public API, but hidden from the
+    /// documentation because it is almost never what newbies are looking for.
+    /// Showing it in rustdoc would cause it to be featured more prominently
+    /// than it deserves.
+    #[doc(hidden)]
+    fn deserialize_in_place<D>(deserializer: D, place: &mut Self) -> Result<(), D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        // Default implementation just delegates to `deserialize` impl.
+        *place = Deserialize::deserialize(deserializer)?;
+        Ok(())
+    }
+}
+
+/// A data structure that can be deserialized without borrowing any data from
+/// the deserializer.
+///
+/// This is primarily useful for trait bounds on functions. For example a
+/// `from_str` function may be able to deserialize a data structure that borrows
+/// from the input string, but a `from_reader` function may only deserialize
+/// owned data.
+///
+/// ```edition2018
+/// # use serde::de::{Deserialize, DeserializeOwned};
+/// # use std::io::{Read, Result};
+/// #
+/// # trait Ignore {
+/// fn from_str<'a, T>(s: &'a str) -> Result<T>
+/// where
+///     T: Deserialize<'a>;
+///
+/// fn from_reader<R, T>(rdr: R) -> Result<T>
+/// where
+///     R: Read,
+///     T: DeserializeOwned;
+/// # }
+/// ```
+///
+/// # Lifetime
+///
+/// The relationship between `Deserialize` and `DeserializeOwned` in trait
+/// bounds is explained in more detail on the page [Understanding deserializer
+/// lifetimes].
+///
+/// [Understanding deserializer lifetimes]: https://serde.rs/lifetimes.html
+pub trait DeserializeOwned: for<'de> Deserialize<'de> {}
+impl<T> DeserializeOwned for T where T: for<'de> Deserialize<'de> {}
+
+/// `DeserializeSeed` is the stateful form of the `Deserialize` trait. If you
+/// ever find yourself looking for a way to pass data into a `Deserialize` impl,
+/// this trait is the way to do it.
+///
+/// As one example of stateful deserialization consider deserializing a JSON
+/// array into an existing buffer. Using the `Deserialize` trait we could
+/// deserialize a JSON array into a `Vec<T>` but it would be a freshly allocated
+/// `Vec<T>`; there is no way for `Deserialize` to reuse a previously allocated
+/// buffer. Using `DeserializeSeed` instead makes this possible as in the
+/// example code below.
+///
+/// The canonical API for stateless deserialization looks like this:
+///
+/// ```edition2018
+/// # use serde::Deserialize;
+/// #
+/// # enum Error {}
+/// #
+/// fn func<'de, T: Deserialize<'de>>() -> Result<T, Error>
+/// # {
+/// #     unimplemented!()
+/// # }
+/// ```
+///
+/// Adjusting an API like this to support stateful deserialization is a matter
+/// of accepting a seed as input:
+///
+/// ```edition2018
+/// # use serde::de::DeserializeSeed;
+/// #
+/// # enum Error {}
+/// #
+/// fn func_seed<'de, T: DeserializeSeed<'de>>(seed: T) -> Result<T::Value, Error>
+/// # {
+/// #     let _ = seed;
+/// #     unimplemented!()
+/// # }
+/// ```
+///
+/// In practice the majority of deserialization is stateless. An API expecting a
+/// seed can be appeased by passing `std::marker::PhantomData` as a seed in the
+/// case of stateless deserialization.
+///
+/// # Lifetime
+///
+/// The `'de` lifetime of this trait is the lifetime of data that may be
+/// borrowed by `Self::Value` when deserialized. See the page [Understanding
+/// deserializer lifetimes] for a more detailed explanation of these lifetimes.
+///
+/// [Understanding deserializer lifetimes]: https://serde.rs/lifetimes.html
+///
+/// # Example
+///
+/// Suppose we have JSON that looks like `[[1, 2], [3, 4, 5], [6]]` and we need
+/// to deserialize it into a flat representation like `vec![1, 2, 3, 4, 5, 6]`.
+/// Allocating a brand new `Vec<T>` for each subarray would be slow. Instead we
+/// would like to allocate a single `Vec<T>` and then deserialize each subarray
+/// into it. This requires stateful deserialization using the `DeserializeSeed`
+/// trait.
+///
+/// ```edition2018
+/// use std::fmt;
+/// use std::marker::PhantomData;
+///
+/// use serde::de::{Deserialize, DeserializeSeed, Deserializer, SeqAccess, Visitor};
+///
+/// // A DeserializeSeed implementation that uses stateful deserialization to
+/// // append array elements onto the end of an existing vector. The preexisting
+/// // state ("seed") in this case is the Vec<T>. The `deserialize` method of
+/// // `ExtendVec` will be traversing the inner arrays of the JSON input and
+/// // appending each integer into the existing Vec.
+/// struct ExtendVec<'a, T: 'a>(&'a mut Vec<T>);
+///
+/// impl<'de, 'a, T> DeserializeSeed<'de> for ExtendVec<'a, T>
+/// where
+///     T: Deserialize<'de>,
+/// {
+///     // The return type of the `deserialize` method. This implementation
+///     // appends onto an existing vector but does not create any new data
+///     // structure, so the return type is ().
+///     type Value = ();
+///
+///     fn deserialize<D>(self, deserializer: D) -> Result<Self::Value, D::Error>
+///     where
+///         D: Deserializer<'de>,
+///     {
+///         // Visitor implementation that will walk an inner array of the JSON
+///         // input.
+///         struct ExtendVecVisitor<'a, T: 'a>(&'a mut Vec<T>);
+///
+///         impl<'de, 'a, T> Visitor<'de> for ExtendVecVisitor<'a, T>
+///         where
+///             T: Deserialize<'de>,
+///         {
+///             type Value = ();
+///
+///             fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+///                 write!(formatter, "an array of integers")
+///             }
+///
+///             fn visit_seq<A>(self, mut seq: A) -> Result<(), A::Error>
+///             where
+///                 A: SeqAccess<'de>,
+///             {
+///                 // Visit each element in the inner array and push it onto
+///                 // the existing vector.
+///                 while let Some(elem) = seq.next_element()? {
+///                     self.0.push(elem);
+///                 }
+///                 Ok(())
+///             }
+///         }
+///
+///         deserializer.deserialize_seq(ExtendVecVisitor(self.0))
+///     }
+/// }
+///
+/// // Visitor implementation that will walk the outer array of the JSON input.
+/// struct FlattenedVecVisitor<T>(PhantomData<T>);
+///
+/// impl<'de, T> Visitor<'de> for FlattenedVecVisitor<T>
+/// where
+///     T: Deserialize<'de>,
+/// {
+///     // This Visitor constructs a single Vec<T> to hold the flattened
+///     // contents of the inner arrays.
+///     type Value = Vec<T>;
+///
+///     fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+///         write!(formatter, "an array of arrays")
+///     }
+///
+///     fn visit_seq<A>(self, mut seq: A) -> Result<Vec<T>, A::Error>
+///     where
+///         A: SeqAccess<'de>,
+///     {
+///         // Create a single Vec to hold the flattened contents.
+///         let mut vec = Vec::new();
+///
+///         // Each iteration through this loop is one inner array.
+///         while let Some(()) = seq.next_element_seed(ExtendVec(&mut vec))? {
+///             // Nothing to do; inner array has been appended into `vec`.
+///         }
+///
+///         // Return the finished vec.
+///         Ok(vec)
+///     }
+/// }
+///
+/// # fn example<'de, D>(deserializer: D) -> Result<(), D::Error>
+/// # where
+/// #     D: Deserializer<'de>,
+/// # {
+/// let visitor = FlattenedVecVisitor(PhantomData);
+/// let flattened: Vec<u64> = deserializer.deserialize_seq(visitor)?;
+/// #     Ok(())
+/// # }
+/// ```
+pub trait DeserializeSeed<'de>: Sized {
+    /// The type produced by using this seed.
+    type Value;
+
+    /// Equivalent to the more common `Deserialize::deserialize` method, except
+    /// with some initial piece of data (the seed) passed in.
+    fn deserialize<D>(self, deserializer: D) -> Result<Self::Value, D::Error>
+    where
+        D: Deserializer<'de>;
+}
+
+impl<'de, T> DeserializeSeed<'de> for PhantomData<T>
+where
+    T: Deserialize<'de>,
+{
+    type Value = T;
+
+    #[inline]
+    fn deserialize<D>(self, deserializer: D) -> Result<T, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        T::deserialize(deserializer)
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+/// A **data format** that can deserialize any data structure supported by
+/// Serde.
+///
+/// The role of this trait is to define the deserialization half of the [Serde
+/// data model], which is a way to categorize every Rust data type into one of
+/// 29 possible types. Each method of the `Deserializer` trait corresponds to one
+/// of the types of the data model.
+///
+/// Implementations of `Deserialize` map themselves into this data model by
+/// passing to the `Deserializer` a `Visitor` implementation that can receive
+/// these various types.
+///
+/// The types that make up the Serde data model are:
+///
+///  - **14 primitive types**
+///    - bool
+///    - i8, i16, i32, i64, i128
+///    - u8, u16, u32, u64, u128
+///    - f32, f64
+///    - char
+///  - **string**
+///    - UTF-8 bytes with a length and no null terminator.
+///    - When serializing, all strings are handled equally. When deserializing,
+///      there are three flavors of strings: transient, owned, and borrowed.
+///  - **byte array** - \[u8\]
+///    - Similar to strings, during deserialization byte arrays can be
+///      transient, owned, or borrowed.
+///  - **option**
+///    - Either none or some value.
+///  - **unit**
+///    - The type of `()` in Rust. It represents an anonymous value containing
+///      no data.
+///  - **unit_struct**
+///    - For example `struct Unit` or `PhantomData<T>`. It represents a named
+///      value containing no data.
+///  - **unit_variant**
+///    - For example the `E::A` and `E::B` in `enum E { A, B }`.
+///  - **newtype_struct**
+///    - For example `struct Millimeters(u8)`.
+///  - **newtype_variant**
+///    - For example the `E::N` in `enum E { N(u8) }`.
+///  - **seq**
+///    - A variably sized heterogeneous sequence of values, for example `Vec<T>`
+///      or `HashSet<T>`. When serializing, the length may or may not be known
+///      before iterating through all the data. When deserializing, the length
+///      is determined by looking at the serialized data.
+///  - **tuple**
+///    - A statically sized heterogeneous sequence of values for which the
+///      length will be known at deserialization time without looking at the
+///      serialized data, for example `(u8,)` or `(String, u64, Vec<T>)` or
+///      `[u64; 10]`.
+///  - **tuple_struct**
+///    - A named tuple, for example `struct Rgb(u8, u8, u8)`.
+///  - **tuple_variant**
+///    - For example the `E::T` in `enum E { T(u8, u8) }`.
+///  - **map**
+///    - A heterogeneous key-value pairing, for example `BTreeMap<K, V>`.
+///  - **struct**
+///    - A heterogeneous key-value pairing in which the keys are strings and
+///      will be known at deserialization time without looking at the serialized
+///      data, for example `struct S { r: u8, g: u8, b: u8 }`.
+///  - **struct_variant**
+///    - For example the `E::S` in `enum E { S { r: u8, g: u8, b: u8 } }`.
+///
+/// The `Deserializer` trait supports two entry point styles which enables
+/// different kinds of deserialization.
+///
+/// 1. The `deserialize` method. Self-describing data formats like JSON are able
+///    to look at the serialized data and tell what it represents. For example
+///    the JSON deserializer may see an opening curly brace (`{`) and know that
+///    it is seeing a map. If the data format supports
+///    `Deserializer::deserialize_any`, it will drive the Visitor using whatever
+///    type it sees in the input. JSON uses this approach when deserializing
+///    `serde_json::Value` which is an enum that can represent any JSON
+///    document. Without knowing what is in a JSON document, we can deserialize
+///    it to `serde_json::Value` by going through
+///    `Deserializer::deserialize_any`.
+///
+/// 2. The various `deserialize_*` methods. Non-self-describing formats like
+///    Bincode need to be told what is in the input in order to deserialize it.
+///    The `deserialize_*` methods are hints to the deserializer for how to
+///    interpret the next piece of input. Non-self-describing formats are not
+///    able to deserialize something like `serde_json::Value` which relies on
+///    `Deserializer::deserialize_any`.
+///
+/// When implementing `Deserialize`, you should avoid relying on
+/// `Deserializer::deserialize_any` unless you need to be told by the
+/// Deserializer what type is in the input. Know that relying on
+/// `Deserializer::deserialize_any` means your data type will be able to
+/// deserialize from self-describing formats only, ruling out Bincode and many
+/// others.
+///
+/// [Serde data model]: https://serde.rs/data-model.html
+///
+/// # Lifetime
+///
+/// The `'de` lifetime of this trait is the lifetime of data that may be
+/// borrowed from the input when deserializing. See the page [Understanding
+/// deserializer lifetimes] for a more detailed explanation of these lifetimes.
+///
+/// [Understanding deserializer lifetimes]: https://serde.rs/lifetimes.html
+///
+/// # Example implementation
+///
+/// The [example data format] presented on the website contains example code for
+/// a basic JSON `Deserializer`.
+///
+/// [example data format]: https://serde.rs/data-format.html
+pub trait Deserializer<'de>: Sized {
+    /// The error type that can be returned if some error occurs during
+    /// deserialization.
+    type Error: Error;
+
+    /// Require the `Deserializer` to figure out how to drive the visitor based
+    /// on what data type is in the input.
+    ///
+    /// When implementing `Deserialize`, you should avoid relying on
+    /// `Deserializer::deserialize_any` unless you need to be told by the
+    /// Deserializer what type is in the input. Know that relying on
+    /// `Deserializer::deserialize_any` means your data type will be able to
+    /// deserialize from self-describing formats only, ruling out Bincode and
+    /// many others.
+    fn deserialize_any<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+    where
+        V: Visitor<'de>;
+
+    /// Hint that the `Deserialize` type is expecting a `bool` value.
+    fn deserialize_bool<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+    where
+        V: Visitor<'de>;
+
+    /// Hint that the `Deserialize` type is expecting an `i8` value.
+    fn deserialize_i8<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+    where
+        V: Visitor<'de>;
+
+    /// Hint that the `Deserialize` type is expecting an `i16` value.
+    fn deserialize_i16<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+    where
+        V: Visitor<'de>;
+
+    /// Hint that the `Deserialize` type is expecting an `i32` value.
+    fn deserialize_i32<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+    where
+        V: Visitor<'de>;
+
+    /// Hint that the `Deserialize` type is expecting an `i64` value.
+    fn deserialize_i64<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+    where
+        V: Visitor<'de>;
+
+    serde_if_integer128! {
+        /// Hint that the `Deserialize` type is expecting an `i128` value.
+        ///
+        /// This method is available only on Rust compiler versions >=1.26. The
+        /// default behavior unconditionally returns an error.
+        fn deserialize_i128<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>
+        {
+            let _ = visitor;
+            Err(Error::custom("i128 is not supported"))
+        }
+    }
+
+    /// Hint that the `Deserialize` type is expecting a `u8` value.
+    fn deserialize_u8<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+    where
+        V: Visitor<'de>;
+
+    /// Hint that the `Deserialize` type is expecting a `u16` value.
+    fn deserialize_u16<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+    where
+        V: Visitor<'de>;
+
+    /// Hint that the `Deserialize` type is expecting a `u32` value.
+    fn deserialize_u32<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+    where
+        V: Visitor<'de>;
+
+    /// Hint that the `Deserialize` type is expecting a `u64` value.
+    fn deserialize_u64<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+    where
+        V: Visitor<'de>;
+
+    serde_if_integer128! {
+        /// Hint that the `Deserialize` type is expecting an `u128` value.
+        ///
+        /// This method is available only on Rust compiler versions >=1.26. The
+        /// default behavior unconditionally returns an error.
+        fn deserialize_u128<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>
+        {
+            let _ = visitor;
+            Err(Error::custom("u128 is not supported"))
+        }
+    }
+
+    /// Hint that the `Deserialize` type is expecting a `f32` value.
+    fn deserialize_f32<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+    where
+        V: Visitor<'de>;
+
+    /// Hint that the `Deserialize` type is expecting a `f64` value.
+    fn deserialize_f64<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+    where
+        V: Visitor<'de>;
+
+    /// Hint that the `Deserialize` type is expecting a `char` value.
+    fn deserialize_char<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+    where
+        V: Visitor<'de>;
+
+    /// Hint that the `Deserialize` type is expecting a string value and does
+    /// not benefit from taking ownership of buffered data owned by the
+    /// `Deserializer`.
+    ///
+    /// If the `Visitor` would benefit from taking ownership of `String` data,
+    /// indiciate this to the `Deserializer` by using `deserialize_string`
+    /// instead.
+    fn deserialize_str<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+    where
+        V: Visitor<'de>;
+
+    /// Hint that the `Deserialize` type is expecting a string value and would
+    /// benefit from taking ownership of buffered data owned by the
+    /// `Deserializer`.
+    ///
+    /// If the `Visitor` would not benefit from taking ownership of `String`
+    /// data, indicate that to the `Deserializer` by using `deserialize_str`
+    /// instead.
+    fn deserialize_string<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+    where
+        V: Visitor<'de>;
+
+    /// Hint that the `Deserialize` type is expecting a byte array and does not
+    /// benefit from taking ownership of buffered data owned by the
+    /// `Deserializer`.
+    ///
+    /// If the `Visitor` would benefit from taking ownership of `Vec<u8>` data,
+    /// indicate this to the `Deserializer` by using `deserialize_byte_buf`
+    /// instead.
+    fn deserialize_bytes<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+    where
+        V: Visitor<'de>;
+
+    /// Hint that the `Deserialize` type is expecting a byte array and would
+    /// benefit from taking ownership of buffered data owned by the
+    /// `Deserializer`.
+    ///
+    /// If the `Visitor` would not benefit from taking ownership of `Vec<u8>`
+    /// data, indicate that to the `Deserializer` by using `deserialize_bytes`
+    /// instead.
+    fn deserialize_byte_buf<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+    where
+        V: Visitor<'de>;
+
+    /// Hint that the `Deserialize` type is expecting an optional value.
+    ///
+    /// This allows deserializers that encode an optional value as a nullable
+    /// value to convert the null value into `None` and a regular value into
+    /// `Some(value)`.
+    fn deserialize_option<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+    where
+        V: Visitor<'de>;
+
+    /// Hint that the `Deserialize` type is expecting a unit value.
+    fn deserialize_unit<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+    where
+        V: Visitor<'de>;
+
+    /// Hint that the `Deserialize` type is expecting a unit struct with a
+    /// particular name.
+    fn deserialize_unit_struct<V>(
+        self,
+        name: &'static str,
+        visitor: V,
+    ) -> Result<V::Value, Self::Error>
+    where
+        V: Visitor<'de>;
+
+    /// Hint that the `Deserialize` type is expecting a newtype struct with a
+    /// particular name.
+    fn deserialize_newtype_struct<V>(
+        self,
+        name: &'static str,
+        visitor: V,
+    ) -> Result<V::Value, Self::Error>
+    where
+        V: Visitor<'de>;
+
+    /// Hint that the `Deserialize` type is expecting a sequence of values.
+    fn deserialize_seq<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+    where
+        V: Visitor<'de>;
+
+    /// Hint that the `Deserialize` type is expecting a sequence of values and
+    /// knows how many values there are without looking at the serialized data.
+    fn deserialize_tuple<V>(self, len: usize, visitor: V) -> Result<V::Value, Self::Error>
+    where
+        V: Visitor<'de>;
+
+    /// Hint that the `Deserialize` type is expecting a tuple struct with a
+    /// particular name and number of fields.
+    fn deserialize_tuple_struct<V>(
+        self,
+        name: &'static str,
+        len: usize,
+        visitor: V,
+    ) -> Result<V::Value, Self::Error>
+    where
+        V: Visitor<'de>;
+
+    /// Hint that the `Deserialize` type is expecting a map of key-value pairs.
+    fn deserialize_map<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+    where
+        V: Visitor<'de>;
+
+    /// Hint that the `Deserialize` type is expecting a struct with a particular
+    /// name and fields.
+    fn deserialize_struct<V>(
+        self,
+        name: &'static str,
+        fields: &'static [&'static str],
+        visitor: V,
+    ) -> Result<V::Value, Self::Error>
+    where
+        V: Visitor<'de>;
+
+    /// Hint that the `Deserialize` type is expecting an enum value with a
+    /// particular name and possible variants.
+    fn deserialize_enum<V>(
+        self,
+        name: &'static str,
+        variants: &'static [&'static str],
+        visitor: V,
+    ) -> Result<V::Value, Self::Error>
+    where
+        V: Visitor<'de>;
+
+    /// Hint that the `Deserialize` type is expecting the name of a struct
+    /// field or the discriminant of an enum variant.
+    fn deserialize_identifier<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+    where
+        V: Visitor<'de>;
+
+    /// Hint that the `Deserialize` type needs to deserialize a value whose type
+    /// doesn't matter because it is ignored.
+    ///
+    /// Deserializers for non-self-describing formats may not support this mode.
+    fn deserialize_ignored_any<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+    where
+        V: Visitor<'de>;
+
+    /// Determine whether `Deserialize` implementations should expect to
+    /// deserialize their human-readable form.
+    ///
+    /// Some types have a human-readable form that may be somewhat expensive to
+    /// construct, as well as a binary form that is compact and efficient.
+    /// Generally text-based formats like JSON and YAML will prefer to use the
+    /// human-readable one and binary formats like Bincode will prefer the
+    /// compact one.
+    ///
+    /// ```edition2018
+    /// # use std::ops::Add;
+    /// # use std::str::FromStr;
+    /// #
+    /// # struct Timestamp;
+    /// #
+    /// # impl Timestamp {
+    /// #     const EPOCH: Timestamp = Timestamp;
+    /// # }
+    /// #
+    /// # impl FromStr for Timestamp {
+    /// #     type Err = String;
+    /// #     fn from_str(_: &str) -> Result<Self, Self::Err> {
+    /// #         unimplemented!()
+    /// #     }
+    /// # }
+    /// #
+    /// # struct Duration;
+    /// #
+    /// # impl Duration {
+    /// #     fn seconds(_: u64) -> Self { unimplemented!() }
+    /// # }
+    /// #
+    /// # impl Add<Duration> for Timestamp {
+    /// #     type Output = Timestamp;
+    /// #     fn add(self, _: Duration) -> Self::Output {
+    /// #         unimplemented!()
+    /// #     }
+    /// # }
+    /// #
+    /// use serde::de::{self, Deserialize, Deserializer};
+    ///
+    /// impl<'de> Deserialize<'de> for Timestamp {
+    ///     fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    ///     where
+    ///         D: Deserializer<'de>,
+    ///     {
+    ///         if deserializer.is_human_readable() {
+    ///             // Deserialize from a human-readable string like "2015-05-15T17:01:00Z".
+    ///             let s = String::deserialize(deserializer)?;
+    ///             Timestamp::from_str(&s).map_err(de::Error::custom)
+    ///         } else {
+    ///             // Deserialize from a compact binary representation, seconds since
+    ///             // the Unix epoch.
+    ///             let n = u64::deserialize(deserializer)?;
+    ///             Ok(Timestamp::EPOCH + Duration::seconds(n))
+    ///         }
+    ///     }
+    /// }
+    /// ```
+    ///
+    /// The default implementation of this method returns `true`. Data formats
+    /// may override this to `false` to request a compact form for types that
+    /// support one. Note that modifying this method to change a format from
+    /// human-readable to compact or vice versa should be regarded as a breaking
+    /// change, as a value serialized in human-readable mode is not required to
+    /// deserialize from the same data in compact mode.
+    #[inline]
+    fn is_human_readable(&self) -> bool {
+        true
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+/// This trait represents a visitor that walks through a deserializer.
+///
+/// # Lifetime
+///
+/// The `'de` lifetime of this trait is the requirement for lifetime of data
+/// that may be borrowed by `Self::Value`. See the page [Understanding
+/// deserializer lifetimes] for a more detailed explanation of these lifetimes.
+///
+/// [Understanding deserializer lifetimes]: https://serde.rs/lifetimes.html
+///
+/// # Example
+///
+/// ```edition2018
+/// # use std::fmt;
+/// #
+/// # use serde::de::{self, Unexpected, Visitor};
+/// #
+/// /// A visitor that deserializes a long string - a string containing at least
+/// /// some minimum number of bytes.
+/// struct LongString {
+///     min: usize,
+/// }
+///
+/// impl<'de> Visitor<'de> for LongString {
+///     type Value = String;
+///
+///     fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+///         write!(formatter, "a string containing at least {} bytes", self.min)
+///     }
+///
+///     fn visit_str<E>(self, s: &str) -> Result<Self::Value, E>
+///     where
+///         E: de::Error,
+///     {
+///         if s.len() >= self.min {
+///             Ok(s.to_owned())
+///         } else {
+///             Err(de::Error::invalid_value(Unexpected::Str(s), &self))
+///         }
+///     }
+/// }
+/// ```
+pub trait Visitor<'de>: Sized {
+    /// The value produced by this visitor.
+    type Value;
+
+    /// Format a message stating what data this Visitor expects to receive.
+    ///
+    /// This is used in error messages. The message should complete the sentence
+    /// "This Visitor expects to receive ...", for example the message could be
+    /// "an integer between 0 and 64". The message should not be capitalized and
+    /// should not end with a period.
+    ///
+    /// ```edition2018
+    /// # use std::fmt;
+    /// #
+    /// # struct S {
+    /// #     max: usize,
+    /// # }
+    /// #
+    /// # impl<'de> serde::de::Visitor<'de> for S {
+    /// #     type Value = ();
+    /// #
+    /// fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+    ///     write!(formatter, "an integer between 0 and {}", self.max)
+    /// }
+    /// # }
+    /// ```
+    fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result;
+
+    /// The input contains a boolean.
+    ///
+    /// The default implementation fails with a type error.
+    fn visit_bool<E>(self, v: bool) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        Err(Error::invalid_type(Unexpected::Bool(v), &self))
+    }
+
+    /// The input contains an `i8`.
+    ///
+    /// The default implementation forwards to [`visit_i64`].
+    ///
+    /// [`visit_i64`]: #method.visit_i64
+    fn visit_i8<E>(self, v: i8) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        self.visit_i64(v as i64)
+    }
+
+    /// The input contains an `i16`.
+    ///
+    /// The default implementation forwards to [`visit_i64`].
+    ///
+    /// [`visit_i64`]: #method.visit_i64
+    fn visit_i16<E>(self, v: i16) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        self.visit_i64(v as i64)
+    }
+
+    /// The input contains an `i32`.
+    ///
+    /// The default implementation forwards to [`visit_i64`].
+    ///
+    /// [`visit_i64`]: #method.visit_i64
+    fn visit_i32<E>(self, v: i32) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        self.visit_i64(v as i64)
+    }
+
+    /// The input contains an `i64`.
+    ///
+    /// The default implementation fails with a type error.
+    fn visit_i64<E>(self, v: i64) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        Err(Error::invalid_type(Unexpected::Signed(v), &self))
+    }
+
+    serde_if_integer128! {
+        /// The input contains a `i128`.
+        ///
+        /// This method is available only on Rust compiler versions >=1.26. The
+        /// default implementation fails with a type error.
+        fn visit_i128<E>(self, v: i128) -> Result<Self::Value, E>
+        where
+            E: Error,
+        {
+            let _ = v;
+            Err(Error::invalid_type(Unexpected::Other("i128"), &self))
+        }
+    }
+
+    /// The input contains a `u8`.
+    ///
+    /// The default implementation forwards to [`visit_u64`].
+    ///
+    /// [`visit_u64`]: #method.visit_u64
+    fn visit_u8<E>(self, v: u8) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        self.visit_u64(v as u64)
+    }
+
+    /// The input contains a `u16`.
+    ///
+    /// The default implementation forwards to [`visit_u64`].
+    ///
+    /// [`visit_u64`]: #method.visit_u64
+    fn visit_u16<E>(self, v: u16) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        self.visit_u64(v as u64)
+    }
+
+    /// The input contains a `u32`.
+    ///
+    /// The default implementation forwards to [`visit_u64`].
+    ///
+    /// [`visit_u64`]: #method.visit_u64
+    fn visit_u32<E>(self, v: u32) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        self.visit_u64(v as u64)
+    }
+
+    /// The input contains a `u64`.
+    ///
+    /// The default implementation fails with a type error.
+    fn visit_u64<E>(self, v: u64) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        Err(Error::invalid_type(Unexpected::Unsigned(v), &self))
+    }
+
+    serde_if_integer128! {
+        /// The input contains a `u128`.
+        ///
+        /// This method is available only on Rust compiler versions >=1.26. The
+        /// default implementation fails with a type error.
+        fn visit_u128<E>(self, v: u128) -> Result<Self::Value, E>
+        where
+            E: Error,
+        {
+            let _ = v;
+            Err(Error::invalid_type(Unexpected::Other("u128"), &self))
+        }
+    }
+
+    /// The input contains an `f32`.
+    ///
+    /// The default implementation forwards to [`visit_f64`].
+    ///
+    /// [`visit_f64`]: #method.visit_f64
+    fn visit_f32<E>(self, v: f32) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        self.visit_f64(v as f64)
+    }
+
+    /// The input contains an `f64`.
+    ///
+    /// The default implementation fails with a type error.
+    fn visit_f64<E>(self, v: f64) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        Err(Error::invalid_type(Unexpected::Float(v), &self))
+    }
+
+    /// The input contains a `char`.
+    ///
+    /// The default implementation forwards to [`visit_str`] as a one-character
+    /// string.
+    ///
+    /// [`visit_str`]: #method.visit_str
+    #[inline]
+    fn visit_char<E>(self, v: char) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        self.visit_str(utf8::encode(v).as_str())
+    }
+
+    /// The input contains a string. The lifetime of the string is ephemeral and
+    /// it may be destroyed after this method returns.
+    ///
+    /// This method allows the `Deserializer` to avoid a copy by retaining
+    /// ownership of any buffered data. `Deserialize` implementations that do
+    /// not benefit from taking ownership of `String` data should indicate that
+    /// to the deserializer by using `Deserializer::deserialize_str` rather than
+    /// `Deserializer::deserialize_string`.
+    ///
+    /// It is never correct to implement `visit_string` without implementing
+    /// `visit_str`. Implement neither, both, or just `visit_str`.
+    fn visit_str<E>(self, v: &str) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        Err(Error::invalid_type(Unexpected::Str(v), &self))
+    }
+
+    /// The input contains a string that lives at least as long as the
+    /// `Deserializer`.
+    ///
+    /// This enables zero-copy deserialization of strings in some formats. For
+    /// example JSON input containing the JSON string `"borrowed"` can be
+    /// deserialized with zero copying into a `&'a str` as long as the input
+    /// data outlives `'a`.
+    ///
+    /// The default implementation forwards to `visit_str`.
+    #[inline]
+    fn visit_borrowed_str<E>(self, v: &'de str) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        self.visit_str(v)
+    }
+
+    /// The input contains a string and ownership of the string is being given
+    /// to the `Visitor`.
+    ///
+    /// This method allows the `Visitor` to avoid a copy by taking ownership of
+    /// a string created by the `Deserializer`. `Deserialize` implementations
+    /// that benefit from taking ownership of `String` data should indicate that
+    /// to the deserializer by using `Deserializer::deserialize_string` rather
+    /// than `Deserializer::deserialize_str`, although not every deserializer
+    /// will honor such a request.
+    ///
+    /// It is never correct to implement `visit_string` without implementing
+    /// `visit_str`. Implement neither, both, or just `visit_str`.
+    ///
+    /// The default implementation forwards to `visit_str` and then drops the
+    /// `String`.
+    #[inline]
+    #[cfg(any(feature = "std", feature = "alloc"))]
+    fn visit_string<E>(self, v: String) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        self.visit_str(&v)
+    }
+
+    /// The input contains a byte array. The lifetime of the byte array is
+    /// ephemeral and it may be destroyed after this method returns.
+    ///
+    /// This method allows the `Deserializer` to avoid a copy by retaining
+    /// ownership of any buffered data. `Deserialize` implementations that do
+    /// not benefit from taking ownership of `Vec<u8>` data should indicate that
+    /// to the deserializer by using `Deserializer::deserialize_bytes` rather
+    /// than `Deserializer::deserialize_byte_buf`.
+    ///
+    /// It is never correct to implement `visit_byte_buf` without implementing
+    /// `visit_bytes`. Implement neither, both, or just `visit_bytes`.
+    fn visit_bytes<E>(self, v: &[u8]) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        let _ = v;
+        Err(Error::invalid_type(Unexpected::Bytes(v), &self))
+    }
+
+    /// The input contains a byte array that lives at least as long as the
+    /// `Deserializer`.
+    ///
+    /// This enables zero-copy deserialization of bytes in some formats. For
+    /// example Bincode data containing bytes can be deserialized with zero
+    /// copying into a `&'a [u8]` as long as the input data outlives `'a`.
+    ///
+    /// The default implementation forwards to `visit_bytes`.
+    #[inline]
+    fn visit_borrowed_bytes<E>(self, v: &'de [u8]) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        self.visit_bytes(v)
+    }
+
+    /// The input contains a byte array and ownership of the byte array is being
+    /// given to the `Visitor`.
+    ///
+    /// This method allows the `Visitor` to avoid a copy by taking ownership of
+    /// a byte buffer created by the `Deserializer`. `Deserialize`
+    /// implementations that benefit from taking ownership of `Vec<u8>` data
+    /// should indicate that to the deserializer by using
+    /// `Deserializer::deserialize_byte_buf` rather than
+    /// `Deserializer::deserialize_bytes`, although not every deserializer will
+    /// honor such a request.
+    ///
+    /// It is never correct to implement `visit_byte_buf` without implementing
+    /// `visit_bytes`. Implement neither, both, or just `visit_bytes`.
+    ///
+    /// The default implementation forwards to `visit_bytes` and then drops the
+    /// `Vec<u8>`.
+    #[cfg(any(feature = "std", feature = "alloc"))]
+    fn visit_byte_buf<E>(self, v: Vec<u8>) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        self.visit_bytes(&v)
+    }
+
+    /// The input contains an optional that is absent.
+    ///
+    /// The default implementation fails with a type error.
+    fn visit_none<E>(self) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        Err(Error::invalid_type(Unexpected::Option, &self))
+    }
+
+    /// The input contains an optional that is present.
+    ///
+    /// The default implementation fails with a type error.
+    fn visit_some<D>(self, deserializer: D) -> Result<Self::Value, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        let _ = deserializer;
+        Err(Error::invalid_type(Unexpected::Option, &self))
+    }
+
+    /// The input contains a unit `()`.
+    ///
+    /// The default implementation fails with a type error.
+    fn visit_unit<E>(self) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        Err(Error::invalid_type(Unexpected::Unit, &self))
+    }
+
+    /// The input contains a newtype struct.
+    ///
+    /// The content of the newtype struct may be read from the given
+    /// `Deserializer`.
+    ///
+    /// The default implementation fails with a type error.
+    fn visit_newtype_struct<D>(self, deserializer: D) -> Result<Self::Value, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        let _ = deserializer;
+        Err(Error::invalid_type(Unexpected::NewtypeStruct, &self))
+    }
+
+    /// The input contains a sequence of elements.
+    ///
+    /// The default implementation fails with a type error.
+    fn visit_seq<A>(self, seq: A) -> Result<Self::Value, A::Error>
+    where
+        A: SeqAccess<'de>,
+    {
+        let _ = seq;
+        Err(Error::invalid_type(Unexpected::Seq, &self))
+    }
+
+    /// The input contains a key-value map.
+    ///
+    /// The default implementation fails with a type error.
+    fn visit_map<A>(self, map: A) -> Result<Self::Value, A::Error>
+    where
+        A: MapAccess<'de>,
+    {
+        let _ = map;
+        Err(Error::invalid_type(Unexpected::Map, &self))
+    }
+
+    /// The input contains an enum.
+    ///
+    /// The default implementation fails with a type error.
+    fn visit_enum<A>(self, data: A) -> Result<Self::Value, A::Error>
+    where
+        A: EnumAccess<'de>,
+    {
+        let _ = data;
+        Err(Error::invalid_type(Unexpected::Enum, &self))
+    }
+
+    // Used when deserializing a flattened Option field. Not public API.
+    #[doc(hidden)]
+    fn __private_visit_untagged_option<D>(self, _: D) -> Result<Self::Value, ()>
+    where
+        D: Deserializer<'de>,
+    {
+        Err(())
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+/// Provides a `Visitor` access to each element of a sequence in the input.
+///
+/// This is a trait that a `Deserializer` passes to a `Visitor` implementation,
+/// which deserializes each item in a sequence.
+///
+/// # Lifetime
+///
+/// The `'de` lifetime of this trait is the lifetime of data that may be
+/// borrowed by deserialized sequence elements. See the page [Understanding
+/// deserializer lifetimes] for a more detailed explanation of these lifetimes.
+///
+/// [Understanding deserializer lifetimes]: https://serde.rs/lifetimes.html
+///
+/// # Example implementation
+///
+/// The [example data format] presented on the website demonstrates an
+/// implementation of `SeqAccess` for a basic JSON data format.
+///
+/// [example data format]: https://serde.rs/data-format.html
+pub trait SeqAccess<'de> {
+    /// The error type that can be returned if some error occurs during
+    /// deserialization.
+    type Error: Error;
+
+    /// This returns `Ok(Some(value))` for the next value in the sequence, or
+    /// `Ok(None)` if there are no more remaining items.
+    ///
+    /// `Deserialize` implementations should typically use
+    /// `SeqAccess::next_element` instead.
+    fn next_element_seed<T>(&mut self, seed: T) -> Result<Option<T::Value>, Self::Error>
+    where
+        T: DeserializeSeed<'de>;
+
+    /// This returns `Ok(Some(value))` for the next value in the sequence, or
+    /// `Ok(None)` if there are no more remaining items.
+    ///
+    /// This method exists as a convenience for `Deserialize` implementations.
+    /// `SeqAccess` implementations should not override the default behavior.
+    #[inline]
+    fn next_element<T>(&mut self) -> Result<Option<T>, Self::Error>
+    where
+        T: Deserialize<'de>,
+    {
+        self.next_element_seed(PhantomData)
+    }
+
+    /// Returns the number of elements remaining in the sequence, if known.
+    #[inline]
+    fn size_hint(&self) -> Option<usize> {
+        None
+    }
+}
+
+impl<'de, 'a, A> SeqAccess<'de> for &'a mut A
+where
+    A: SeqAccess<'de>,
+{
+    type Error = A::Error;
+
+    #[inline]
+    fn next_element_seed<T>(&mut self, seed: T) -> Result<Option<T::Value>, Self::Error>
+    where
+        T: DeserializeSeed<'de>,
+    {
+        (**self).next_element_seed(seed)
+    }
+
+    #[inline]
+    fn next_element<T>(&mut self) -> Result<Option<T>, Self::Error>
+    where
+        T: Deserialize<'de>,
+    {
+        (**self).next_element()
+    }
+
+    #[inline]
+    fn size_hint(&self) -> Option<usize> {
+        (**self).size_hint()
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+/// Provides a `Visitor` access to each entry of a map in the input.
+///
+/// This is a trait that a `Deserializer` passes to a `Visitor` implementation.
+///
+/// # Lifetime
+///
+/// The `'de` lifetime of this trait is the lifetime of data that may be
+/// borrowed by deserialized map entries. See the page [Understanding
+/// deserializer lifetimes] for a more detailed explanation of these lifetimes.
+///
+/// [Understanding deserializer lifetimes]: https://serde.rs/lifetimes.html
+///
+/// # Example implementation
+///
+/// The [example data format] presented on the website demonstrates an
+/// implementation of `MapAccess` for a basic JSON data format.
+///
+/// [example data format]: https://serde.rs/data-format.html
+pub trait MapAccess<'de> {
+    /// The error type that can be returned if some error occurs during
+    /// deserialization.
+    type Error: Error;
+
+    /// This returns `Ok(Some(key))` for the next key in the map, or `Ok(None)`
+    /// if there are no more remaining entries.
+    ///
+    /// `Deserialize` implementations should typically use
+    /// `MapAccess::next_key` or `MapAccess::next_entry` instead.
+    fn next_key_seed<K>(&mut self, seed: K) -> Result<Option<K::Value>, Self::Error>
+    where
+        K: DeserializeSeed<'de>;
+
+    /// This returns a `Ok(value)` for the next value in the map.
+    ///
+    /// `Deserialize` implementations should typically use
+    /// `MapAccess::next_value` instead.
+    ///
+    /// # Panics
+    ///
+    /// Calling `next_value_seed` before `next_key_seed` is incorrect and is
+    /// allowed to panic or return bogus results.
+    fn next_value_seed<V>(&mut self, seed: V) -> Result<V::Value, Self::Error>
+    where
+        V: DeserializeSeed<'de>;
+
+    /// This returns `Ok(Some((key, value)))` for the next (key-value) pair in
+    /// the map, or `Ok(None)` if there are no more remaining items.
+    ///
+    /// `MapAccess` implementations should override the default behavior if a
+    /// more efficient implementation is possible.
+    ///
+    /// `Deserialize` implementations should typically use
+    /// `MapAccess::next_entry` instead.
+    #[inline]
+    fn next_entry_seed<K, V>(
+        &mut self,
+        kseed: K,
+        vseed: V,
+    ) -> Result<Option<(K::Value, V::Value)>, Self::Error>
+    where
+        K: DeserializeSeed<'de>,
+        V: DeserializeSeed<'de>,
+    {
+        match try!(self.next_key_seed(kseed)) {
+            Some(key) => {
+                let value = try!(self.next_value_seed(vseed));
+                Ok(Some((key, value)))
+            }
+            None => Ok(None),
+        }
+    }
+
+    /// This returns `Ok(Some(key))` for the next key in the map, or `Ok(None)`
+    /// if there are no more remaining entries.
+    ///
+    /// This method exists as a convenience for `Deserialize` implementations.
+    /// `MapAccess` implementations should not override the default behavior.
+    #[inline]
+    fn next_key<K>(&mut self) -> Result<Option<K>, Self::Error>
+    where
+        K: Deserialize<'de>,
+    {
+        self.next_key_seed(PhantomData)
+    }
+
+    /// This returns a `Ok(value)` for the next value in the map.
+    ///
+    /// This method exists as a convenience for `Deserialize` implementations.
+    /// `MapAccess` implementations should not override the default behavior.
+    ///
+    /// # Panics
+    ///
+    /// Calling `next_value` before `next_key` is incorrect and is allowed to
+    /// panic or return bogus results.
+    #[inline]
+    fn next_value<V>(&mut self) -> Result<V, Self::Error>
+    where
+        V: Deserialize<'de>,
+    {
+        self.next_value_seed(PhantomData)
+    }
+
+    /// This returns `Ok(Some((key, value)))` for the next (key-value) pair in
+    /// the map, or `Ok(None)` if there are no more remaining items.
+    ///
+    /// This method exists as a convenience for `Deserialize` implementations.
+    /// `MapAccess` implementations should not override the default behavior.
+    #[inline]
+    fn next_entry<K, V>(&mut self) -> Result<Option<(K, V)>, Self::Error>
+    where
+        K: Deserialize<'de>,
+        V: Deserialize<'de>,
+    {
+        self.next_entry_seed(PhantomData, PhantomData)
+    }
+
+    /// Returns the number of entries remaining in the map, if known.
+    #[inline]
+    fn size_hint(&self) -> Option<usize> {
+        None
+    }
+}
+
+impl<'de, 'a, A> MapAccess<'de> for &'a mut A
+where
+    A: MapAccess<'de>,
+{
+    type Error = A::Error;
+
+    #[inline]
+    fn next_key_seed<K>(&mut self, seed: K) -> Result<Option<K::Value>, Self::Error>
+    where
+        K: DeserializeSeed<'de>,
+    {
+        (**self).next_key_seed(seed)
+    }
+
+    #[inline]
+    fn next_value_seed<V>(&mut self, seed: V) -> Result<V::Value, Self::Error>
+    where
+        V: DeserializeSeed<'de>,
+    {
+        (**self).next_value_seed(seed)
+    }
+
+    #[inline]
+    fn next_entry_seed<K, V>(
+        &mut self,
+        kseed: K,
+        vseed: V,
+    ) -> Result<Option<(K::Value, V::Value)>, Self::Error>
+    where
+        K: DeserializeSeed<'de>,
+        V: DeserializeSeed<'de>,
+    {
+        (**self).next_entry_seed(kseed, vseed)
+    }
+
+    #[inline]
+    fn next_entry<K, V>(&mut self) -> Result<Option<(K, V)>, Self::Error>
+    where
+        K: Deserialize<'de>,
+        V: Deserialize<'de>,
+    {
+        (**self).next_entry()
+    }
+
+    #[inline]
+    fn next_key<K>(&mut self) -> Result<Option<K>, Self::Error>
+    where
+        K: Deserialize<'de>,
+    {
+        (**self).next_key()
+    }
+
+    #[inline]
+    fn next_value<V>(&mut self) -> Result<V, Self::Error>
+    where
+        V: Deserialize<'de>,
+    {
+        (**self).next_value()
+    }
+
+    #[inline]
+    fn size_hint(&self) -> Option<usize> {
+        (**self).size_hint()
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+/// Provides a `Visitor` access to the data of an enum in the input.
+///
+/// `EnumAccess` is created by the `Deserializer` and passed to the
+/// `Visitor` in order to identify which variant of an enum to deserialize.
+///
+/// # Lifetime
+///
+/// The `'de` lifetime of this trait is the lifetime of data that may be
+/// borrowed by the deserialized enum variant. See the page [Understanding
+/// deserializer lifetimes] for a more detailed explanation of these lifetimes.
+///
+/// [Understanding deserializer lifetimes]: https://serde.rs/lifetimes.html
+///
+/// # Example implementation
+///
+/// The [example data format] presented on the website demonstrates an
+/// implementation of `EnumAccess` for a basic JSON data format.
+///
+/// [example data format]: https://serde.rs/data-format.html
+pub trait EnumAccess<'de>: Sized {
+    /// The error type that can be returned if some error occurs during
+    /// deserialization.
+    type Error: Error;
+    /// The `Visitor` that will be used to deserialize the content of the enum
+    /// variant.
+    type Variant: VariantAccess<'de, Error = Self::Error>;
+
+    /// `variant` is called to identify which variant to deserialize.
+    ///
+    /// `Deserialize` implementations should typically use `EnumAccess::variant`
+    /// instead.
+    fn variant_seed<V>(self, seed: V) -> Result<(V::Value, Self::Variant), Self::Error>
+    where
+        V: DeserializeSeed<'de>;
+
+    /// `variant` is called to identify which variant to deserialize.
+    ///
+    /// This method exists as a convenience for `Deserialize` implementations.
+    /// `EnumAccess` implementations should not override the default behavior.
+    #[inline]
+    fn variant<V>(self) -> Result<(V, Self::Variant), Self::Error>
+    where
+        V: Deserialize<'de>,
+    {
+        self.variant_seed(PhantomData)
+    }
+}
+
+/// `VariantAccess` is a visitor that is created by the `Deserializer` and
+/// passed to the `Deserialize` to deserialize the content of a particular enum
+/// variant.
+///
+/// # Lifetime
+///
+/// The `'de` lifetime of this trait is the lifetime of data that may be
+/// borrowed by the deserialized enum variant. See the page [Understanding
+/// deserializer lifetimes] for a more detailed explanation of these lifetimes.
+///
+/// [Understanding deserializer lifetimes]: https://serde.rs/lifetimes.html
+///
+/// # Example implementation
+///
+/// The [example data format] presented on the website demonstrates an
+/// implementation of `VariantAccess` for a basic JSON data format.
+///
+/// [example data format]: https://serde.rs/data-format.html
+pub trait VariantAccess<'de>: Sized {
+    /// The error type that can be returned if some error occurs during
+    /// deserialization. Must match the error type of our `EnumAccess`.
+    type Error: Error;
+
+    /// Called when deserializing a variant with no values.
+    ///
+    /// If the data contains a different type of variant, the following
+    /// `invalid_type` error should be constructed:
+    ///
+    /// ```edition2018
+    /// # use serde::de::{self, value, DeserializeSeed, Visitor, VariantAccess, Unexpected};
+    /// #
+    /// # struct X;
+    /// #
+    /// # impl<'de> VariantAccess<'de> for X {
+    /// #     type Error = value::Error;
+    /// #
+    /// fn unit_variant(self) -> Result<(), Self::Error> {
+    ///     // What the data actually contained; suppose it is a tuple variant.
+    ///     let unexp = Unexpected::TupleVariant;
+    ///     Err(de::Error::invalid_type(unexp, &"unit variant"))
+    /// }
+    /// #
+    /// #     fn newtype_variant_seed<T>(self, _: T) -> Result<T::Value, Self::Error>
+    /// #     where
+    /// #         T: DeserializeSeed<'de>,
+    /// #     { unimplemented!() }
+    /// #
+    /// #     fn tuple_variant<V>(self, _: usize, _: V) -> Result<V::Value, Self::Error>
+    /// #     where
+    /// #         V: Visitor<'de>,
+    /// #     { unimplemented!() }
+    /// #
+    /// #     fn struct_variant<V>(self, _: &[&str], _: V) -> Result<V::Value, Self::Error>
+    /// #     where
+    /// #         V: Visitor<'de>,
+    /// #     { unimplemented!() }
+    /// # }
+    /// ```
+    fn unit_variant(self) -> Result<(), Self::Error>;
+
+    /// Called when deserializing a variant with a single value.
+    ///
+    /// `Deserialize` implementations should typically use
+    /// `VariantAccess::newtype_variant` instead.
+    ///
+    /// If the data contains a different type of variant, the following
+    /// `invalid_type` error should be constructed:
+    ///
+    /// ```edition2018
+    /// # use serde::de::{self, value, DeserializeSeed, Visitor, VariantAccess, Unexpected};
+    /// #
+    /// # struct X;
+    /// #
+    /// # impl<'de> VariantAccess<'de> for X {
+    /// #     type Error = value::Error;
+    /// #
+    /// #     fn unit_variant(self) -> Result<(), Self::Error> {
+    /// #         unimplemented!()
+    /// #     }
+    /// #
+    /// fn newtype_variant_seed<T>(self, _seed: T) -> Result<T::Value, Self::Error>
+    /// where
+    ///     T: DeserializeSeed<'de>,
+    /// {
+    ///     // What the data actually contained; suppose it is a unit variant.
+    ///     let unexp = Unexpected::UnitVariant;
+    ///     Err(de::Error::invalid_type(unexp, &"newtype variant"))
+    /// }
+    /// #
+    /// #     fn tuple_variant<V>(self, _: usize, _: V) -> Result<V::Value, Self::Error>
+    /// #     where
+    /// #         V: Visitor<'de>,
+    /// #     { unimplemented!() }
+    /// #
+    /// #     fn struct_variant<V>(self, _: &[&str], _: V) -> Result<V::Value, Self::Error>
+    /// #     where
+    /// #         V: Visitor<'de>,
+    /// #     { unimplemented!() }
+    /// # }
+    /// ```
+    fn newtype_variant_seed<T>(self, seed: T) -> Result<T::Value, Self::Error>
+    where
+        T: DeserializeSeed<'de>;
+
+    /// Called when deserializing a variant with a single value.
+    ///
+    /// This method exists as a convenience for `Deserialize` implementations.
+    /// `VariantAccess` implementations should not override the default
+    /// behavior.
+    #[inline]
+    fn newtype_variant<T>(self) -> Result<T, Self::Error>
+    where
+        T: Deserialize<'de>,
+    {
+        self.newtype_variant_seed(PhantomData)
+    }
+
+    /// Called when deserializing a tuple-like variant.
+    ///
+    /// The `len` is the number of fields expected in the tuple variant.
+    ///
+    /// If the data contains a different type of variant, the following
+    /// `invalid_type` error should be constructed:
+    ///
+    /// ```edition2018
+    /// # use serde::de::{self, value, DeserializeSeed, Visitor, VariantAccess, Unexpected};
+    /// #
+    /// # struct X;
+    /// #
+    /// # impl<'de> VariantAccess<'de> for X {
+    /// #     type Error = value::Error;
+    /// #
+    /// #     fn unit_variant(self) -> Result<(), Self::Error> {
+    /// #         unimplemented!()
+    /// #     }
+    /// #
+    /// #     fn newtype_variant_seed<T>(self, _: T) -> Result<T::Value, Self::Error>
+    /// #     where
+    /// #         T: DeserializeSeed<'de>,
+    /// #     { unimplemented!() }
+    /// #
+    /// fn tuple_variant<V>(
+    ///     self,
+    ///     _len: usize,
+    ///     _visitor: V,
+    /// ) -> Result<V::Value, Self::Error>
+    /// where
+    ///     V: Visitor<'de>,
+    /// {
+    ///     // What the data actually contained; suppose it is a unit variant.
+    ///     let unexp = Unexpected::UnitVariant;
+    ///     Err(de::Error::invalid_type(unexp, &"tuple variant"))
+    /// }
+    /// #
+    /// #     fn struct_variant<V>(self, _: &[&str], _: V) -> Result<V::Value, Self::Error>
+    /// #     where
+    /// #         V: Visitor<'de>,
+    /// #     { unimplemented!() }
+    /// # }
+    /// ```
+    fn tuple_variant<V>(self, len: usize, visitor: V) -> Result<V::Value, Self::Error>
+    where
+        V: Visitor<'de>;
+
+    /// Called when deserializing a struct-like variant.
+    ///
+    /// The `fields` are the names of the fields of the struct variant.
+    ///
+    /// If the data contains a different type of variant, the following
+    /// `invalid_type` error should be constructed:
+    ///
+    /// ```edition2018
+    /// # use serde::de::{self, value, DeserializeSeed, Visitor, VariantAccess, Unexpected};
+    /// #
+    /// # struct X;
+    /// #
+    /// # impl<'de> VariantAccess<'de> for X {
+    /// #     type Error = value::Error;
+    /// #
+    /// #     fn unit_variant(self) -> Result<(), Self::Error> {
+    /// #         unimplemented!()
+    /// #     }
+    /// #
+    /// #     fn newtype_variant_seed<T>(self, _: T) -> Result<T::Value, Self::Error>
+    /// #     where
+    /// #         T: DeserializeSeed<'de>,
+    /// #     { unimplemented!() }
+    /// #
+    /// #     fn tuple_variant<V>(self, _: usize, _: V) -> Result<V::Value, Self::Error>
+    /// #     where
+    /// #         V: Visitor<'de>,
+    /// #     { unimplemented!() }
+    /// #
+    /// fn struct_variant<V>(
+    ///     self,
+    ///     _fields: &'static [&'static str],
+    ///     _visitor: V,
+    /// ) -> Result<V::Value, Self::Error>
+    /// where
+    ///     V: Visitor<'de>,
+    /// {
+    ///     // What the data actually contained; suppose it is a unit variant.
+    ///     let unexp = Unexpected::UnitVariant;
+    ///     Err(de::Error::invalid_type(unexp, &"struct variant"))
+    /// }
+    /// # }
+    /// ```
+    fn struct_variant<V>(
+        self,
+        fields: &'static [&'static str],
+        visitor: V,
+    ) -> Result<V::Value, Self::Error>
+    where
+        V: Visitor<'de>;
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+/// Converts an existing value into a `Deserializer` from which other values can
+/// be deserialized.
+///
+/// # Lifetime
+///
+/// The `'de` lifetime of this trait is the lifetime of data that may be
+/// borrowed from the resulting `Deserializer`. See the page [Understanding
+/// deserializer lifetimes] for a more detailed explanation of these lifetimes.
+///
+/// [Understanding deserializer lifetimes]: https://serde.rs/lifetimes.html
+///
+/// # Example
+///
+/// ```edition2018
+/// use std::str::FromStr;
+/// use serde::Deserialize;
+/// use serde::de::{value, IntoDeserializer};
+///
+/// #[derive(Deserialize)]
+/// enum Setting {
+///     On,
+///     Off,
+/// }
+///
+/// impl FromStr for Setting {
+///     type Err = value::Error;
+///
+///     fn from_str(s: &str) -> Result<Self, Self::Err> {
+///         Self::deserialize(s.into_deserializer())
+///     }
+/// }
+/// ```
+pub trait IntoDeserializer<'de, E: Error = value::Error> {
+    /// The type of the deserializer being converted into.
+    type Deserializer: Deserializer<'de, Error = E>;
+
+    /// Convert this value into a deserializer.
+    fn into_deserializer(self) -> Self::Deserializer;
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+/// Used in error messages.
+///
+/// - expected `a`
+/// - expected `a` or `b`
+/// - expected one of `a`, `b`, `c`
+///
+/// The slice of names must not be empty.
+struct OneOf {
+    names: &'static [&'static str],
+}
+
+impl Display for OneOf {
+    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        match self.names.len() {
+            0 => panic!(), // special case elsewhere
+            1 => write!(formatter, "`{}`", self.names[0]),
+            2 => write!(formatter, "`{}` or `{}`", self.names[0], self.names[1]),
+            _ => {
+                try!(write!(formatter, "one of "));
+                for (i, alt) in self.names.iter().enumerate() {
+                    if i > 0 {
+                        try!(write!(formatter, ", "));
+                    }
+                    try!(write!(formatter, "`{}`", alt));
+                }
+                Ok(())
+            }
+        }
+    }
+}
diff --git a/src/de/utf8.rs b/src/de/utf8.rs
new file mode 100644
index 0000000..576fd03
--- /dev/null
+++ b/src/de/utf8.rs
@@ -0,0 +1,46 @@
+use lib::*;
+
+const TAG_CONT: u8 = 0b1000_0000;
+const TAG_TWO_B: u8 = 0b1100_0000;
+const TAG_THREE_B: u8 = 0b1110_0000;
+const TAG_FOUR_B: u8 = 0b1111_0000;
+const MAX_ONE_B: u32 = 0x80;
+const MAX_TWO_B: u32 = 0x800;
+const MAX_THREE_B: u32 = 0x10000;
+
+#[inline]
+pub fn encode(c: char) -> Encode {
+    let code = c as u32;
+    let mut buf = [0; 4];
+    let pos = if code < MAX_ONE_B {
+        buf[3] = code as u8;
+        3
+    } else if code < MAX_TWO_B {
+        buf[2] = (code >> 6 & 0x1F) as u8 | TAG_TWO_B;
+        buf[3] = (code & 0x3F) as u8 | TAG_CONT;
+        2
+    } else if code < MAX_THREE_B {
+        buf[1] = (code >> 12 & 0x0F) as u8 | TAG_THREE_B;
+        buf[2] = (code >> 6 & 0x3F) as u8 | TAG_CONT;
+        buf[3] = (code & 0x3F) as u8 | TAG_CONT;
+        1
+    } else {
+        buf[0] = (code >> 18 & 0x07) as u8 | TAG_FOUR_B;
+        buf[1] = (code >> 12 & 0x3F) as u8 | TAG_CONT;
+        buf[2] = (code >> 6 & 0x3F) as u8 | TAG_CONT;
+        buf[3] = (code & 0x3F) as u8 | TAG_CONT;
+        0
+    };
+    Encode { buf: buf, pos: pos }
+}
+
+pub struct Encode {
+    buf: [u8; 4],
+    pos: usize,
+}
+
+impl Encode {
+    pub fn as_str(&self) -> &str {
+        str::from_utf8(&self.buf[self.pos..]).unwrap()
+    }
+}
diff --git a/src/de/value.rs b/src/de/value.rs
new file mode 100644
index 0000000..d0a185d
--- /dev/null
+++ b/src/de/value.rs
@@ -0,0 +1,1497 @@
+//! Building blocks for deserializing basic values using the `IntoDeserializer`
+//! trait.
+//!
+//! ```edition2018
+//! use std::str::FromStr;
+//! use serde::Deserialize;
+//! use serde::de::{value, IntoDeserializer};
+//!
+//! #[derive(Deserialize)]
+//! enum Setting {
+//!     On,
+//!     Off,
+//! }
+//!
+//! impl FromStr for Setting {
+//!     type Err = value::Error;
+//!
+//!     fn from_str(s: &str) -> Result<Self, Self::Err> {
+//!         Self::deserialize(s.into_deserializer())
+//!     }
+//! }
+//! ```
+
+use lib::*;
+
+use self::private::{First, Second};
+use de::{self, Expected, IntoDeserializer, SeqAccess};
+use private::de::size_hint;
+use ser;
+
+////////////////////////////////////////////////////////////////////////////////
+
+// For structs that contain a PhantomData. We do not want the trait
+// bound `E: Clone` inferred by derive(Clone).
+macro_rules! impl_copy_clone {
+    ($ty:ident $(<$lifetime:tt>)*) => {
+        impl<$($lifetime,)* E> Copy for $ty<$($lifetime,)* E> {}
+
+        impl<$($lifetime,)* E> Clone for $ty<$($lifetime,)* E> {
+            fn clone(&self) -> Self {
+                *self
+            }
+        }
+    };
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+/// A minimal representation of all possible errors that can occur using the
+/// `IntoDeserializer` trait.
+#[derive(Clone, Debug, PartialEq)]
+pub struct Error {
+    err: ErrorImpl,
+}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+type ErrorImpl = Box<str>;
+#[cfg(not(any(feature = "std", feature = "alloc")))]
+type ErrorImpl = ();
+
+impl de::Error for Error {
+    #[cfg(any(feature = "std", feature = "alloc"))]
+    #[cold]
+    fn custom<T>(msg: T) -> Self
+    where
+        T: Display,
+    {
+        Error {
+            err: msg.to_string().into_boxed_str(),
+        }
+    }
+
+    #[cfg(not(any(feature = "std", feature = "alloc")))]
+    #[cold]
+    fn custom<T>(msg: T) -> Self
+    where
+        T: Display,
+    {
+        let _ = msg;
+        Error { err: () }
+    }
+}
+
+impl ser::Error for Error {
+    #[cold]
+    fn custom<T>(msg: T) -> Self
+    where
+        T: Display,
+    {
+        de::Error::custom(msg)
+    }
+}
+
+impl Display for Error {
+    #[cfg(any(feature = "std", feature = "alloc"))]
+    fn fmt(&self, formatter: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+        formatter.write_str(&self.err)
+    }
+
+    #[cfg(not(any(feature = "std", feature = "alloc")))]
+    fn fmt(&self, formatter: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+        formatter.write_str("Serde deserialization error")
+    }
+}
+
+#[cfg(feature = "std")]
+impl error::Error for Error {
+    fn description(&self) -> &str {
+        &self.err
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+impl<'de, E> IntoDeserializer<'de, E> for ()
+where
+    E: de::Error,
+{
+    type Deserializer = UnitDeserializer<E>;
+
+    fn into_deserializer(self) -> UnitDeserializer<E> {
+        UnitDeserializer {
+            marker: PhantomData,
+        }
+    }
+}
+
+/// A deserializer holding a `()`.
+#[derive(Debug)]
+pub struct UnitDeserializer<E> {
+    marker: PhantomData<E>,
+}
+
+impl_copy_clone!(UnitDeserializer);
+
+impl<'de, E> de::Deserializer<'de> for UnitDeserializer<E>
+where
+    E: de::Error,
+{
+    type Error = E;
+
+    forward_to_deserialize_any! {
+        bool i8 i16 i32 i64 i128 u8 u16 u32 u64 u128 f32 f64 char str string
+        bytes byte_buf unit unit_struct newtype_struct seq tuple tuple_struct
+        map struct enum identifier ignored_any
+    }
+
+    fn deserialize_any<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+    where
+        V: de::Visitor<'de>,
+    {
+        visitor.visit_unit()
+    }
+
+    fn deserialize_option<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+    where
+        V: de::Visitor<'de>,
+    {
+        visitor.visit_none()
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+/// A deserializer that cannot be instantiated.
+#[cfg(feature = "unstable")]
+pub struct NeverDeserializer<E> {
+    never: !,
+    marker: PhantomData<E>,
+}
+
+#[cfg(feature = "unstable")]
+impl<'de, E> IntoDeserializer<'de, E> for !
+where
+    E: de::Error,
+{
+    type Deserializer = NeverDeserializer<E>;
+
+    fn into_deserializer(self) -> Self::Deserializer {
+        self
+    }
+}
+
+#[cfg(feature = "unstable")]
+impl<'de, E> de::Deserializer<'de> for NeverDeserializer<E>
+where
+    E: de::Error,
+{
+    type Error = E;
+
+    fn deserialize_any<V>(self, _visitor: V) -> Result<V::Value, Self::Error>
+    where
+        V: de::Visitor<'de>,
+    {
+        self.never
+    }
+
+    forward_to_deserialize_any! {
+        bool i8 i16 i32 i64 i128 u8 u16 u32 u64 u128 f32 f64 char str string
+        bytes byte_buf option unit unit_struct newtype_struct seq tuple
+        tuple_struct map struct enum identifier ignored_any
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+macro_rules! primitive_deserializer {
+    ($ty:ty, $doc:tt, $name:ident, $method:ident $($cast:tt)*) => {
+        #[doc = "A deserializer holding"]
+        #[doc = $doc]
+        #[derive(Debug)]
+        pub struct $name<E> {
+            value: $ty,
+            marker: PhantomData<E>
+        }
+
+        impl_copy_clone!($name);
+
+        impl<'de, E> IntoDeserializer<'de, E> for $ty
+        where
+            E: de::Error,
+        {
+            type Deserializer = $name<E>;
+
+            fn into_deserializer(self) -> $name<E> {
+                $name {
+                    value: self,
+                    marker: PhantomData,
+                }
+            }
+        }
+
+        impl<'de, E> de::Deserializer<'de> for $name<E>
+        where
+            E: de::Error,
+        {
+            type Error = E;
+
+            forward_to_deserialize_any! {
+                bool i8 i16 i32 i64 i128 u8 u16 u32 u64 u128 f32 f64 char str
+                string bytes byte_buf option unit unit_struct newtype_struct seq
+                tuple tuple_struct map struct enum identifier ignored_any
+            }
+
+            fn deserialize_any<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+            where
+                V: de::Visitor<'de>,
+            {
+                visitor.$method(self.value $($cast)*)
+            }
+        }
+    }
+}
+
+primitive_deserializer!(bool, "a `bool`.", BoolDeserializer, visit_bool);
+primitive_deserializer!(i8, "an `i8`.", I8Deserializer, visit_i8);
+primitive_deserializer!(i16, "an `i16`.", I16Deserializer, visit_i16);
+primitive_deserializer!(i32, "an `i32`.", I32Deserializer, visit_i32);
+primitive_deserializer!(i64, "an `i64`.", I64Deserializer, visit_i64);
+primitive_deserializer!(isize, "an `isize`.", IsizeDeserializer, visit_i64 as i64);
+primitive_deserializer!(u8, "a `u8`.", U8Deserializer, visit_u8);
+primitive_deserializer!(u16, "a `u16`.", U16Deserializer, visit_u16);
+primitive_deserializer!(u64, "a `u64`.", U64Deserializer, visit_u64);
+primitive_deserializer!(usize, "a `usize`.", UsizeDeserializer, visit_u64 as u64);
+primitive_deserializer!(f32, "an `f32`.", F32Deserializer, visit_f32);
+primitive_deserializer!(f64, "an `f64`.", F64Deserializer, visit_f64);
+primitive_deserializer!(char, "a `char`.", CharDeserializer, visit_char);
+
+serde_if_integer128! {
+    primitive_deserializer!(i128, "an `i128`.", I128Deserializer, visit_i128);
+    primitive_deserializer!(u128, "a `u128`.", U128Deserializer, visit_u128);
+}
+
+/// A deserializer holding a `u32`.
+#[derive(Debug)]
+pub struct U32Deserializer<E> {
+    value: u32,
+    marker: PhantomData<E>,
+}
+
+impl_copy_clone!(U32Deserializer);
+
+impl<'de, E> IntoDeserializer<'de, E> for u32
+where
+    E: de::Error,
+{
+    type Deserializer = U32Deserializer<E>;
+
+    fn into_deserializer(self) -> U32Deserializer<E> {
+        U32Deserializer {
+            value: self,
+            marker: PhantomData,
+        }
+    }
+}
+
+impl<'de, E> de::Deserializer<'de> for U32Deserializer<E>
+where
+    E: de::Error,
+{
+    type Error = E;
+
+    forward_to_deserialize_any! {
+        bool i8 i16 i32 i64 i128 u8 u16 u32 u64 u128 f32 f64 char str string
+        bytes byte_buf option unit unit_struct newtype_struct seq tuple
+        tuple_struct map struct identifier ignored_any
+    }
+
+    fn deserialize_any<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+    where
+        V: de::Visitor<'de>,
+    {
+        visitor.visit_u32(self.value)
+    }
+
+    fn deserialize_enum<V>(
+        self,
+        name: &str,
+        variants: &'static [&'static str],
+        visitor: V,
+    ) -> Result<V::Value, Self::Error>
+    where
+        V: de::Visitor<'de>,
+    {
+        let _ = name;
+        let _ = variants;
+        visitor.visit_enum(self)
+    }
+}
+
+impl<'de, E> de::EnumAccess<'de> for U32Deserializer<E>
+where
+    E: de::Error,
+{
+    type Error = E;
+    type Variant = private::UnitOnly<E>;
+
+    fn variant_seed<T>(self, seed: T) -> Result<(T::Value, Self::Variant), Self::Error>
+    where
+        T: de::DeserializeSeed<'de>,
+    {
+        seed.deserialize(self).map(private::unit_only)
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+/// A deserializer holding a `&str`.
+#[derive(Debug)]
+pub struct StrDeserializer<'a, E> {
+    value: &'a str,
+    marker: PhantomData<E>,
+}
+
+impl_copy_clone!(StrDeserializer<'de>);
+
+impl<'de, 'a, E> IntoDeserializer<'de, E> for &'a str
+where
+    E: de::Error,
+{
+    type Deserializer = StrDeserializer<'a, E>;
+
+    fn into_deserializer(self) -> StrDeserializer<'a, E> {
+        StrDeserializer {
+            value: self,
+            marker: PhantomData,
+        }
+    }
+}
+
+impl<'de, 'a, E> de::Deserializer<'de> for StrDeserializer<'a, E>
+where
+    E: de::Error,
+{
+    type Error = E;
+
+    fn deserialize_any<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+    where
+        V: de::Visitor<'de>,
+    {
+        visitor.visit_str(self.value)
+    }
+
+    fn deserialize_enum<V>(
+        self,
+        name: &str,
+        variants: &'static [&'static str],
+        visitor: V,
+    ) -> Result<V::Value, Self::Error>
+    where
+        V: de::Visitor<'de>,
+    {
+        let _ = name;
+        let _ = variants;
+        visitor.visit_enum(self)
+    }
+
+    forward_to_deserialize_any! {
+        bool i8 i16 i32 i64 i128 u8 u16 u32 u64 u128 f32 f64 char str string
+        bytes byte_buf option unit unit_struct newtype_struct seq tuple
+        tuple_struct map struct identifier ignored_any
+    }
+}
+
+impl<'de, 'a, E> de::EnumAccess<'de> for StrDeserializer<'a, E>
+where
+    E: de::Error,
+{
+    type Error = E;
+    type Variant = private::UnitOnly<E>;
+
+    fn variant_seed<T>(self, seed: T) -> Result<(T::Value, Self::Variant), Self::Error>
+    where
+        T: de::DeserializeSeed<'de>,
+    {
+        seed.deserialize(self).map(private::unit_only)
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+/// A deserializer holding a `&str` with a lifetime tied to another
+/// deserializer.
+#[derive(Debug)]
+pub struct BorrowedStrDeserializer<'de, E> {
+    value: &'de str,
+    marker: PhantomData<E>,
+}
+
+impl_copy_clone!(BorrowedStrDeserializer<'de>);
+
+impl<'de, E> BorrowedStrDeserializer<'de, E> {
+    /// Create a new borrowed deserializer from the given string.
+    pub fn new(value: &'de str) -> BorrowedStrDeserializer<'de, E> {
+        BorrowedStrDeserializer {
+            value: value,
+            marker: PhantomData,
+        }
+    }
+}
+
+impl<'de, E> de::Deserializer<'de> for BorrowedStrDeserializer<'de, E>
+where
+    E: de::Error,
+{
+    type Error = E;
+
+    fn deserialize_any<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+    where
+        V: de::Visitor<'de>,
+    {
+        visitor.visit_borrowed_str(self.value)
+    }
+
+    fn deserialize_enum<V>(
+        self,
+        name: &str,
+        variants: &'static [&'static str],
+        visitor: V,
+    ) -> Result<V::Value, Self::Error>
+    where
+        V: de::Visitor<'de>,
+    {
+        let _ = name;
+        let _ = variants;
+        visitor.visit_enum(self)
+    }
+
+    forward_to_deserialize_any! {
+        bool i8 i16 i32 i64 i128 u8 u16 u32 u64 u128 f32 f64 char str string
+        bytes byte_buf option unit unit_struct newtype_struct seq tuple
+        tuple_struct map struct identifier ignored_any
+    }
+}
+
+impl<'de, E> de::EnumAccess<'de> for BorrowedStrDeserializer<'de, E>
+where
+    E: de::Error,
+{
+    type Error = E;
+    type Variant = private::UnitOnly<E>;
+
+    fn variant_seed<T>(self, seed: T) -> Result<(T::Value, Self::Variant), Self::Error>
+    where
+        T: de::DeserializeSeed<'de>,
+    {
+        seed.deserialize(self).map(private::unit_only)
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+/// A deserializer holding a `String`.
+#[cfg(any(feature = "std", feature = "alloc"))]
+#[derive(Debug)]
+pub struct StringDeserializer<E> {
+    value: String,
+    marker: PhantomData<E>,
+}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+impl<E> Clone for StringDeserializer<E> {
+    fn clone(&self) -> Self {
+        StringDeserializer {
+            value: self.value.clone(),
+            marker: PhantomData,
+        }
+    }
+}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+impl<'de, E> IntoDeserializer<'de, E> for String
+where
+    E: de::Error,
+{
+    type Deserializer = StringDeserializer<E>;
+
+    fn into_deserializer(self) -> StringDeserializer<E> {
+        StringDeserializer {
+            value: self,
+            marker: PhantomData,
+        }
+    }
+}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+impl<'de, E> de::Deserializer<'de> for StringDeserializer<E>
+where
+    E: de::Error,
+{
+    type Error = E;
+
+    fn deserialize_any<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+    where
+        V: de::Visitor<'de>,
+    {
+        visitor.visit_string(self.value)
+    }
+
+    fn deserialize_enum<V>(
+        self,
+        name: &str,
+        variants: &'static [&'static str],
+        visitor: V,
+    ) -> Result<V::Value, Self::Error>
+    where
+        V: de::Visitor<'de>,
+    {
+        let _ = name;
+        let _ = variants;
+        visitor.visit_enum(self)
+    }
+
+    forward_to_deserialize_any! {
+        bool i8 i16 i32 i64 i128 u8 u16 u32 u64 u128 f32 f64 char str string
+        bytes byte_buf option unit unit_struct newtype_struct seq tuple
+        tuple_struct map struct identifier ignored_any
+    }
+}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+impl<'de, 'a, E> de::EnumAccess<'de> for StringDeserializer<E>
+where
+    E: de::Error,
+{
+    type Error = E;
+    type Variant = private::UnitOnly<E>;
+
+    fn variant_seed<T>(self, seed: T) -> Result<(T::Value, Self::Variant), Self::Error>
+    where
+        T: de::DeserializeSeed<'de>,
+    {
+        seed.deserialize(self).map(private::unit_only)
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+/// A deserializer holding a `Cow<str>`.
+#[cfg(any(feature = "std", feature = "alloc"))]
+#[derive(Debug)]
+pub struct CowStrDeserializer<'a, E> {
+    value: Cow<'a, str>,
+    marker: PhantomData<E>,
+}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+impl<'a, E> Clone for CowStrDeserializer<'a, E> {
+    fn clone(&self) -> Self {
+        CowStrDeserializer {
+            value: self.value.clone(),
+            marker: PhantomData,
+        }
+    }
+}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+impl<'de, 'a, E> IntoDeserializer<'de, E> for Cow<'a, str>
+where
+    E: de::Error,
+{
+    type Deserializer = CowStrDeserializer<'a, E>;
+
+    fn into_deserializer(self) -> CowStrDeserializer<'a, E> {
+        CowStrDeserializer {
+            value: self,
+            marker: PhantomData,
+        }
+    }
+}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+impl<'de, 'a, E> de::Deserializer<'de> for CowStrDeserializer<'a, E>
+where
+    E: de::Error,
+{
+    type Error = E;
+
+    fn deserialize_any<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+    where
+        V: de::Visitor<'de>,
+    {
+        match self.value {
+            Cow::Borrowed(string) => visitor.visit_str(string),
+            Cow::Owned(string) => visitor.visit_string(string),
+        }
+    }
+
+    fn deserialize_enum<V>(
+        self,
+        name: &str,
+        variants: &'static [&'static str],
+        visitor: V,
+    ) -> Result<V::Value, Self::Error>
+    where
+        V: de::Visitor<'de>,
+    {
+        let _ = name;
+        let _ = variants;
+        visitor.visit_enum(self)
+    }
+
+    forward_to_deserialize_any! {
+        bool i8 i16 i32 i64 i128 u8 u16 u32 u64 u128 f32 f64 char str string
+        bytes byte_buf option unit unit_struct newtype_struct seq tuple
+        tuple_struct map struct identifier ignored_any
+    }
+}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+impl<'de, 'a, E> de::EnumAccess<'de> for CowStrDeserializer<'a, E>
+where
+    E: de::Error,
+{
+    type Error = E;
+    type Variant = private::UnitOnly<E>;
+
+    fn variant_seed<T>(self, seed: T) -> Result<(T::Value, Self::Variant), Self::Error>
+    where
+        T: de::DeserializeSeed<'de>,
+    {
+        seed.deserialize(self).map(private::unit_only)
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+/// A deserializer holding a `&[u8]` with a lifetime tied to another
+/// deserializer.
+#[derive(Debug)]
+pub struct BorrowedBytesDeserializer<'de, E> {
+    value: &'de [u8],
+    marker: PhantomData<E>,
+}
+
+impl_copy_clone!(BorrowedBytesDeserializer<'de>);
+
+impl<'de, E> BorrowedBytesDeserializer<'de, E> {
+    /// Create a new borrowed deserializer from the given byte slice.
+    pub fn new(value: &'de [u8]) -> BorrowedBytesDeserializer<'de, E> {
+        BorrowedBytesDeserializer {
+            value: value,
+            marker: PhantomData,
+        }
+    }
+}
+
+impl<'de, E> de::Deserializer<'de> for BorrowedBytesDeserializer<'de, E>
+where
+    E: de::Error,
+{
+    type Error = E;
+
+    fn deserialize_any<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+    where
+        V: de::Visitor<'de>,
+    {
+        visitor.visit_borrowed_bytes(self.value)
+    }
+
+    forward_to_deserialize_any! {
+        bool i8 i16 i32 i64 i128 u8 u16 u32 u64 u128 f32 f64 char str string
+        bytes byte_buf option unit unit_struct newtype_struct seq tuple
+        tuple_struct map struct identifier ignored_any enum
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+/// A deserializer that iterates over a sequence.
+#[derive(Clone, Debug)]
+pub struct SeqDeserializer<I, E> {
+    iter: iter::Fuse<I>,
+    count: usize,
+    marker: PhantomData<E>,
+}
+
+impl<I, E> SeqDeserializer<I, E>
+where
+    I: Iterator,
+{
+    /// Construct a new `SeqDeserializer<I, E>`.
+    pub fn new(iter: I) -> Self {
+        SeqDeserializer {
+            iter: iter.fuse(),
+            count: 0,
+            marker: PhantomData,
+        }
+    }
+}
+
+impl<I, E> SeqDeserializer<I, E>
+where
+    I: Iterator,
+    E: de::Error,
+{
+    /// Check for remaining elements after passing a `SeqDeserializer` to
+    /// `Visitor::visit_seq`.
+    pub fn end(self) -> Result<(), E> {
+        let remaining = self.iter.count();
+        if remaining == 0 {
+            Ok(())
+        } else {
+            // First argument is the number of elements in the data, second
+            // argument is the number of elements expected by the Deserialize.
+            Err(de::Error::invalid_length(
+                self.count + remaining,
+                &ExpectedInSeq(self.count),
+            ))
+        }
+    }
+}
+
+impl<'de, I, T, E> de::Deserializer<'de> for SeqDeserializer<I, E>
+where
+    I: Iterator<Item = T>,
+    T: IntoDeserializer<'de, E>,
+    E: de::Error,
+{
+    type Error = E;
+
+    fn deserialize_any<V>(mut self, visitor: V) -> Result<V::Value, Self::Error>
+    where
+        V: de::Visitor<'de>,
+    {
+        let v = try!(visitor.visit_seq(&mut self));
+        try!(self.end());
+        Ok(v)
+    }
+
+    forward_to_deserialize_any! {
+        bool i8 i16 i32 i64 i128 u8 u16 u32 u64 u128 f32 f64 char str string
+        bytes byte_buf option unit unit_struct newtype_struct seq tuple
+        tuple_struct map struct enum identifier ignored_any
+    }
+}
+
+impl<'de, I, T, E> de::SeqAccess<'de> for SeqDeserializer<I, E>
+where
+    I: Iterator<Item = T>,
+    T: IntoDeserializer<'de, E>,
+    E: de::Error,
+{
+    type Error = E;
+
+    fn next_element_seed<V>(&mut self, seed: V) -> Result<Option<V::Value>, Self::Error>
+    where
+        V: de::DeserializeSeed<'de>,
+    {
+        match self.iter.next() {
+            Some(value) => {
+                self.count += 1;
+                seed.deserialize(value.into_deserializer()).map(Some)
+            }
+            None => Ok(None),
+        }
+    }
+
+    fn size_hint(&self) -> Option<usize> {
+        size_hint::from_bounds(&self.iter)
+    }
+}
+
+struct ExpectedInSeq(usize);
+
+impl Expected for ExpectedInSeq {
+    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        if self.0 == 1 {
+            write!(formatter, "1 element in sequence")
+        } else {
+            write!(formatter, "{} elements in sequence", self.0)
+        }
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+impl<'de, T, E> IntoDeserializer<'de, E> for Vec<T>
+where
+    T: IntoDeserializer<'de, E>,
+    E: de::Error,
+{
+    type Deserializer = SeqDeserializer<<Self as IntoIterator>::IntoIter, E>;
+
+    fn into_deserializer(self) -> Self::Deserializer {
+        SeqDeserializer::new(self.into_iter())
+    }
+}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+impl<'de, T, E> IntoDeserializer<'de, E> for BTreeSet<T>
+where
+    T: IntoDeserializer<'de, E> + Eq + Ord,
+    E: de::Error,
+{
+    type Deserializer = SeqDeserializer<<Self as IntoIterator>::IntoIter, E>;
+
+    fn into_deserializer(self) -> Self::Deserializer {
+        SeqDeserializer::new(self.into_iter())
+    }
+}
+
+#[cfg(feature = "std")]
+impl<'de, T, S, E> IntoDeserializer<'de, E> for HashSet<T, S>
+where
+    T: IntoDeserializer<'de, E> + Eq + Hash,
+    S: BuildHasher,
+    E: de::Error,
+{
+    type Deserializer = SeqDeserializer<<Self as IntoIterator>::IntoIter, E>;
+
+    fn into_deserializer(self) -> Self::Deserializer {
+        SeqDeserializer::new(self.into_iter())
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+/// A deserializer holding a `SeqAccess`.
+#[derive(Clone, Debug)]
+pub struct SeqAccessDeserializer<A> {
+    seq: A,
+}
+
+impl<A> SeqAccessDeserializer<A> {
+    /// Construct a new `SeqAccessDeserializer<A>`.
+    pub fn new(seq: A) -> Self {
+        SeqAccessDeserializer { seq: seq }
+    }
+}
+
+impl<'de, A> de::Deserializer<'de> for SeqAccessDeserializer<A>
+where
+    A: de::SeqAccess<'de>,
+{
+    type Error = A::Error;
+
+    fn deserialize_any<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+    where
+        V: de::Visitor<'de>,
+    {
+        visitor.visit_seq(self.seq)
+    }
+
+    forward_to_deserialize_any! {
+        bool i8 i16 i32 i64 i128 u8 u16 u32 u64 u128 f32 f64 char str string
+        bytes byte_buf option unit unit_struct newtype_struct seq tuple
+        tuple_struct map struct enum identifier ignored_any
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+/// A deserializer that iterates over a map.
+pub struct MapDeserializer<'de, I, E>
+where
+    I: Iterator,
+    I::Item: private::Pair,
+{
+    iter: iter::Fuse<I>,
+    value: Option<Second<I::Item>>,
+    count: usize,
+    lifetime: PhantomData<&'de ()>,
+    error: PhantomData<E>,
+}
+
+impl<'de, I, E> MapDeserializer<'de, I, E>
+where
+    I: Iterator,
+    I::Item: private::Pair,
+{
+    /// Construct a new `MapDeserializer<I, E>`.
+    pub fn new(iter: I) -> Self {
+        MapDeserializer {
+            iter: iter.fuse(),
+            value: None,
+            count: 0,
+            lifetime: PhantomData,
+            error: PhantomData,
+        }
+    }
+}
+
+impl<'de, I, E> MapDeserializer<'de, I, E>
+where
+    I: Iterator,
+    I::Item: private::Pair,
+    E: de::Error,
+{
+    /// Check for remaining elements after passing a `MapDeserializer` to
+    /// `Visitor::visit_map`.
+    pub fn end(self) -> Result<(), E> {
+        let remaining = self.iter.count();
+        if remaining == 0 {
+            Ok(())
+        } else {
+            // First argument is the number of elements in the data, second
+            // argument is the number of elements expected by the Deserialize.
+            Err(de::Error::invalid_length(
+                self.count + remaining,
+                &ExpectedInMap(self.count),
+            ))
+        }
+    }
+}
+
+impl<'de, I, E> MapDeserializer<'de, I, E>
+where
+    I: Iterator,
+    I::Item: private::Pair,
+{
+    fn next_pair(&mut self) -> Option<(First<I::Item>, Second<I::Item>)> {
+        match self.iter.next() {
+            Some(kv) => {
+                self.count += 1;
+                Some(private::Pair::split(kv))
+            }
+            None => None,
+        }
+    }
+}
+
+impl<'de, I, E> de::Deserializer<'de> for MapDeserializer<'de, I, E>
+where
+    I: Iterator,
+    I::Item: private::Pair,
+    First<I::Item>: IntoDeserializer<'de, E>,
+    Second<I::Item>: IntoDeserializer<'de, E>,
+    E: de::Error,
+{
+    type Error = E;
+
+    fn deserialize_any<V>(mut self, visitor: V) -> Result<V::Value, Self::Error>
+    where
+        V: de::Visitor<'de>,
+    {
+        let value = try!(visitor.visit_map(&mut self));
+        try!(self.end());
+        Ok(value)
+    }
+
+    fn deserialize_seq<V>(mut self, visitor: V) -> Result<V::Value, Self::Error>
+    where
+        V: de::Visitor<'de>,
+    {
+        let value = try!(visitor.visit_seq(&mut self));
+        try!(self.end());
+        Ok(value)
+    }
+
+    fn deserialize_tuple<V>(self, len: usize, visitor: V) -> Result<V::Value, Self::Error>
+    where
+        V: de::Visitor<'de>,
+    {
+        let _ = len;
+        self.deserialize_seq(visitor)
+    }
+
+    forward_to_deserialize_any! {
+        bool i8 i16 i32 i64 i128 u8 u16 u32 u64 u128 f32 f64 char str string
+        bytes byte_buf option unit unit_struct newtype_struct tuple_struct map
+        struct enum identifier ignored_any
+    }
+}
+
+impl<'de, I, E> de::MapAccess<'de> for MapDeserializer<'de, I, E>
+where
+    I: Iterator,
+    I::Item: private::Pair,
+    First<I::Item>: IntoDeserializer<'de, E>,
+    Second<I::Item>: IntoDeserializer<'de, E>,
+    E: de::Error,
+{
+    type Error = E;
+
+    fn next_key_seed<T>(&mut self, seed: T) -> Result<Option<T::Value>, Self::Error>
+    where
+        T: de::DeserializeSeed<'de>,
+    {
+        match self.next_pair() {
+            Some((key, value)) => {
+                self.value = Some(value);
+                seed.deserialize(key.into_deserializer()).map(Some)
+            }
+            None => Ok(None),
+        }
+    }
+
+    fn next_value_seed<T>(&mut self, seed: T) -> Result<T::Value, Self::Error>
+    where
+        T: de::DeserializeSeed<'de>,
+    {
+        let value = self.value.take();
+        // Panic because this indicates a bug in the program rather than an
+        // expected failure.
+        let value = value.expect("MapAccess::visit_value called before visit_key");
+        seed.deserialize(value.into_deserializer())
+    }
+
+    fn next_entry_seed<TK, TV>(
+        &mut self,
+        kseed: TK,
+        vseed: TV,
+    ) -> Result<Option<(TK::Value, TV::Value)>, Self::Error>
+    where
+        TK: de::DeserializeSeed<'de>,
+        TV: de::DeserializeSeed<'de>,
+    {
+        match self.next_pair() {
+            Some((key, value)) => {
+                let key = try!(kseed.deserialize(key.into_deserializer()));
+                let value = try!(vseed.deserialize(value.into_deserializer()));
+                Ok(Some((key, value)))
+            }
+            None => Ok(None),
+        }
+    }
+
+    fn size_hint(&self) -> Option<usize> {
+        size_hint::from_bounds(&self.iter)
+    }
+}
+
+impl<'de, I, E> de::SeqAccess<'de> for MapDeserializer<'de, I, E>
+where
+    I: Iterator,
+    I::Item: private::Pair,
+    First<I::Item>: IntoDeserializer<'de, E>,
+    Second<I::Item>: IntoDeserializer<'de, E>,
+    E: de::Error,
+{
+    type Error = E;
+
+    fn next_element_seed<T>(&mut self, seed: T) -> Result<Option<T::Value>, Self::Error>
+    where
+        T: de::DeserializeSeed<'de>,
+    {
+        match self.next_pair() {
+            Some((k, v)) => {
+                let de = PairDeserializer(k, v, PhantomData);
+                seed.deserialize(de).map(Some)
+            }
+            None => Ok(None),
+        }
+    }
+
+    fn size_hint(&self) -> Option<usize> {
+        size_hint::from_bounds(&self.iter)
+    }
+}
+
+// Cannot #[derive(Clone)] because of the bound `Second<I::Item>: Clone`.
+impl<'de, I, E> Clone for MapDeserializer<'de, I, E>
+where
+    I: Iterator + Clone,
+    I::Item: private::Pair,
+    Second<I::Item>: Clone,
+{
+    fn clone(&self) -> Self {
+        MapDeserializer {
+            iter: self.iter.clone(),
+            value: self.value.clone(),
+            count: self.count,
+            lifetime: self.lifetime,
+            error: self.error,
+        }
+    }
+}
+
+// Cannot #[derive(Debug)] because of the bound `Second<I::Item>: Debug`.
+impl<'de, I, E> Debug for MapDeserializer<'de, I, E>
+where
+    I: Iterator + Debug,
+    I::Item: private::Pair,
+    Second<I::Item>: Debug,
+{
+    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        formatter
+            .debug_struct("MapDeserializer")
+            .field("iter", &self.iter)
+            .field("value", &self.value)
+            .field("count", &self.count)
+            .field("lifetime", &self.lifetime)
+            .field("error", &self.error)
+            .finish()
+    }
+}
+
+// Used in the `impl SeqAccess for MapDeserializer` to visit the map as a
+// sequence of pairs.
+struct PairDeserializer<A, B, E>(A, B, PhantomData<E>);
+
+impl<'de, A, B, E> de::Deserializer<'de> for PairDeserializer<A, B, E>
+where
+    A: IntoDeserializer<'de, E>,
+    B: IntoDeserializer<'de, E>,
+    E: de::Error,
+{
+    type Error = E;
+
+    forward_to_deserialize_any! {
+        bool i8 i16 i32 i64 i128 u8 u16 u32 u64 u128 f32 f64 char str string
+        bytes byte_buf option unit unit_struct newtype_struct tuple_struct map
+        struct enum identifier ignored_any
+    }
+
+    fn deserialize_any<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+    where
+        V: de::Visitor<'de>,
+    {
+        self.deserialize_seq(visitor)
+    }
+
+    fn deserialize_seq<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+    where
+        V: de::Visitor<'de>,
+    {
+        let mut pair_visitor = PairVisitor(Some(self.0), Some(self.1), PhantomData);
+        let pair = try!(visitor.visit_seq(&mut pair_visitor));
+        if pair_visitor.1.is_none() {
+            Ok(pair)
+        } else {
+            let remaining = pair_visitor.size_hint().unwrap();
+            // First argument is the number of elements in the data, second
+            // argument is the number of elements expected by the Deserialize.
+            Err(de::Error::invalid_length(2, &ExpectedInSeq(2 - remaining)))
+        }
+    }
+
+    fn deserialize_tuple<V>(self, len: usize, visitor: V) -> Result<V::Value, Self::Error>
+    where
+        V: de::Visitor<'de>,
+    {
+        if len == 2 {
+            self.deserialize_seq(visitor)
+        } else {
+            // First argument is the number of elements in the data, second
+            // argument is the number of elements expected by the Deserialize.
+            Err(de::Error::invalid_length(2, &ExpectedInSeq(len)))
+        }
+    }
+}
+
+struct PairVisitor<A, B, E>(Option<A>, Option<B>, PhantomData<E>);
+
+impl<'de, A, B, E> de::SeqAccess<'de> for PairVisitor<A, B, E>
+where
+    A: IntoDeserializer<'de, E>,
+    B: IntoDeserializer<'de, E>,
+    E: de::Error,
+{
+    type Error = E;
+
+    fn next_element_seed<T>(&mut self, seed: T) -> Result<Option<T::Value>, Self::Error>
+    where
+        T: de::DeserializeSeed<'de>,
+    {
+        if let Some(k) = self.0.take() {
+            seed.deserialize(k.into_deserializer()).map(Some)
+        } else if let Some(v) = self.1.take() {
+            seed.deserialize(v.into_deserializer()).map(Some)
+        } else {
+            Ok(None)
+        }
+    }
+
+    fn size_hint(&self) -> Option<usize> {
+        if self.0.is_some() {
+            Some(2)
+        } else if self.1.is_some() {
+            Some(1)
+        } else {
+            Some(0)
+        }
+    }
+}
+
+struct ExpectedInMap(usize);
+
+impl Expected for ExpectedInMap {
+    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        if self.0 == 1 {
+            write!(formatter, "1 element in map")
+        } else {
+            write!(formatter, "{} elements in map", self.0)
+        }
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+impl<'de, K, V, E> IntoDeserializer<'de, E> for BTreeMap<K, V>
+where
+    K: IntoDeserializer<'de, E> + Eq + Ord,
+    V: IntoDeserializer<'de, E>,
+    E: de::Error,
+{
+    type Deserializer = MapDeserializer<'de, <Self as IntoIterator>::IntoIter, E>;
+
+    fn into_deserializer(self) -> Self::Deserializer {
+        MapDeserializer::new(self.into_iter())
+    }
+}
+
+#[cfg(feature = "std")]
+impl<'de, K, V, S, E> IntoDeserializer<'de, E> for HashMap<K, V, S>
+where
+    K: IntoDeserializer<'de, E> + Eq + Hash,
+    V: IntoDeserializer<'de, E>,
+    S: BuildHasher,
+    E: de::Error,
+{
+    type Deserializer = MapDeserializer<'de, <Self as IntoIterator>::IntoIter, E>;
+
+    fn into_deserializer(self) -> Self::Deserializer {
+        MapDeserializer::new(self.into_iter())
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+/// A deserializer holding a `MapAccess`.
+#[derive(Clone, Debug)]
+pub struct MapAccessDeserializer<A> {
+    map: A,
+}
+
+impl<A> MapAccessDeserializer<A> {
+    /// Construct a new `MapAccessDeserializer<A>`.
+    pub fn new(map: A) -> Self {
+        MapAccessDeserializer { map: map }
+    }
+}
+
+impl<'de, A> de::Deserializer<'de> for MapAccessDeserializer<A>
+where
+    A: de::MapAccess<'de>,
+{
+    type Error = A::Error;
+
+    fn deserialize_any<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+    where
+        V: de::Visitor<'de>,
+    {
+        visitor.visit_map(self.map)
+    }
+
+    fn deserialize_enum<V>(
+        self,
+        _name: &str,
+        _variants: &'static [&'static str],
+        visitor: V,
+    ) -> Result<V::Value, Self::Error>
+    where
+        V: de::Visitor<'de>,
+    {
+        visitor.visit_enum(self)
+    }
+
+    forward_to_deserialize_any! {
+        bool i8 i16 i32 i64 i128 u8 u16 u32 u64 u128 f32 f64 char str string
+        bytes byte_buf option unit unit_struct newtype_struct seq tuple
+        tuple_struct map struct identifier ignored_any
+    }
+}
+
+impl<'de, A> de::EnumAccess<'de> for MapAccessDeserializer<A>
+where
+    A: de::MapAccess<'de>,
+{
+    type Error = A::Error;
+    type Variant = private::MapAsEnum<A>;
+
+    fn variant_seed<T>(mut self, seed: T) -> Result<(T::Value, Self::Variant), Self::Error>
+    where
+        T: de::DeserializeSeed<'de>,
+    {
+        match self.map.next_key_seed(seed)? {
+            Some(key) => Ok((key, private::map_as_enum(self.map))),
+            None => Err(de::Error::invalid_type(de::Unexpected::Map, &"enum")),
+        }
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+mod private {
+    use lib::*;
+
+    use de::{self, DeserializeSeed, Deserializer, MapAccess, Unexpected, VariantAccess, Visitor};
+
+    #[derive(Clone, Debug)]
+    pub struct UnitOnly<E> {
+        marker: PhantomData<E>,
+    }
+
+    pub fn unit_only<T, E>(t: T) -> (T, UnitOnly<E>) {
+        (
+            t,
+            UnitOnly {
+                marker: PhantomData,
+            },
+        )
+    }
+
+    impl<'de, E> de::VariantAccess<'de> for UnitOnly<E>
+    where
+        E: de::Error,
+    {
+        type Error = E;
+
+        fn unit_variant(self) -> Result<(), Self::Error> {
+            Ok(())
+        }
+
+        fn newtype_variant_seed<T>(self, _seed: T) -> Result<T::Value, Self::Error>
+        where
+            T: de::DeserializeSeed<'de>,
+        {
+            Err(de::Error::invalid_type(
+                Unexpected::UnitVariant,
+                &"newtype variant",
+            ))
+        }
+
+        fn tuple_variant<V>(self, _len: usize, _visitor: V) -> Result<V::Value, Self::Error>
+        where
+            V: de::Visitor<'de>,
+        {
+            Err(de::Error::invalid_type(
+                Unexpected::UnitVariant,
+                &"tuple variant",
+            ))
+        }
+
+        fn struct_variant<V>(
+            self,
+            _fields: &'static [&'static str],
+            _visitor: V,
+        ) -> Result<V::Value, Self::Error>
+        where
+            V: de::Visitor<'de>,
+        {
+            Err(de::Error::invalid_type(
+                Unexpected::UnitVariant,
+                &"struct variant",
+            ))
+        }
+    }
+
+    #[derive(Clone, Debug)]
+    pub struct MapAsEnum<A> {
+        map: A,
+    }
+
+    pub fn map_as_enum<A>(map: A) -> MapAsEnum<A> {
+        MapAsEnum { map: map }
+    }
+
+    impl<'de, A> VariantAccess<'de> for MapAsEnum<A>
+    where
+        A: MapAccess<'de>,
+    {
+        type Error = A::Error;
+
+        fn unit_variant(mut self) -> Result<(), Self::Error> {
+            self.map.next_value()
+        }
+
+        fn newtype_variant_seed<T>(mut self, seed: T) -> Result<T::Value, Self::Error>
+        where
+            T: DeserializeSeed<'de>,
+        {
+            self.map.next_value_seed(seed)
+        }
+
+        fn tuple_variant<V>(mut self, len: usize, visitor: V) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            self.map.next_value_seed(SeedTupleVariant {
+                len: len,
+                visitor: visitor,
+            })
+        }
+
+        fn struct_variant<V>(
+            mut self,
+            _fields: &'static [&'static str],
+            visitor: V,
+        ) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            self.map
+                .next_value_seed(SeedStructVariant { visitor: visitor })
+        }
+    }
+
+    struct SeedTupleVariant<V> {
+        len: usize,
+        visitor: V,
+    }
+
+    impl<'de, V> DeserializeSeed<'de> for SeedTupleVariant<V>
+    where
+        V: Visitor<'de>,
+    {
+        type Value = V::Value;
+
+        fn deserialize<D>(self, deserializer: D) -> Result<Self::Value, D::Error>
+        where
+            D: Deserializer<'de>,
+        {
+            deserializer.deserialize_tuple(self.len, self.visitor)
+        }
+    }
+
+    struct SeedStructVariant<V> {
+        visitor: V,
+    }
+
+    impl<'de, V> DeserializeSeed<'de> for SeedStructVariant<V>
+    where
+        V: Visitor<'de>,
+    {
+        type Value = V::Value;
+
+        fn deserialize<D>(self, deserializer: D) -> Result<Self::Value, D::Error>
+        where
+            D: Deserializer<'de>,
+        {
+            deserializer.deserialize_map(self.visitor)
+        }
+    }
+
+    /// Avoid having to restate the generic types on `MapDeserializer`. The
+    /// `Iterator::Item` contains enough information to figure out K and V.
+    pub trait Pair {
+        type First;
+        type Second;
+        fn split(self) -> (Self::First, Self::Second);
+    }
+
+    impl<A, B> Pair for (A, B) {
+        type First = A;
+        type Second = B;
+        fn split(self) -> (A, B) {
+            self
+        }
+    }
+
+    pub type First<T> = <T as Pair>::First;
+    pub type Second<T> = <T as Pair>::Second;
+}
diff --git a/src/export.rs b/src/export.rs
new file mode 100644
index 0000000..caa1f4d
--- /dev/null
+++ b/src/export.rs
@@ -0,0 +1,39 @@
+pub use lib::clone::Clone;
+pub use lib::convert::{From, Into};
+pub use lib::default::Default;
+pub use lib::fmt::{self, Formatter};
+pub use lib::marker::PhantomData;
+pub use lib::option::Option::{self, None, Some};
+pub use lib::result::Result::{self, Err, Ok};
+
+pub use self::string::from_utf8_lossy;
+
+#[cfg(any(feature = "alloc", feature = "std"))]
+pub use lib::{ToString, Vec};
+
+#[cfg(core_try_from)]
+pub use lib::convert::TryFrom;
+
+mod string {
+    use lib::*;
+
+    #[cfg(any(feature = "std", feature = "alloc"))]
+    pub fn from_utf8_lossy(bytes: &[u8]) -> Cow<str> {
+        String::from_utf8_lossy(bytes)
+    }
+
+    // The generated code calls this like:
+    //
+    //     let value = &_serde::export::from_utf8_lossy(bytes);
+    //     Err(_serde::de::Error::unknown_variant(value, VARIANTS))
+    //
+    // so it is okay for the return type to be different from the std case as long
+    // as the above works.
+    #[cfg(not(any(feature = "std", feature = "alloc")))]
+    pub fn from_utf8_lossy(bytes: &[u8]) -> &str {
+        // Three unicode replacement characters if it fails. They look like a
+        // white-on-black question mark. The user will recognize it as invalid
+        // UTF-8.
+        str::from_utf8(bytes).unwrap_or("\u{fffd}\u{fffd}\u{fffd}")
+    }
+}
diff --git a/src/integer128.rs b/src/integer128.rs
new file mode 100644
index 0000000..01d6f34
--- /dev/null
+++ b/src/integer128.rs
@@ -0,0 +1,82 @@
+/// Conditional compilation depending on whether Serde is built with support for
+/// 128-bit integers.
+///
+/// Data formats that wish to support Rust compiler versions older than 1.26
+/// (or targets that lack 128-bit integers) may place the i128 / u128 methods
+/// of their Serializer and Deserializer behind this macro.
+///
+/// Data formats that require a minimum Rust compiler version of at least 1.26,
+/// or do not target platforms that lack 128-bit integers, do not need to
+/// bother with this macro and may assume support for 128-bit integers.
+///
+/// ```edition2018
+/// # use serde::private::ser::Error;
+/// #
+/// # struct MySerializer;
+/// #
+/// use serde::{serde_if_integer128, Serializer};
+///
+/// impl Serializer for MySerializer {
+///     type Ok = ();
+///     type Error = Error;
+///
+///     fn serialize_i64(self, v: i64) -> Result<Self::Ok, Self::Error> {
+///         /* ... */
+/// #         unimplemented!()
+///     }
+///
+///     /* ... */
+///
+///     serde_if_integer128! {
+///         fn serialize_i128(self, v: i128) -> Result<Self::Ok, Self::Error> {
+///             /* ... */
+/// #             unimplemented!()
+///         }
+///
+///         fn serialize_u128(self, v: u128) -> Result<Self::Ok, Self::Error> {
+///             /* ... */
+/// #             unimplemented!()
+///         }
+///     }
+/// #
+/// #     serde::__serialize_unimplemented! {
+/// #         bool i8 i16 i32 u8 u16 u32 u64 f32 f64 char str bytes none some
+/// #         unit unit_struct unit_variant newtype_struct newtype_variant seq
+/// #         tuple tuple_struct tuple_variant map struct struct_variant
+/// #     }
+/// }
+/// ```
+///
+/// When Serde is built with support for 128-bit integers, this macro expands
+/// transparently into just the input tokens.
+///
+/// ```edition2018
+/// macro_rules! serde_if_integer128 {
+///     ($($tt:tt)*) => {
+///         $($tt)*
+///     };
+/// }
+/// ```
+///
+/// When built without support for 128-bit integers, this macro expands to
+/// nothing.
+///
+/// ```edition2018
+/// macro_rules! serde_if_integer128 {
+///     ($($tt:tt)*) => {};
+/// }
+/// ```
+#[cfg(integer128)]
+#[macro_export]
+macro_rules! serde_if_integer128 {
+    ($($tt:tt)*) => {
+        $($tt)*
+    };
+}
+
+#[cfg(not(integer128))]
+#[macro_export]
+#[doc(hidden)]
+macro_rules! serde_if_integer128 {
+    ($($tt:tt)*) => {};
+}
diff --git a/src/lib.rs b/src/lib.rs
new file mode 100644
index 0000000..7c4733c
--- /dev/null
+++ b/src/lib.rs
@@ -0,0 +1,288 @@
+//! # Serde
+//!
+//! Serde is a framework for ***ser***ializing and ***de***serializing Rust data
+//! structures efficiently and generically.
+//!
+//! The Serde ecosystem consists of data structures that know how to serialize
+//! and deserialize themselves along with data formats that know how to
+//! serialize and deserialize other things. Serde provides the layer by which
+//! these two groups interact with each other, allowing any supported data
+//! structure to be serialized and deserialized using any supported data format.
+//!
+//! See the Serde website [https://serde.rs/] for additional documentation and
+//! usage examples.
+//!
+//! [https://serde.rs/]: https://serde.rs/
+//!
+//! ## Design
+//!
+//! Where many other languages rely on runtime reflection for serializing data,
+//! Serde is instead built on Rust's powerful trait system. A data structure
+//! that knows how to serialize and deserialize itself is one that implements
+//! Serde's `Serialize` and `Deserialize` traits (or uses Serde's derive
+//! attribute to automatically generate implementations at compile time). This
+//! avoids any overhead of reflection or runtime type information. In fact in
+//! many situations the interaction between data structure and data format can
+//! be completely optimized away by the Rust compiler, leaving Serde
+//! serialization to perform the same speed as a handwritten serializer for the
+//! specific selection of data structure and data format.
+//!
+//! ## Data formats
+//!
+//! The following is a partial list of data formats that have been implemented
+//! for Serde by the community.
+//!
+//! - [JSON], the ubiquitous JavaScript Object Notation used by many HTTP APIs.
+//! - [Bincode], a compact binary format
+//!   used for IPC within the Servo rendering engine.
+//! - [CBOR], a Concise Binary Object Representation designed for small message
+//!   size without the need for version negotiation.
+//! - [YAML], a self-proclaimed human-friendly configuration language that ain't
+//!   markup language.
+//! - [MessagePack], an efficient binary format that resembles a compact JSON.
+//! - [TOML], a minimal configuration format used by [Cargo].
+//! - [Pickle], a format common in the Python world.
+//! - [RON], a Rusty Object Notation.
+//! - [BSON], the data storage and network transfer format used by MongoDB.
+//! - [Avro], a binary format used within Apache Hadoop, with support for schema
+//!   definition.
+//! - [JSON5], A superset of JSON including some productions from ES5.
+//! - [Postcard], a no\_std and embedded-systems friendly compact binary format.
+//! - [URL], the x-www-form-urlencoded format.
+//! - [Envy], a way to deserialize environment variables into Rust structs.
+//!   *(deserialization only)*
+//! - [Envy Store], a way to deserialize [AWS Parameter Store] parameters into
+//!   Rust structs. *(deserialization only)*
+//! - [S-expressions], the textual representation of code and data used by the
+//!   Lisp language family.
+//! - [D-Bus]'s binary wire format.
+//! - [FlexBuffers], the schemaless cousin of Google's FlatBuffers zero-copy serialization format.
+//!
+//! [JSON]: https://github.com/serde-rs/json
+//! [Bincode]: https://github.com/TyOverby/bincode
+//! [CBOR]: https://github.com/pyfisch/cbor
+//! [YAML]: https://github.com/dtolnay/serde-yaml
+//! [MessagePack]: https://github.com/3Hren/msgpack-rust
+//! [TOML]: https://github.com/alexcrichton/toml-rs
+//! [Pickle]: https://github.com/birkenfeld/serde-pickle
+//! [RON]: https://github.com/ron-rs/ron
+//! [BSON]: https://github.com/zonyitoo/bson-rs
+//! [Avro]: https://github.com/flavray/avro-rs
+//! [JSON5]: https://github.com/callum-oakley/json5-rs
+//! [Postcard]: https://github.com/jamesmunns/postcard
+//! [URL]: https://github.com/nox/serde_urlencoded
+//! [Envy]: https://github.com/softprops/envy
+//! [Envy Store]: https://github.com/softprops/envy-store
+//! [Cargo]: http://doc.crates.io/manifest.html
+//! [AWS Parameter Store]: https://docs.aws.amazon.com/systems-manager/latest/userguide/systems-manager-paramstore.html
+//! [S-expressions]: https://github.com/rotty/lexpr-rs
+//! [D-Bus]: https://docs.rs/zvariant
+//! [FlexBuffers]: https://github.com/google/flatbuffers/tree/master/rust/flexbuffers
+
+////////////////////////////////////////////////////////////////////////////////
+
+// Serde types in rustdoc of other crates get linked to here.
+#![doc(html_root_url = "https://docs.rs/serde/1.0.115")]
+// Support using Serde without the standard library!
+#![cfg_attr(not(feature = "std"), no_std)]
+// Unstable functionality only if the user asks for it. For tracking and
+// discussion of these features please refer to this issue:
+//
+//    https://github.com/serde-rs/serde/issues/812
+#![cfg_attr(feature = "unstable", feature(never_type))]
+#![allow(unknown_lints, bare_trait_objects, deprecated)]
+#![cfg_attr(feature = "cargo-clippy", allow(renamed_and_removed_lints))]
+#![cfg_attr(feature = "cargo-clippy", deny(clippy, clippy_pedantic))]
+// Ignored clippy and clippy_pedantic lints
+#![cfg_attr(
+    feature = "cargo-clippy",
+    allow(
+        // clippy bug: https://github.com/rust-lang/rust-clippy/issues/5704
+        unnested_or_patterns,
+        // not available in our oldest supported compiler
+        checked_conversions,
+        empty_enum,
+        redundant_field_names,
+        redundant_static_lifetimes,
+        // integer and float ser/de requires these sorts of casts
+        cast_possible_truncation,
+        cast_possible_wrap,
+        cast_sign_loss,
+        // things are often more readable this way
+        cast_lossless,
+        module_name_repetitions,
+        option_if_let_else,
+        single_match_else,
+        type_complexity,
+        use_self,
+        zero_prefixed_literal,
+        // correctly used
+        enum_glob_use,
+        wildcard_imports,
+        // not practical
+        needless_pass_by_value,
+        similar_names,
+        too_many_lines,
+        // preference
+        doc_markdown,
+        unseparated_literal_suffix,
+        // false positive
+        needless_doctest_main,
+        // noisy
+        missing_errors_doc,
+        must_use_candidate,
+    )
+)]
+// Rustc lints.
+#![forbid(unsafe_code)]
+#![deny(missing_docs, unused_imports)]
+
+////////////////////////////////////////////////////////////////////////////////
+
+#[cfg(feature = "alloc")]
+extern crate alloc;
+
+/// A facade around all the types we need from the `std`, `core`, and `alloc`
+/// crates. This avoids elaborate import wrangling having to happen in every
+/// module.
+mod lib {
+    mod core {
+        #[cfg(not(feature = "std"))]
+        pub use core::*;
+        #[cfg(feature = "std")]
+        pub use std::*;
+    }
+
+    pub use self::core::{cmp, iter, mem, num, slice, str};
+    pub use self::core::{f32, f64};
+    pub use self::core::{i16, i32, i64, i8, isize};
+    pub use self::core::{u16, u32, u64, u8, usize};
+
+    pub use self::core::cell::{Cell, RefCell};
+    pub use self::core::clone::{self, Clone};
+    pub use self::core::convert::{self, From, Into};
+    pub use self::core::default::{self, Default};
+    pub use self::core::fmt::{self, Debug, Display};
+    pub use self::core::marker::{self, PhantomData};
+    pub use self::core::ops::Range;
+    pub use self::core::option::{self, Option};
+    pub use self::core::result::{self, Result};
+
+    #[cfg(all(feature = "alloc", not(feature = "std")))]
+    pub use alloc::borrow::{Cow, ToOwned};
+    #[cfg(feature = "std")]
+    pub use std::borrow::{Cow, ToOwned};
+
+    #[cfg(all(feature = "alloc", not(feature = "std")))]
+    pub use alloc::string::{String, ToString};
+    #[cfg(feature = "std")]
+    pub use std::string::{String, ToString};
+
+    #[cfg(all(feature = "alloc", not(feature = "std")))]
+    pub use alloc::vec::Vec;
+    #[cfg(feature = "std")]
+    pub use std::vec::Vec;
+
+    #[cfg(all(feature = "alloc", not(feature = "std")))]
+    pub use alloc::boxed::Box;
+    #[cfg(feature = "std")]
+    pub use std::boxed::Box;
+
+    #[cfg(all(feature = "rc", feature = "alloc", not(feature = "std")))]
+    pub use alloc::rc::{Rc, Weak as RcWeak};
+    #[cfg(all(feature = "rc", feature = "std"))]
+    pub use std::rc::{Rc, Weak as RcWeak};
+
+    #[cfg(all(feature = "rc", feature = "alloc", not(feature = "std")))]
+    pub use alloc::sync::{Arc, Weak as ArcWeak};
+    #[cfg(all(feature = "rc", feature = "std"))]
+    pub use std::sync::{Arc, Weak as ArcWeak};
+
+    #[cfg(all(feature = "alloc", not(feature = "std")))]
+    pub use alloc::collections::{BTreeMap, BTreeSet, BinaryHeap, LinkedList, VecDeque};
+    #[cfg(feature = "std")]
+    pub use std::collections::{BTreeMap, BTreeSet, BinaryHeap, LinkedList, VecDeque};
+
+    #[cfg(feature = "std")]
+    pub use std::{error, net};
+
+    #[cfg(feature = "std")]
+    pub use std::collections::{HashMap, HashSet};
+    #[cfg(feature = "std")]
+    pub use std::ffi::{CStr, CString, OsStr, OsString};
+    #[cfg(feature = "std")]
+    pub use std::hash::{BuildHasher, Hash};
+    #[cfg(feature = "std")]
+    pub use std::io::Write;
+    #[cfg(feature = "std")]
+    pub use std::num::Wrapping;
+    #[cfg(feature = "std")]
+    pub use std::path::{Path, PathBuf};
+    #[cfg(feature = "std")]
+    pub use std::sync::{Mutex, RwLock};
+    #[cfg(feature = "std")]
+    pub use std::time::{SystemTime, UNIX_EPOCH};
+
+    #[cfg(all(feature = "std", collections_bound))]
+    pub use std::collections::Bound;
+
+    #[cfg(core_reverse)]
+    pub use self::core::cmp::Reverse;
+
+    #[cfg(ops_bound)]
+    pub use self::core::ops::Bound;
+
+    #[cfg(range_inclusive)]
+    pub use self::core::ops::RangeInclusive;
+
+    #[cfg(all(feature = "std", std_atomic))]
+    pub use std::sync::atomic::{
+        AtomicBool, AtomicI16, AtomicI32, AtomicI8, AtomicIsize, AtomicU16, AtomicU32, AtomicU8,
+        AtomicUsize, Ordering,
+    };
+    #[cfg(all(feature = "std", std_atomic64))]
+    pub use std::sync::atomic::{AtomicI64, AtomicU64};
+
+    #[cfg(any(core_duration, feature = "std"))]
+    pub use self::core::time::Duration;
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+#[macro_use]
+mod macros;
+
+#[macro_use]
+mod integer128;
+
+pub mod de;
+pub mod ser;
+
+#[doc(inline)]
+pub use de::{Deserialize, Deserializer};
+#[doc(inline)]
+pub use ser::{Serialize, Serializer};
+
+// Generated code uses these to support no_std. Not public API.
+#[doc(hidden)]
+pub mod export;
+
+// Helpers used by generated code and doc tests. Not public API.
+#[doc(hidden)]
+pub mod private;
+
+#[cfg(not(feature = "std"))]
+mod std_error;
+
+// Re-export #[derive(Serialize, Deserialize)].
+//
+// The reason re-exporting is not enabled by default is that disabling it would
+// be annoying for crates that provide handwritten impls or data formats. They
+// would need to disable default features and then explicitly re-enable std.
+#[cfg(feature = "serde_derive")]
+#[allow(unused_imports)]
+#[macro_use]
+extern crate serde_derive;
+#[cfg(feature = "serde_derive")]
+#[doc(hidden)]
+pub use serde_derive::*;
diff --git a/src/macros.rs b/src/macros.rs
new file mode 100644
index 0000000..57600a5
--- /dev/null
+++ b/src/macros.rs
@@ -0,0 +1,236 @@
+// Super explicit first paragraph because this shows up at the top level and
+// trips up people who are just looking for basic Serialize / Deserialize
+// documentation.
+//
+/// Helper macro when implementing the `Deserializer` part of a new data format
+/// for Serde.
+///
+/// Some [`Deserializer`] implementations for self-describing formats do not
+/// care what hint the [`Visitor`] gives them, they just want to blindly call
+/// the [`Visitor`] method corresponding to the data they can tell is in the
+/// input. This requires repetitive implementations of all the [`Deserializer`]
+/// trait methods.
+///
+/// ```edition2018
+/// # use serde::forward_to_deserialize_any;
+/// # use serde::de::{value, Deserializer, Visitor};
+/// #
+/// # struct MyDeserializer;
+/// #
+/// # impl<'de> Deserializer<'de> for MyDeserializer {
+/// #     type Error = value::Error;
+/// #
+/// #     fn deserialize_any<V>(self, _: V) -> Result<V::Value, Self::Error>
+/// #     where
+/// #         V: Visitor<'de>,
+/// #     {
+/// #         unimplemented!()
+/// #     }
+/// #
+/// #[inline]
+/// fn deserialize_bool<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+/// where
+///     V: Visitor<'de>,
+/// {
+///     self.deserialize_any(visitor)
+/// }
+/// #
+/// #     forward_to_deserialize_any! {
+/// #         i8 i16 i32 i64 i128 u8 u16 u32 u64 u128 f32 f64 char str string
+/// #         bytes byte_buf option unit unit_struct newtype_struct seq tuple
+/// #         tuple_struct map struct enum identifier ignored_any
+/// #     }
+/// # }
+/// ```
+///
+/// The `forward_to_deserialize_any!` macro implements these simple forwarding
+/// methods so that they forward directly to [`Deserializer::deserialize_any`].
+/// You can choose which methods to forward.
+///
+/// ```edition2018
+/// # use serde::forward_to_deserialize_any;
+/// # use serde::de::{value, Deserializer, Visitor};
+/// #
+/// # struct MyDeserializer;
+/// #
+/// impl<'de> Deserializer<'de> for MyDeserializer {
+/// #   type Error = value::Error;
+/// #
+///     fn deserialize_any<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+///     where
+///         V: Visitor<'de>,
+///     {
+///         /* ... */
+/// #       let _ = visitor;
+/// #       unimplemented!()
+///     }
+///
+///     forward_to_deserialize_any! {
+///         bool i8 i16 i32 i64 i128 u8 u16 u32 u64 u128 f32 f64 char str string
+///         bytes byte_buf option unit unit_struct newtype_struct seq tuple
+///         tuple_struct map struct enum identifier ignored_any
+///     }
+/// }
+/// ```
+///
+/// The macro assumes the convention that your `Deserializer` lifetime parameter
+/// is called `'de` and that the `Visitor` type parameters on each method are
+/// called `V`. A different type parameter and a different lifetime can be
+/// specified explicitly if necessary.
+///
+/// ```edition2018
+/// # use std::marker::PhantomData;
+/// #
+/// # use serde::forward_to_deserialize_any;
+/// # use serde::de::{value, Deserializer, Visitor};
+/// #
+/// # struct MyDeserializer<V>(PhantomData<V>);
+/// #
+/// # impl<'q, V> Deserializer<'q> for MyDeserializer<V> {
+/// #     type Error = value::Error;
+/// #
+/// #     fn deserialize_any<W>(self, visitor: W) -> Result<W::Value, Self::Error>
+/// #     where
+/// #         W: Visitor<'q>,
+/// #     {
+/// #         unimplemented!()
+/// #     }
+/// #
+/// forward_to_deserialize_any! {
+///     <W: Visitor<'q>>
+///     bool i8 i16 i32 i64 i128 u8 u16 u32 u64 u128 f32 f64 char str string
+///     bytes byte_buf option unit unit_struct newtype_struct seq tuple
+///     tuple_struct map struct enum identifier ignored_any
+/// }
+/// # }
+/// ```
+///
+/// [`Deserializer`]: trait.Deserializer.html
+/// [`Visitor`]: de/trait.Visitor.html
+/// [`Deserializer::deserialize_any`]: trait.Deserializer.html#tymethod.deserialize_any
+#[macro_export(local_inner_macros)]
+macro_rules! forward_to_deserialize_any {
+    (<$visitor:ident: Visitor<$lifetime:tt>> $($func:ident)*) => {
+        $(forward_to_deserialize_any_helper!{$func<$lifetime, $visitor>})*
+    };
+    // This case must be after the previous one.
+    ($($func:ident)*) => {
+        $(forward_to_deserialize_any_helper!{$func<'de, V>})*
+    };
+}
+
+#[doc(hidden)]
+#[macro_export]
+macro_rules! forward_to_deserialize_any_method {
+    ($func:ident<$l:tt, $v:ident>($($arg:ident : $ty:ty),*)) => {
+        #[inline]
+        fn $func<$v>(self, $($arg: $ty,)* visitor: $v) -> $crate::export::Result<$v::Value, Self::Error>
+        where
+            $v: $crate::de::Visitor<$l>,
+        {
+            $(
+                let _ = $arg;
+            )*
+            self.deserialize_any(visitor)
+        }
+    };
+}
+
+#[doc(hidden)]
+#[macro_export(local_inner_macros)]
+macro_rules! forward_to_deserialize_any_helper {
+    (bool<$l:tt, $v:ident>) => {
+        forward_to_deserialize_any_method!{deserialize_bool<$l, $v>()}
+    };
+    (i8<$l:tt, $v:ident>) => {
+        forward_to_deserialize_any_method!{deserialize_i8<$l, $v>()}
+    };
+    (i16<$l:tt, $v:ident>) => {
+        forward_to_deserialize_any_method!{deserialize_i16<$l, $v>()}
+    };
+    (i32<$l:tt, $v:ident>) => {
+        forward_to_deserialize_any_method!{deserialize_i32<$l, $v>()}
+    };
+    (i64<$l:tt, $v:ident>) => {
+        forward_to_deserialize_any_method!{deserialize_i64<$l, $v>()}
+    };
+    (i128<$l:tt, $v:ident>) => {
+        serde_if_integer128! {
+            forward_to_deserialize_any_method!{deserialize_i128<$l, $v>()}
+        }
+    };
+    (u8<$l:tt, $v:ident>) => {
+        forward_to_deserialize_any_method!{deserialize_u8<$l, $v>()}
+    };
+    (u16<$l:tt, $v:ident>) => {
+        forward_to_deserialize_any_method!{deserialize_u16<$l, $v>()}
+    };
+    (u32<$l:tt, $v:ident>) => {
+        forward_to_deserialize_any_method!{deserialize_u32<$l, $v>()}
+    };
+    (u64<$l:tt, $v:ident>) => {
+        forward_to_deserialize_any_method!{deserialize_u64<$l, $v>()}
+    };
+    (u128<$l:tt, $v:ident>) => {
+        serde_if_integer128! {
+            forward_to_deserialize_any_method!{deserialize_u128<$l, $v>()}
+        }
+    };
+    (f32<$l:tt, $v:ident>) => {
+        forward_to_deserialize_any_method!{deserialize_f32<$l, $v>()}
+    };
+    (f64<$l:tt, $v:ident>) => {
+        forward_to_deserialize_any_method!{deserialize_f64<$l, $v>()}
+    };
+    (char<$l:tt, $v:ident>) => {
+        forward_to_deserialize_any_method!{deserialize_char<$l, $v>()}
+    };
+    (str<$l:tt, $v:ident>) => {
+        forward_to_deserialize_any_method!{deserialize_str<$l, $v>()}
+    };
+    (string<$l:tt, $v:ident>) => {
+        forward_to_deserialize_any_method!{deserialize_string<$l, $v>()}
+    };
+    (bytes<$l:tt, $v:ident>) => {
+        forward_to_deserialize_any_method!{deserialize_bytes<$l, $v>()}
+    };
+    (byte_buf<$l:tt, $v:ident>) => {
+        forward_to_deserialize_any_method!{deserialize_byte_buf<$l, $v>()}
+    };
+    (option<$l:tt, $v:ident>) => {
+        forward_to_deserialize_any_method!{deserialize_option<$l, $v>()}
+    };
+    (unit<$l:tt, $v:ident>) => {
+        forward_to_deserialize_any_method!{deserialize_unit<$l, $v>()}
+    };
+    (unit_struct<$l:tt, $v:ident>) => {
+        forward_to_deserialize_any_method!{deserialize_unit_struct<$l, $v>(name: &'static str)}
+    };
+    (newtype_struct<$l:tt, $v:ident>) => {
+        forward_to_deserialize_any_method!{deserialize_newtype_struct<$l, $v>(name: &'static str)}
+    };
+    (seq<$l:tt, $v:ident>) => {
+        forward_to_deserialize_any_method!{deserialize_seq<$l, $v>()}
+    };
+    (tuple<$l:tt, $v:ident>) => {
+        forward_to_deserialize_any_method!{deserialize_tuple<$l, $v>(len: usize)}
+    };
+    (tuple_struct<$l:tt, $v:ident>) => {
+        forward_to_deserialize_any_method!{deserialize_tuple_struct<$l, $v>(name: &'static str, len: usize)}
+    };
+    (map<$l:tt, $v:ident>) => {
+        forward_to_deserialize_any_method!{deserialize_map<$l, $v>()}
+    };
+    (struct<$l:tt, $v:ident>) => {
+        forward_to_deserialize_any_method!{deserialize_struct<$l, $v>(name: &'static str, fields: &'static [&'static str])}
+    };
+    (enum<$l:tt, $v:ident>) => {
+        forward_to_deserialize_any_method!{deserialize_enum<$l, $v>(name: &'static str, variants: &'static [&'static str])}
+    };
+    (identifier<$l:tt, $v:ident>) => {
+        forward_to_deserialize_any_method!{deserialize_identifier<$l, $v>()}
+    };
+    (ignored_any<$l:tt, $v:ident>) => {
+        forward_to_deserialize_any_method!{deserialize_ignored_any<$l, $v>()}
+    };
+}
diff --git a/src/private/de.rs b/src/private/de.rs
new file mode 100644
index 0000000..bcb964a
--- /dev/null
+++ b/src/private/de.rs
@@ -0,0 +1,2954 @@
+use lib::*;
+
+use de::{Deserialize, DeserializeSeed, Deserializer, Error, IntoDeserializer, Visitor};
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+use de::{MapAccess, Unexpected};
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+pub use self::content::{
+    Content, ContentDeserializer, ContentRefDeserializer, EnumDeserializer,
+    InternallyTaggedUnitVisitor, TagContentOtherField, TagContentOtherFieldVisitor,
+    TagOrContentField, TagOrContentFieldVisitor, TaggedContentVisitor, UntaggedUnitVisitor,
+};
+
+/// If the missing field is of type `Option<T>` then treat is as `None`,
+/// otherwise it is an error.
+pub fn missing_field<'de, V, E>(field: &'static str) -> Result<V, E>
+where
+    V: Deserialize<'de>,
+    E: Error,
+{
+    struct MissingFieldDeserializer<E>(&'static str, PhantomData<E>);
+
+    impl<'de, E> Deserializer<'de> for MissingFieldDeserializer<E>
+    where
+        E: Error,
+    {
+        type Error = E;
+
+        fn deserialize_any<V>(self, _visitor: V) -> Result<V::Value, E>
+        where
+            V: Visitor<'de>,
+        {
+            Err(Error::missing_field(self.0))
+        }
+
+        fn deserialize_option<V>(self, visitor: V) -> Result<V::Value, E>
+        where
+            V: Visitor<'de>,
+        {
+            visitor.visit_none()
+        }
+
+        forward_to_deserialize_any! {
+            bool i8 i16 i32 i64 i128 u8 u16 u32 u64 u128 f32 f64 char str string
+            bytes byte_buf unit unit_struct newtype_struct seq tuple
+            tuple_struct map struct enum identifier ignored_any
+        }
+    }
+
+    let deserializer = MissingFieldDeserializer(field, PhantomData);
+    Deserialize::deserialize(deserializer)
+}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+pub fn borrow_cow_str<'de: 'a, 'a, D, R>(deserializer: D) -> Result<R, D::Error>
+where
+    D: Deserializer<'de>,
+    R: From<Cow<'a, str>>,
+{
+    struct CowStrVisitor;
+
+    impl<'a> Visitor<'a> for CowStrVisitor {
+        type Value = Cow<'a, str>;
+
+        fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+            formatter.write_str("a string")
+        }
+
+        fn visit_str<E>(self, v: &str) -> Result<Self::Value, E>
+        where
+            E: Error,
+        {
+            Ok(Cow::Owned(v.to_owned()))
+        }
+
+        fn visit_borrowed_str<E>(self, v: &'a str) -> Result<Self::Value, E>
+        where
+            E: Error,
+        {
+            Ok(Cow::Borrowed(v))
+        }
+
+        fn visit_string<E>(self, v: String) -> Result<Self::Value, E>
+        where
+            E: Error,
+        {
+            Ok(Cow::Owned(v))
+        }
+
+        fn visit_bytes<E>(self, v: &[u8]) -> Result<Self::Value, E>
+        where
+            E: Error,
+        {
+            match str::from_utf8(v) {
+                Ok(s) => Ok(Cow::Owned(s.to_owned())),
+                Err(_) => Err(Error::invalid_value(Unexpected::Bytes(v), &self)),
+            }
+        }
+
+        fn visit_borrowed_bytes<E>(self, v: &'a [u8]) -> Result<Self::Value, E>
+        where
+            E: Error,
+        {
+            match str::from_utf8(v) {
+                Ok(s) => Ok(Cow::Borrowed(s)),
+                Err(_) => Err(Error::invalid_value(Unexpected::Bytes(v), &self)),
+            }
+        }
+
+        fn visit_byte_buf<E>(self, v: Vec<u8>) -> Result<Self::Value, E>
+        where
+            E: Error,
+        {
+            match String::from_utf8(v) {
+                Ok(s) => Ok(Cow::Owned(s)),
+                Err(e) => Err(Error::invalid_value(
+                    Unexpected::Bytes(&e.into_bytes()),
+                    &self,
+                )),
+            }
+        }
+    }
+
+    deserializer.deserialize_str(CowStrVisitor).map(From::from)
+}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+pub fn borrow_cow_bytes<'de: 'a, 'a, D, R>(deserializer: D) -> Result<R, D::Error>
+where
+    D: Deserializer<'de>,
+    R: From<Cow<'a, [u8]>>,
+{
+    struct CowBytesVisitor;
+
+    impl<'a> Visitor<'a> for CowBytesVisitor {
+        type Value = Cow<'a, [u8]>;
+
+        fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+            formatter.write_str("a byte array")
+        }
+
+        fn visit_str<E>(self, v: &str) -> Result<Self::Value, E>
+        where
+            E: Error,
+        {
+            Ok(Cow::Owned(v.as_bytes().to_vec()))
+        }
+
+        fn visit_borrowed_str<E>(self, v: &'a str) -> Result<Self::Value, E>
+        where
+            E: Error,
+        {
+            Ok(Cow::Borrowed(v.as_bytes()))
+        }
+
+        fn visit_string<E>(self, v: String) -> Result<Self::Value, E>
+        where
+            E: Error,
+        {
+            Ok(Cow::Owned(v.into_bytes()))
+        }
+
+        fn visit_bytes<E>(self, v: &[u8]) -> Result<Self::Value, E>
+        where
+            E: Error,
+        {
+            Ok(Cow::Owned(v.to_vec()))
+        }
+
+        fn visit_borrowed_bytes<E>(self, v: &'a [u8]) -> Result<Self::Value, E>
+        where
+            E: Error,
+        {
+            Ok(Cow::Borrowed(v))
+        }
+
+        fn visit_byte_buf<E>(self, v: Vec<u8>) -> Result<Self::Value, E>
+        where
+            E: Error,
+        {
+            Ok(Cow::Owned(v))
+        }
+    }
+
+    deserializer
+        .deserialize_bytes(CowBytesVisitor)
+        .map(From::from)
+}
+
+pub mod size_hint {
+    use lib::*;
+
+    pub fn from_bounds<I>(iter: &I) -> Option<usize>
+    where
+        I: Iterator,
+    {
+        helper(iter.size_hint())
+    }
+
+    #[inline]
+    pub fn cautious(hint: Option<usize>) -> usize {
+        cmp::min(hint.unwrap_or(0), 4096)
+    }
+
+    fn helper(bounds: (usize, Option<usize>)) -> Option<usize> {
+        match bounds {
+            (lower, Some(upper)) if lower == upper => Some(upper),
+            _ => None,
+        }
+    }
+}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+mod content {
+    // This module is private and nothing here should be used outside of
+    // generated code.
+    //
+    // We will iterate on the implementation for a few releases and only have to
+    // worry about backward compatibility for the `untagged` and `tag` attributes
+    // rather than for this entire mechanism.
+    //
+    // This issue is tracking making some of this stuff public:
+    // https://github.com/serde-rs/serde/issues/741
+
+    use lib::*;
+
+    use super::size_hint;
+    use de::{
+        self, Deserialize, DeserializeSeed, Deserializer, EnumAccess, Expected, IgnoredAny,
+        MapAccess, SeqAccess, Unexpected, Visitor,
+    };
+
+    /// Used from generated code to buffer the contents of the Deserializer when
+    /// deserializing untagged enums and internally tagged enums.
+    ///
+    /// Not public API. Use serde-value instead.
+    #[derive(Debug)]
+    pub enum Content<'de> {
+        Bool(bool),
+
+        U8(u8),
+        U16(u16),
+        U32(u32),
+        U64(u64),
+
+        I8(i8),
+        I16(i16),
+        I32(i32),
+        I64(i64),
+
+        F32(f32),
+        F64(f64),
+
+        Char(char),
+        String(String),
+        Str(&'de str),
+        ByteBuf(Vec<u8>),
+        Bytes(&'de [u8]),
+
+        None,
+        Some(Box<Content<'de>>),
+
+        Unit,
+        Newtype(Box<Content<'de>>),
+        Seq(Vec<Content<'de>>),
+        Map(Vec<(Content<'de>, Content<'de>)>),
+    }
+
+    impl<'de> Content<'de> {
+        pub fn as_str(&self) -> Option<&str> {
+            match *self {
+                Content::Str(x) => Some(x),
+                Content::String(ref x) => Some(x),
+                Content::Bytes(x) => str::from_utf8(x).ok(),
+                Content::ByteBuf(ref x) => str::from_utf8(x).ok(),
+                _ => None,
+            }
+        }
+
+        #[cold]
+        fn unexpected(&self) -> Unexpected {
+            match *self {
+                Content::Bool(b) => Unexpected::Bool(b),
+                Content::U8(n) => Unexpected::Unsigned(n as u64),
+                Content::U16(n) => Unexpected::Unsigned(n as u64),
+                Content::U32(n) => Unexpected::Unsigned(n as u64),
+                Content::U64(n) => Unexpected::Unsigned(n),
+                Content::I8(n) => Unexpected::Signed(n as i64),
+                Content::I16(n) => Unexpected::Signed(n as i64),
+                Content::I32(n) => Unexpected::Signed(n as i64),
+                Content::I64(n) => Unexpected::Signed(n),
+                Content::F32(f) => Unexpected::Float(f as f64),
+                Content::F64(f) => Unexpected::Float(f),
+                Content::Char(c) => Unexpected::Char(c),
+                Content::String(ref s) => Unexpected::Str(s),
+                Content::Str(s) => Unexpected::Str(s),
+                Content::ByteBuf(ref b) => Unexpected::Bytes(b),
+                Content::Bytes(b) => Unexpected::Bytes(b),
+                Content::None | Content::Some(_) => Unexpected::Option,
+                Content::Unit => Unexpected::Unit,
+                Content::Newtype(_) => Unexpected::NewtypeStruct,
+                Content::Seq(_) => Unexpected::Seq,
+                Content::Map(_) => Unexpected::Map,
+            }
+        }
+    }
+
+    impl<'de> Deserialize<'de> for Content<'de> {
+        fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+        where
+            D: Deserializer<'de>,
+        {
+            // Untagged and internally tagged enums are only supported in
+            // self-describing formats.
+            let visitor = ContentVisitor { value: PhantomData };
+            deserializer.deserialize_any(visitor)
+        }
+    }
+
+    struct ContentVisitor<'de> {
+        value: PhantomData<Content<'de>>,
+    }
+
+    impl<'de> ContentVisitor<'de> {
+        fn new() -> Self {
+            ContentVisitor { value: PhantomData }
+        }
+    }
+
+    impl<'de> Visitor<'de> for ContentVisitor<'de> {
+        type Value = Content<'de>;
+
+        fn expecting(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+            fmt.write_str("any value")
+        }
+
+        fn visit_bool<F>(self, value: bool) -> Result<Self::Value, F>
+        where
+            F: de::Error,
+        {
+            Ok(Content::Bool(value))
+        }
+
+        fn visit_i8<F>(self, value: i8) -> Result<Self::Value, F>
+        where
+            F: de::Error,
+        {
+            Ok(Content::I8(value))
+        }
+
+        fn visit_i16<F>(self, value: i16) -> Result<Self::Value, F>
+        where
+            F: de::Error,
+        {
+            Ok(Content::I16(value))
+        }
+
+        fn visit_i32<F>(self, value: i32) -> Result<Self::Value, F>
+        where
+            F: de::Error,
+        {
+            Ok(Content::I32(value))
+        }
+
+        fn visit_i64<F>(self, value: i64) -> Result<Self::Value, F>
+        where
+            F: de::Error,
+        {
+            Ok(Content::I64(value))
+        }
+
+        fn visit_u8<F>(self, value: u8) -> Result<Self::Value, F>
+        where
+            F: de::Error,
+        {
+            Ok(Content::U8(value))
+        }
+
+        fn visit_u16<F>(self, value: u16) -> Result<Self::Value, F>
+        where
+            F: de::Error,
+        {
+            Ok(Content::U16(value))
+        }
+
+        fn visit_u32<F>(self, value: u32) -> Result<Self::Value, F>
+        where
+            F: de::Error,
+        {
+            Ok(Content::U32(value))
+        }
+
+        fn visit_u64<F>(self, value: u64) -> Result<Self::Value, F>
+        where
+            F: de::Error,
+        {
+            Ok(Content::U64(value))
+        }
+
+        fn visit_f32<F>(self, value: f32) -> Result<Self::Value, F>
+        where
+            F: de::Error,
+        {
+            Ok(Content::F32(value))
+        }
+
+        fn visit_f64<F>(self, value: f64) -> Result<Self::Value, F>
+        where
+            F: de::Error,
+        {
+            Ok(Content::F64(value))
+        }
+
+        fn visit_char<F>(self, value: char) -> Result<Self::Value, F>
+        where
+            F: de::Error,
+        {
+            Ok(Content::Char(value))
+        }
+
+        fn visit_str<F>(self, value: &str) -> Result<Self::Value, F>
+        where
+            F: de::Error,
+        {
+            Ok(Content::String(value.into()))
+        }
+
+        fn visit_borrowed_str<F>(self, value: &'de str) -> Result<Self::Value, F>
+        where
+            F: de::Error,
+        {
+            Ok(Content::Str(value))
+        }
+
+        fn visit_string<F>(self, value: String) -> Result<Self::Value, F>
+        where
+            F: de::Error,
+        {
+            Ok(Content::String(value))
+        }
+
+        fn visit_bytes<F>(self, value: &[u8]) -> Result<Self::Value, F>
+        where
+            F: de::Error,
+        {
+            Ok(Content::ByteBuf(value.into()))
+        }
+
+        fn visit_borrowed_bytes<F>(self, value: &'de [u8]) -> Result<Self::Value, F>
+        where
+            F: de::Error,
+        {
+            Ok(Content::Bytes(value))
+        }
+
+        fn visit_byte_buf<F>(self, value: Vec<u8>) -> Result<Self::Value, F>
+        where
+            F: de::Error,
+        {
+            Ok(Content::ByteBuf(value))
+        }
+
+        fn visit_unit<F>(self) -> Result<Self::Value, F>
+        where
+            F: de::Error,
+        {
+            Ok(Content::Unit)
+        }
+
+        fn visit_none<F>(self) -> Result<Self::Value, F>
+        where
+            F: de::Error,
+        {
+            Ok(Content::None)
+        }
+
+        fn visit_some<D>(self, deserializer: D) -> Result<Self::Value, D::Error>
+        where
+            D: Deserializer<'de>,
+        {
+            Deserialize::deserialize(deserializer).map(|v| Content::Some(Box::new(v)))
+        }
+
+        fn visit_newtype_struct<D>(self, deserializer: D) -> Result<Self::Value, D::Error>
+        where
+            D: Deserializer<'de>,
+        {
+            Deserialize::deserialize(deserializer).map(|v| Content::Newtype(Box::new(v)))
+        }
+
+        fn visit_seq<V>(self, mut visitor: V) -> Result<Self::Value, V::Error>
+        where
+            V: SeqAccess<'de>,
+        {
+            let mut vec = Vec::with_capacity(size_hint::cautious(visitor.size_hint()));
+            while let Some(e) = try!(visitor.next_element()) {
+                vec.push(e);
+            }
+            Ok(Content::Seq(vec))
+        }
+
+        fn visit_map<V>(self, mut visitor: V) -> Result<Self::Value, V::Error>
+        where
+            V: MapAccess<'de>,
+        {
+            let mut vec = Vec::with_capacity(size_hint::cautious(visitor.size_hint()));
+            while let Some(kv) = try!(visitor.next_entry()) {
+                vec.push(kv);
+            }
+            Ok(Content::Map(vec))
+        }
+
+        fn visit_enum<V>(self, _visitor: V) -> Result<Self::Value, V::Error>
+        where
+            V: EnumAccess<'de>,
+        {
+            Err(de::Error::custom(
+                "untagged and internally tagged enums do not support enum input",
+            ))
+        }
+    }
+
+    /// This is the type of the map keys in an internally tagged enum.
+    ///
+    /// Not public API.
+    pub enum TagOrContent<'de> {
+        Tag,
+        Content(Content<'de>),
+    }
+
+    struct TagOrContentVisitor<'de> {
+        name: &'static str,
+        value: PhantomData<TagOrContent<'de>>,
+    }
+
+    impl<'de> TagOrContentVisitor<'de> {
+        fn new(name: &'static str) -> Self {
+            TagOrContentVisitor {
+                name: name,
+                value: PhantomData,
+            }
+        }
+    }
+
+    impl<'de> DeserializeSeed<'de> for TagOrContentVisitor<'de> {
+        type Value = TagOrContent<'de>;
+
+        fn deserialize<D>(self, deserializer: D) -> Result<Self::Value, D::Error>
+        where
+            D: Deserializer<'de>,
+        {
+            // Internally tagged enums are only supported in self-describing
+            // formats.
+            deserializer.deserialize_any(self)
+        }
+    }
+
+    impl<'de> Visitor<'de> for TagOrContentVisitor<'de> {
+        type Value = TagOrContent<'de>;
+
+        fn expecting(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+            write!(fmt, "a type tag `{}` or any other value", self.name)
+        }
+
+        fn visit_bool<F>(self, value: bool) -> Result<Self::Value, F>
+        where
+            F: de::Error,
+        {
+            ContentVisitor::new()
+                .visit_bool(value)
+                .map(TagOrContent::Content)
+        }
+
+        fn visit_i8<F>(self, value: i8) -> Result<Self::Value, F>
+        where
+            F: de::Error,
+        {
+            ContentVisitor::new()
+                .visit_i8(value)
+                .map(TagOrContent::Content)
+        }
+
+        fn visit_i16<F>(self, value: i16) -> Result<Self::Value, F>
+        where
+            F: de::Error,
+        {
+            ContentVisitor::new()
+                .visit_i16(value)
+                .map(TagOrContent::Content)
+        }
+
+        fn visit_i32<F>(self, value: i32) -> Result<Self::Value, F>
+        where
+            F: de::Error,
+        {
+            ContentVisitor::new()
+                .visit_i32(value)
+                .map(TagOrContent::Content)
+        }
+
+        fn visit_i64<F>(self, value: i64) -> Result<Self::Value, F>
+        where
+            F: de::Error,
+        {
+            ContentVisitor::new()
+                .visit_i64(value)
+                .map(TagOrContent::Content)
+        }
+
+        fn visit_u8<F>(self, value: u8) -> Result<Self::Value, F>
+        where
+            F: de::Error,
+        {
+            ContentVisitor::new()
+                .visit_u8(value)
+                .map(TagOrContent::Content)
+        }
+
+        fn visit_u16<F>(self, value: u16) -> Result<Self::Value, F>
+        where
+            F: de::Error,
+        {
+            ContentVisitor::new()
+                .visit_u16(value)
+                .map(TagOrContent::Content)
+        }
+
+        fn visit_u32<F>(self, value: u32) -> Result<Self::Value, F>
+        where
+            F: de::Error,
+        {
+            ContentVisitor::new()
+                .visit_u32(value)
+                .map(TagOrContent::Content)
+        }
+
+        fn visit_u64<F>(self, value: u64) -> Result<Self::Value, F>
+        where
+            F: de::Error,
+        {
+            ContentVisitor::new()
+                .visit_u64(value)
+                .map(TagOrContent::Content)
+        }
+
+        fn visit_f32<F>(self, value: f32) -> Result<Self::Value, F>
+        where
+            F: de::Error,
+        {
+            ContentVisitor::new()
+                .visit_f32(value)
+                .map(TagOrContent::Content)
+        }
+
+        fn visit_f64<F>(self, value: f64) -> Result<Self::Value, F>
+        where
+            F: de::Error,
+        {
+            ContentVisitor::new()
+                .visit_f64(value)
+                .map(TagOrContent::Content)
+        }
+
+        fn visit_char<F>(self, value: char) -> Result<Self::Value, F>
+        where
+            F: de::Error,
+        {
+            ContentVisitor::new()
+                .visit_char(value)
+                .map(TagOrContent::Content)
+        }
+
+        fn visit_str<F>(self, value: &str) -> Result<Self::Value, F>
+        where
+            F: de::Error,
+        {
+            if value == self.name {
+                Ok(TagOrContent::Tag)
+            } else {
+                ContentVisitor::new()
+                    .visit_str(value)
+                    .map(TagOrContent::Content)
+            }
+        }
+
+        fn visit_borrowed_str<F>(self, value: &'de str) -> Result<Self::Value, F>
+        where
+            F: de::Error,
+        {
+            if value == self.name {
+                Ok(TagOrContent::Tag)
+            } else {
+                ContentVisitor::new()
+                    .visit_borrowed_str(value)
+                    .map(TagOrContent::Content)
+            }
+        }
+
+        fn visit_string<F>(self, value: String) -> Result<Self::Value, F>
+        where
+            F: de::Error,
+        {
+            if value == self.name {
+                Ok(TagOrContent::Tag)
+            } else {
+                ContentVisitor::new()
+                    .visit_string(value)
+                    .map(TagOrContent::Content)
+            }
+        }
+
+        fn visit_bytes<F>(self, value: &[u8]) -> Result<Self::Value, F>
+        where
+            F: de::Error,
+        {
+            if value == self.name.as_bytes() {
+                Ok(TagOrContent::Tag)
+            } else {
+                ContentVisitor::new()
+                    .visit_bytes(value)
+                    .map(TagOrContent::Content)
+            }
+        }
+
+        fn visit_borrowed_bytes<F>(self, value: &'de [u8]) -> Result<Self::Value, F>
+        where
+            F: de::Error,
+        {
+            if value == self.name.as_bytes() {
+                Ok(TagOrContent::Tag)
+            } else {
+                ContentVisitor::new()
+                    .visit_borrowed_bytes(value)
+                    .map(TagOrContent::Content)
+            }
+        }
+
+        fn visit_byte_buf<F>(self, value: Vec<u8>) -> Result<Self::Value, F>
+        where
+            F: de::Error,
+        {
+            if value == self.name.as_bytes() {
+                Ok(TagOrContent::Tag)
+            } else {
+                ContentVisitor::new()
+                    .visit_byte_buf(value)
+                    .map(TagOrContent::Content)
+            }
+        }
+
+        fn visit_unit<F>(self) -> Result<Self::Value, F>
+        where
+            F: de::Error,
+        {
+            ContentVisitor::new()
+                .visit_unit()
+                .map(TagOrContent::Content)
+        }
+
+        fn visit_none<F>(self) -> Result<Self::Value, F>
+        where
+            F: de::Error,
+        {
+            ContentVisitor::new()
+                .visit_none()
+                .map(TagOrContent::Content)
+        }
+
+        fn visit_some<D>(self, deserializer: D) -> Result<Self::Value, D::Error>
+        where
+            D: Deserializer<'de>,
+        {
+            ContentVisitor::new()
+                .visit_some(deserializer)
+                .map(TagOrContent::Content)
+        }
+
+        fn visit_newtype_struct<D>(self, deserializer: D) -> Result<Self::Value, D::Error>
+        where
+            D: Deserializer<'de>,
+        {
+            ContentVisitor::new()
+                .visit_newtype_struct(deserializer)
+                .map(TagOrContent::Content)
+        }
+
+        fn visit_seq<V>(self, visitor: V) -> Result<Self::Value, V::Error>
+        where
+            V: SeqAccess<'de>,
+        {
+            ContentVisitor::new()
+                .visit_seq(visitor)
+                .map(TagOrContent::Content)
+        }
+
+        fn visit_map<V>(self, visitor: V) -> Result<Self::Value, V::Error>
+        where
+            V: MapAccess<'de>,
+        {
+            ContentVisitor::new()
+                .visit_map(visitor)
+                .map(TagOrContent::Content)
+        }
+
+        fn visit_enum<V>(self, visitor: V) -> Result<Self::Value, V::Error>
+        where
+            V: EnumAccess<'de>,
+        {
+            ContentVisitor::new()
+                .visit_enum(visitor)
+                .map(TagOrContent::Content)
+        }
+    }
+
+    /// Used by generated code to deserialize an internally tagged enum.
+    ///
+    /// Not public API.
+    pub struct TaggedContent<'de, T> {
+        pub tag: T,
+        pub content: Content<'de>,
+    }
+
+    /// Not public API.
+    pub struct TaggedContentVisitor<'de, T> {
+        tag_name: &'static str,
+        value: PhantomData<TaggedContent<'de, T>>,
+    }
+
+    impl<'de, T> TaggedContentVisitor<'de, T> {
+        /// Visitor for the content of an internally tagged enum with the given
+        /// tag name.
+        pub fn new(name: &'static str) -> Self {
+            TaggedContentVisitor {
+                tag_name: name,
+                value: PhantomData,
+            }
+        }
+    }
+
+    impl<'de, T> DeserializeSeed<'de> for TaggedContentVisitor<'de, T>
+    where
+        T: Deserialize<'de>,
+    {
+        type Value = TaggedContent<'de, T>;
+
+        fn deserialize<D>(self, deserializer: D) -> Result<Self::Value, D::Error>
+        where
+            D: Deserializer<'de>,
+        {
+            // Internally tagged enums are only supported in self-describing
+            // formats.
+            deserializer.deserialize_any(self)
+        }
+    }
+
+    impl<'de, T> Visitor<'de> for TaggedContentVisitor<'de, T>
+    where
+        T: Deserialize<'de>,
+    {
+        type Value = TaggedContent<'de, T>;
+
+        fn expecting(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+            fmt.write_str("internally tagged enum")
+        }
+
+        fn visit_seq<S>(self, mut seq: S) -> Result<Self::Value, S::Error>
+        where
+            S: SeqAccess<'de>,
+        {
+            let tag = match try!(seq.next_element()) {
+                Some(tag) => tag,
+                None => {
+                    return Err(de::Error::missing_field(self.tag_name));
+                }
+            };
+            let rest = de::value::SeqAccessDeserializer::new(seq);
+            Ok(TaggedContent {
+                tag: tag,
+                content: try!(Content::deserialize(rest)),
+            })
+        }
+
+        fn visit_map<M>(self, mut map: M) -> Result<Self::Value, M::Error>
+        where
+            M: MapAccess<'de>,
+        {
+            let mut tag = None;
+            let mut vec = Vec::with_capacity(size_hint::cautious(map.size_hint()));
+            while let Some(k) = try!(map.next_key_seed(TagOrContentVisitor::new(self.tag_name))) {
+                match k {
+                    TagOrContent::Tag => {
+                        if tag.is_some() {
+                            return Err(de::Error::duplicate_field(self.tag_name));
+                        }
+                        tag = Some(try!(map.next_value()));
+                    }
+                    TagOrContent::Content(k) => {
+                        let v = try!(map.next_value());
+                        vec.push((k, v));
+                    }
+                }
+            }
+            match tag {
+                None => Err(de::Error::missing_field(self.tag_name)),
+                Some(tag) => Ok(TaggedContent {
+                    tag: tag,
+                    content: Content::Map(vec),
+                }),
+            }
+        }
+    }
+
+    /// Used by generated code to deserialize an adjacently tagged enum.
+    ///
+    /// Not public API.
+    pub enum TagOrContentField {
+        Tag,
+        Content,
+    }
+
+    /// Not public API.
+    pub struct TagOrContentFieldVisitor {
+        pub tag: &'static str,
+        pub content: &'static str,
+    }
+
+    impl<'de> DeserializeSeed<'de> for TagOrContentFieldVisitor {
+        type Value = TagOrContentField;
+
+        fn deserialize<D>(self, deserializer: D) -> Result<Self::Value, D::Error>
+        where
+            D: Deserializer<'de>,
+        {
+            deserializer.deserialize_str(self)
+        }
+    }
+
+    impl<'de> Visitor<'de> for TagOrContentFieldVisitor {
+        type Value = TagOrContentField;
+
+        fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+            write!(formatter, "{:?} or {:?}", self.tag, self.content)
+        }
+
+        fn visit_str<E>(self, field: &str) -> Result<Self::Value, E>
+        where
+            E: de::Error,
+        {
+            if field == self.tag {
+                Ok(TagOrContentField::Tag)
+            } else if field == self.content {
+                Ok(TagOrContentField::Content)
+            } else {
+                Err(de::Error::invalid_value(Unexpected::Str(field), &self))
+            }
+        }
+    }
+
+    /// Used by generated code to deserialize an adjacently tagged enum when
+    /// ignoring unrelated fields is allowed.
+    ///
+    /// Not public API.
+    pub enum TagContentOtherField {
+        Tag,
+        Content,
+        Other,
+    }
+
+    /// Not public API.
+    pub struct TagContentOtherFieldVisitor {
+        pub tag: &'static str,
+        pub content: &'static str,
+    }
+
+    impl<'de> DeserializeSeed<'de> for TagContentOtherFieldVisitor {
+        type Value = TagContentOtherField;
+
+        fn deserialize<D>(self, deserializer: D) -> Result<Self::Value, D::Error>
+        where
+            D: Deserializer<'de>,
+        {
+            deserializer.deserialize_str(self)
+        }
+    }
+
+    impl<'de> Visitor<'de> for TagContentOtherFieldVisitor {
+        type Value = TagContentOtherField;
+
+        fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+            write!(
+                formatter,
+                "{:?}, {:?}, or other ignored fields",
+                self.tag, self.content
+            )
+        }
+
+        fn visit_str<E>(self, field: &str) -> Result<Self::Value, E>
+        where
+            E: de::Error,
+        {
+            if field == self.tag {
+                Ok(TagContentOtherField::Tag)
+            } else if field == self.content {
+                Ok(TagContentOtherField::Content)
+            } else {
+                Ok(TagContentOtherField::Other)
+            }
+        }
+    }
+
+    /// Not public API
+    pub struct ContentDeserializer<'de, E> {
+        content: Content<'de>,
+        err: PhantomData<E>,
+    }
+
+    impl<'de, E> ContentDeserializer<'de, E>
+    where
+        E: de::Error,
+    {
+        #[cold]
+        fn invalid_type(self, exp: &Expected) -> E {
+            de::Error::invalid_type(self.content.unexpected(), exp)
+        }
+
+        fn deserialize_integer<V>(self, visitor: V) -> Result<V::Value, E>
+        where
+            V: Visitor<'de>,
+        {
+            match self.content {
+                Content::U8(v) => visitor.visit_u8(v),
+                Content::U16(v) => visitor.visit_u16(v),
+                Content::U32(v) => visitor.visit_u32(v),
+                Content::U64(v) => visitor.visit_u64(v),
+                Content::I8(v) => visitor.visit_i8(v),
+                Content::I16(v) => visitor.visit_i16(v),
+                Content::I32(v) => visitor.visit_i32(v),
+                Content::I64(v) => visitor.visit_i64(v),
+                _ => Err(self.invalid_type(&visitor)),
+            }
+        }
+    }
+
+    fn visit_content_seq<'de, V, E>(content: Vec<Content<'de>>, visitor: V) -> Result<V::Value, E>
+    where
+        V: Visitor<'de>,
+        E: de::Error,
+    {
+        let seq = content.into_iter().map(ContentDeserializer::new);
+        let mut seq_visitor = de::value::SeqDeserializer::new(seq);
+        let value = try!(visitor.visit_seq(&mut seq_visitor));
+        try!(seq_visitor.end());
+        Ok(value)
+    }
+
+    fn visit_content_map<'de, V, E>(
+        content: Vec<(Content<'de>, Content<'de>)>,
+        visitor: V,
+    ) -> Result<V::Value, E>
+    where
+        V: Visitor<'de>,
+        E: de::Error,
+    {
+        let map = content
+            .into_iter()
+            .map(|(k, v)| (ContentDeserializer::new(k), ContentDeserializer::new(v)));
+        let mut map_visitor = de::value::MapDeserializer::new(map);
+        let value = try!(visitor.visit_map(&mut map_visitor));
+        try!(map_visitor.end());
+        Ok(value)
+    }
+
+    /// Used when deserializing an internally tagged enum because the content
+    /// will be used exactly once.
+    impl<'de, E> Deserializer<'de> for ContentDeserializer<'de, E>
+    where
+        E: de::Error,
+    {
+        type Error = E;
+
+        fn deserialize_any<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            match self.content {
+                Content::Bool(v) => visitor.visit_bool(v),
+                Content::U8(v) => visitor.visit_u8(v),
+                Content::U16(v) => visitor.visit_u16(v),
+                Content::U32(v) => visitor.visit_u32(v),
+                Content::U64(v) => visitor.visit_u64(v),
+                Content::I8(v) => visitor.visit_i8(v),
+                Content::I16(v) => visitor.visit_i16(v),
+                Content::I32(v) => visitor.visit_i32(v),
+                Content::I64(v) => visitor.visit_i64(v),
+                Content::F32(v) => visitor.visit_f32(v),
+                Content::F64(v) => visitor.visit_f64(v),
+                Content::Char(v) => visitor.visit_char(v),
+                Content::String(v) => visitor.visit_string(v),
+                Content::Str(v) => visitor.visit_borrowed_str(v),
+                Content::ByteBuf(v) => visitor.visit_byte_buf(v),
+                Content::Bytes(v) => visitor.visit_borrowed_bytes(v),
+                Content::Unit => visitor.visit_unit(),
+                Content::None => visitor.visit_none(),
+                Content::Some(v) => visitor.visit_some(ContentDeserializer::new(*v)),
+                Content::Newtype(v) => visitor.visit_newtype_struct(ContentDeserializer::new(*v)),
+                Content::Seq(v) => visit_content_seq(v, visitor),
+                Content::Map(v) => visit_content_map(v, visitor),
+            }
+        }
+
+        fn deserialize_bool<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            match self.content {
+                Content::Bool(v) => visitor.visit_bool(v),
+                _ => Err(self.invalid_type(&visitor)),
+            }
+        }
+
+        fn deserialize_i8<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            self.deserialize_integer(visitor)
+        }
+
+        fn deserialize_i16<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            self.deserialize_integer(visitor)
+        }
+
+        fn deserialize_i32<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            self.deserialize_integer(visitor)
+        }
+
+        fn deserialize_i64<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            self.deserialize_integer(visitor)
+        }
+
+        fn deserialize_u8<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            self.deserialize_integer(visitor)
+        }
+
+        fn deserialize_u16<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            self.deserialize_integer(visitor)
+        }
+
+        fn deserialize_u32<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            self.deserialize_integer(visitor)
+        }
+
+        fn deserialize_u64<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            self.deserialize_integer(visitor)
+        }
+
+        fn deserialize_f32<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            match self.content {
+                Content::F32(v) => visitor.visit_f32(v),
+                Content::F64(v) => visitor.visit_f64(v),
+                Content::U64(v) => visitor.visit_u64(v),
+                Content::I64(v) => visitor.visit_i64(v),
+                _ => Err(self.invalid_type(&visitor)),
+            }
+        }
+
+        fn deserialize_f64<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            match self.content {
+                Content::F64(v) => visitor.visit_f64(v),
+                Content::U64(v) => visitor.visit_u64(v),
+                Content::I64(v) => visitor.visit_i64(v),
+                _ => Err(self.invalid_type(&visitor)),
+            }
+        }
+
+        fn deserialize_char<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            match self.content {
+                Content::Char(v) => visitor.visit_char(v),
+                Content::String(v) => visitor.visit_string(v),
+                Content::Str(v) => visitor.visit_borrowed_str(v),
+                _ => Err(self.invalid_type(&visitor)),
+            }
+        }
+
+        fn deserialize_str<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            self.deserialize_string(visitor)
+        }
+
+        fn deserialize_string<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            match self.content {
+                Content::String(v) => visitor.visit_string(v),
+                Content::Str(v) => visitor.visit_borrowed_str(v),
+                Content::ByteBuf(v) => visitor.visit_byte_buf(v),
+                Content::Bytes(v) => visitor.visit_borrowed_bytes(v),
+                _ => Err(self.invalid_type(&visitor)),
+            }
+        }
+
+        fn deserialize_bytes<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            self.deserialize_byte_buf(visitor)
+        }
+
+        fn deserialize_byte_buf<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            match self.content {
+                Content::String(v) => visitor.visit_string(v),
+                Content::Str(v) => visitor.visit_borrowed_str(v),
+                Content::ByteBuf(v) => visitor.visit_byte_buf(v),
+                Content::Bytes(v) => visitor.visit_borrowed_bytes(v),
+                Content::Seq(v) => visit_content_seq(v, visitor),
+                _ => Err(self.invalid_type(&visitor)),
+            }
+        }
+
+        fn deserialize_option<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            match self.content {
+                Content::None => visitor.visit_none(),
+                Content::Some(v) => visitor.visit_some(ContentDeserializer::new(*v)),
+                Content::Unit => visitor.visit_unit(),
+                _ => visitor.visit_some(self),
+            }
+        }
+
+        fn deserialize_unit<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            match self.content {
+                Content::Unit => visitor.visit_unit(),
+                _ => Err(self.invalid_type(&visitor)),
+            }
+        }
+
+        fn deserialize_unit_struct<V>(
+            self,
+            _name: &'static str,
+            visitor: V,
+        ) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            match self.content {
+                // As a special case, allow deserializing untagged newtype
+                // variant containing unit struct.
+                //
+                //     #[derive(Deserialize)]
+                //     struct Info;
+                //
+                //     #[derive(Deserialize)]
+                //     #[serde(tag = "topic")]
+                //     enum Message {
+                //         Info(Info),
+                //     }
+                //
+                // We want {"topic":"Info"} to deserialize even though
+                // ordinarily unit structs do not deserialize from empty map.
+                Content::Map(ref v) if v.is_empty() => visitor.visit_unit(),
+                _ => self.deserialize_any(visitor),
+            }
+        }
+
+        fn deserialize_newtype_struct<V>(
+            self,
+            _name: &str,
+            visitor: V,
+        ) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            match self.content {
+                Content::Newtype(v) => visitor.visit_newtype_struct(ContentDeserializer::new(*v)),
+                _ => visitor.visit_newtype_struct(self),
+            }
+        }
+
+        fn deserialize_seq<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            match self.content {
+                Content::Seq(v) => visit_content_seq(v, visitor),
+                _ => Err(self.invalid_type(&visitor)),
+            }
+        }
+
+        fn deserialize_tuple<V>(self, _len: usize, visitor: V) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            self.deserialize_seq(visitor)
+        }
+
+        fn deserialize_tuple_struct<V>(
+            self,
+            _name: &'static str,
+            _len: usize,
+            visitor: V,
+        ) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            self.deserialize_seq(visitor)
+        }
+
+        fn deserialize_map<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            match self.content {
+                Content::Map(v) => visit_content_map(v, visitor),
+                _ => Err(self.invalid_type(&visitor)),
+            }
+        }
+
+        fn deserialize_struct<V>(
+            self,
+            _name: &'static str,
+            _fields: &'static [&'static str],
+            visitor: V,
+        ) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            match self.content {
+                Content::Seq(v) => visit_content_seq(v, visitor),
+                Content::Map(v) => visit_content_map(v, visitor),
+                _ => Err(self.invalid_type(&visitor)),
+            }
+        }
+
+        fn deserialize_enum<V>(
+            self,
+            _name: &str,
+            _variants: &'static [&'static str],
+            visitor: V,
+        ) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            let (variant, value) = match self.content {
+                Content::Map(value) => {
+                    let mut iter = value.into_iter();
+                    let (variant, value) = match iter.next() {
+                        Some(v) => v,
+                        None => {
+                            return Err(de::Error::invalid_value(
+                                de::Unexpected::Map,
+                                &"map with a single key",
+                            ));
+                        }
+                    };
+                    // enums are encoded in json as maps with a single key:value pair
+                    if iter.next().is_some() {
+                        return Err(de::Error::invalid_value(
+                            de::Unexpected::Map,
+                            &"map with a single key",
+                        ));
+                    }
+                    (variant, Some(value))
+                }
+                s @ Content::String(_) | s @ Content::Str(_) => (s, None),
+                other => {
+                    return Err(de::Error::invalid_type(
+                        other.unexpected(),
+                        &"string or map",
+                    ));
+                }
+            };
+
+            visitor.visit_enum(EnumDeserializer::new(variant, value))
+        }
+
+        fn deserialize_identifier<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            match self.content {
+                Content::String(v) => visitor.visit_string(v),
+                Content::Str(v) => visitor.visit_borrowed_str(v),
+                Content::ByteBuf(v) => visitor.visit_byte_buf(v),
+                Content::Bytes(v) => visitor.visit_borrowed_bytes(v),
+                Content::U8(v) => visitor.visit_u8(v),
+                Content::U64(v) => visitor.visit_u64(v),
+                _ => Err(self.invalid_type(&visitor)),
+            }
+        }
+
+        fn deserialize_ignored_any<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            drop(self);
+            visitor.visit_unit()
+        }
+    }
+
+    impl<'de, E> ContentDeserializer<'de, E> {
+        /// private API, don't use
+        pub fn new(content: Content<'de>) -> Self {
+            ContentDeserializer {
+                content: content,
+                err: PhantomData,
+            }
+        }
+    }
+
+    pub struct EnumDeserializer<'de, E>
+    where
+        E: de::Error,
+    {
+        variant: Content<'de>,
+        value: Option<Content<'de>>,
+        err: PhantomData<E>,
+    }
+
+    impl<'de, E> EnumDeserializer<'de, E>
+    where
+        E: de::Error,
+    {
+        pub fn new(variant: Content<'de>, value: Option<Content<'de>>) -> EnumDeserializer<'de, E> {
+            EnumDeserializer {
+                variant: variant,
+                value: value,
+                err: PhantomData,
+            }
+        }
+    }
+
+    impl<'de, E> de::EnumAccess<'de> for EnumDeserializer<'de, E>
+    where
+        E: de::Error,
+    {
+        type Error = E;
+        type Variant = VariantDeserializer<'de, Self::Error>;
+
+        fn variant_seed<V>(self, seed: V) -> Result<(V::Value, Self::Variant), E>
+        where
+            V: de::DeserializeSeed<'de>,
+        {
+            let visitor = VariantDeserializer {
+                value: self.value,
+                err: PhantomData,
+            };
+            seed.deserialize(ContentDeserializer::new(self.variant))
+                .map(|v| (v, visitor))
+        }
+    }
+
+    pub struct VariantDeserializer<'de, E>
+    where
+        E: de::Error,
+    {
+        value: Option<Content<'de>>,
+        err: PhantomData<E>,
+    }
+
+    impl<'de, E> de::VariantAccess<'de> for VariantDeserializer<'de, E>
+    where
+        E: de::Error,
+    {
+        type Error = E;
+
+        fn unit_variant(self) -> Result<(), E> {
+            match self.value {
+                Some(value) => de::Deserialize::deserialize(ContentDeserializer::new(value)),
+                None => Ok(()),
+            }
+        }
+
+        fn newtype_variant_seed<T>(self, seed: T) -> Result<T::Value, E>
+        where
+            T: de::DeserializeSeed<'de>,
+        {
+            match self.value {
+                Some(value) => seed.deserialize(ContentDeserializer::new(value)),
+                None => Err(de::Error::invalid_type(
+                    de::Unexpected::UnitVariant,
+                    &"newtype variant",
+                )),
+            }
+        }
+
+        fn tuple_variant<V>(self, _len: usize, visitor: V) -> Result<V::Value, Self::Error>
+        where
+            V: de::Visitor<'de>,
+        {
+            match self.value {
+                Some(Content::Seq(v)) => {
+                    de::Deserializer::deserialize_any(SeqDeserializer::new(v), visitor)
+                }
+                Some(other) => Err(de::Error::invalid_type(
+                    other.unexpected(),
+                    &"tuple variant",
+                )),
+                None => Err(de::Error::invalid_type(
+                    de::Unexpected::UnitVariant,
+                    &"tuple variant",
+                )),
+            }
+        }
+
+        fn struct_variant<V>(
+            self,
+            _fields: &'static [&'static str],
+            visitor: V,
+        ) -> Result<V::Value, Self::Error>
+        where
+            V: de::Visitor<'de>,
+        {
+            match self.value {
+                Some(Content::Map(v)) => {
+                    de::Deserializer::deserialize_any(MapDeserializer::new(v), visitor)
+                }
+                Some(Content::Seq(v)) => {
+                    de::Deserializer::deserialize_any(SeqDeserializer::new(v), visitor)
+                }
+                Some(other) => Err(de::Error::invalid_type(
+                    other.unexpected(),
+                    &"struct variant",
+                )),
+                None => Err(de::Error::invalid_type(
+                    de::Unexpected::UnitVariant,
+                    &"struct variant",
+                )),
+            }
+        }
+    }
+
+    struct SeqDeserializer<'de, E>
+    where
+        E: de::Error,
+    {
+        iter: <Vec<Content<'de>> as IntoIterator>::IntoIter,
+        err: PhantomData<E>,
+    }
+
+    impl<'de, E> SeqDeserializer<'de, E>
+    where
+        E: de::Error,
+    {
+        fn new(vec: Vec<Content<'de>>) -> Self {
+            SeqDeserializer {
+                iter: vec.into_iter(),
+                err: PhantomData,
+            }
+        }
+    }
+
+    impl<'de, E> de::Deserializer<'de> for SeqDeserializer<'de, E>
+    where
+        E: de::Error,
+    {
+        type Error = E;
+
+        #[inline]
+        fn deserialize_any<V>(mut self, visitor: V) -> Result<V::Value, Self::Error>
+        where
+            V: de::Visitor<'de>,
+        {
+            let len = self.iter.len();
+            if len == 0 {
+                visitor.visit_unit()
+            } else {
+                let ret = try!(visitor.visit_seq(&mut self));
+                let remaining = self.iter.len();
+                if remaining == 0 {
+                    Ok(ret)
+                } else {
+                    Err(de::Error::invalid_length(len, &"fewer elements in array"))
+                }
+            }
+        }
+
+        forward_to_deserialize_any! {
+            bool i8 i16 i32 i64 i128 u8 u16 u32 u64 u128 f32 f64 char str string
+            bytes byte_buf option unit unit_struct newtype_struct seq tuple
+            tuple_struct map struct enum identifier ignored_any
+        }
+    }
+
+    impl<'de, E> de::SeqAccess<'de> for SeqDeserializer<'de, E>
+    where
+        E: de::Error,
+    {
+        type Error = E;
+
+        fn next_element_seed<T>(&mut self, seed: T) -> Result<Option<T::Value>, Self::Error>
+        where
+            T: de::DeserializeSeed<'de>,
+        {
+            match self.iter.next() {
+                Some(value) => seed.deserialize(ContentDeserializer::new(value)).map(Some),
+                None => Ok(None),
+            }
+        }
+
+        fn size_hint(&self) -> Option<usize> {
+            size_hint::from_bounds(&self.iter)
+        }
+    }
+
+    struct MapDeserializer<'de, E>
+    where
+        E: de::Error,
+    {
+        iter: <Vec<(Content<'de>, Content<'de>)> as IntoIterator>::IntoIter,
+        value: Option<Content<'de>>,
+        err: PhantomData<E>,
+    }
+
+    impl<'de, E> MapDeserializer<'de, E>
+    where
+        E: de::Error,
+    {
+        fn new(map: Vec<(Content<'de>, Content<'de>)>) -> Self {
+            MapDeserializer {
+                iter: map.into_iter(),
+                value: None,
+                err: PhantomData,
+            }
+        }
+    }
+
+    impl<'de, E> de::MapAccess<'de> for MapDeserializer<'de, E>
+    where
+        E: de::Error,
+    {
+        type Error = E;
+
+        fn next_key_seed<T>(&mut self, seed: T) -> Result<Option<T::Value>, Self::Error>
+        where
+            T: de::DeserializeSeed<'de>,
+        {
+            match self.iter.next() {
+                Some((key, value)) => {
+                    self.value = Some(value);
+                    seed.deserialize(ContentDeserializer::new(key)).map(Some)
+                }
+                None => Ok(None),
+            }
+        }
+
+        fn next_value_seed<T>(&mut self, seed: T) -> Result<T::Value, Self::Error>
+        where
+            T: de::DeserializeSeed<'de>,
+        {
+            match self.value.take() {
+                Some(value) => seed.deserialize(ContentDeserializer::new(value)),
+                None => Err(de::Error::custom("value is missing")),
+            }
+        }
+
+        fn size_hint(&self) -> Option<usize> {
+            size_hint::from_bounds(&self.iter)
+        }
+    }
+
+    impl<'de, E> de::Deserializer<'de> for MapDeserializer<'de, E>
+    where
+        E: de::Error,
+    {
+        type Error = E;
+
+        #[inline]
+        fn deserialize_any<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+        where
+            V: de::Visitor<'de>,
+        {
+            visitor.visit_map(self)
+        }
+
+        forward_to_deserialize_any! {
+            bool i8 i16 i32 i64 i128 u8 u16 u32 u64 u128 f32 f64 char str string
+            bytes byte_buf option unit unit_struct newtype_struct seq tuple
+            tuple_struct map struct enum identifier ignored_any
+        }
+    }
+
+    /// Not public API.
+    pub struct ContentRefDeserializer<'a, 'de: 'a, E> {
+        content: &'a Content<'de>,
+        err: PhantomData<E>,
+    }
+
+    impl<'a, 'de, E> ContentRefDeserializer<'a, 'de, E>
+    where
+        E: de::Error,
+    {
+        #[cold]
+        fn invalid_type(self, exp: &Expected) -> E {
+            de::Error::invalid_type(self.content.unexpected(), exp)
+        }
+
+        fn deserialize_integer<V>(self, visitor: V) -> Result<V::Value, E>
+        where
+            V: Visitor<'de>,
+        {
+            match *self.content {
+                Content::U8(v) => visitor.visit_u8(v),
+                Content::U16(v) => visitor.visit_u16(v),
+                Content::U32(v) => visitor.visit_u32(v),
+                Content::U64(v) => visitor.visit_u64(v),
+                Content::I8(v) => visitor.visit_i8(v),
+                Content::I16(v) => visitor.visit_i16(v),
+                Content::I32(v) => visitor.visit_i32(v),
+                Content::I64(v) => visitor.visit_i64(v),
+                _ => Err(self.invalid_type(&visitor)),
+            }
+        }
+    }
+
+    fn visit_content_seq_ref<'a, 'de, V, E>(
+        content: &'a [Content<'de>],
+        visitor: V,
+    ) -> Result<V::Value, E>
+    where
+        V: Visitor<'de>,
+        E: de::Error,
+    {
+        let seq = content.iter().map(ContentRefDeserializer::new);
+        let mut seq_visitor = de::value::SeqDeserializer::new(seq);
+        let value = try!(visitor.visit_seq(&mut seq_visitor));
+        try!(seq_visitor.end());
+        Ok(value)
+    }
+
+    fn visit_content_map_ref<'a, 'de, V, E>(
+        content: &'a [(Content<'de>, Content<'de>)],
+        visitor: V,
+    ) -> Result<V::Value, E>
+    where
+        V: Visitor<'de>,
+        E: de::Error,
+    {
+        let map = content.iter().map(|&(ref k, ref v)| {
+            (
+                ContentRefDeserializer::new(k),
+                ContentRefDeserializer::new(v),
+            )
+        });
+        let mut map_visitor = de::value::MapDeserializer::new(map);
+        let value = try!(visitor.visit_map(&mut map_visitor));
+        try!(map_visitor.end());
+        Ok(value)
+    }
+
+    /// Used when deserializing an untagged enum because the content may need
+    /// to be used more than once.
+    impl<'de, 'a, E> Deserializer<'de> for ContentRefDeserializer<'a, 'de, E>
+    where
+        E: de::Error,
+    {
+        type Error = E;
+
+        fn deserialize_any<V>(self, visitor: V) -> Result<V::Value, E>
+        where
+            V: Visitor<'de>,
+        {
+            match *self.content {
+                Content::Bool(v) => visitor.visit_bool(v),
+                Content::U8(v) => visitor.visit_u8(v),
+                Content::U16(v) => visitor.visit_u16(v),
+                Content::U32(v) => visitor.visit_u32(v),
+                Content::U64(v) => visitor.visit_u64(v),
+                Content::I8(v) => visitor.visit_i8(v),
+                Content::I16(v) => visitor.visit_i16(v),
+                Content::I32(v) => visitor.visit_i32(v),
+                Content::I64(v) => visitor.visit_i64(v),
+                Content::F32(v) => visitor.visit_f32(v),
+                Content::F64(v) => visitor.visit_f64(v),
+                Content::Char(v) => visitor.visit_char(v),
+                Content::String(ref v) => visitor.visit_str(v),
+                Content::Str(v) => visitor.visit_borrowed_str(v),
+                Content::ByteBuf(ref v) => visitor.visit_bytes(v),
+                Content::Bytes(v) => visitor.visit_borrowed_bytes(v),
+                Content::Unit => visitor.visit_unit(),
+                Content::None => visitor.visit_none(),
+                Content::Some(ref v) => visitor.visit_some(ContentRefDeserializer::new(v)),
+                Content::Newtype(ref v) => {
+                    visitor.visit_newtype_struct(ContentRefDeserializer::new(v))
+                }
+                Content::Seq(ref v) => visit_content_seq_ref(v, visitor),
+                Content::Map(ref v) => visit_content_map_ref(v, visitor),
+            }
+        }
+
+        fn deserialize_bool<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            match *self.content {
+                Content::Bool(v) => visitor.visit_bool(v),
+                _ => Err(self.invalid_type(&visitor)),
+            }
+        }
+
+        fn deserialize_i8<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            self.deserialize_integer(visitor)
+        }
+
+        fn deserialize_i16<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            self.deserialize_integer(visitor)
+        }
+
+        fn deserialize_i32<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            self.deserialize_integer(visitor)
+        }
+
+        fn deserialize_i64<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            self.deserialize_integer(visitor)
+        }
+
+        fn deserialize_u8<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            self.deserialize_integer(visitor)
+        }
+
+        fn deserialize_u16<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            self.deserialize_integer(visitor)
+        }
+
+        fn deserialize_u32<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            self.deserialize_integer(visitor)
+        }
+
+        fn deserialize_u64<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            self.deserialize_integer(visitor)
+        }
+
+        fn deserialize_f32<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            match *self.content {
+                Content::F32(v) => visitor.visit_f32(v),
+                Content::F64(v) => visitor.visit_f64(v),
+                Content::U64(v) => visitor.visit_u64(v),
+                Content::I64(v) => visitor.visit_i64(v),
+                _ => Err(self.invalid_type(&visitor)),
+            }
+        }
+
+        fn deserialize_f64<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            match *self.content {
+                Content::F64(v) => visitor.visit_f64(v),
+                Content::U64(v) => visitor.visit_u64(v),
+                Content::I64(v) => visitor.visit_i64(v),
+                _ => Err(self.invalid_type(&visitor)),
+            }
+        }
+
+        fn deserialize_char<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            match *self.content {
+                Content::Char(v) => visitor.visit_char(v),
+                Content::String(ref v) => visitor.visit_str(v),
+                Content::Str(v) => visitor.visit_borrowed_str(v),
+                _ => Err(self.invalid_type(&visitor)),
+            }
+        }
+
+        fn deserialize_str<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            match *self.content {
+                Content::String(ref v) => visitor.visit_str(v),
+                Content::Str(v) => visitor.visit_borrowed_str(v),
+                Content::ByteBuf(ref v) => visitor.visit_bytes(v),
+                Content::Bytes(v) => visitor.visit_borrowed_bytes(v),
+                _ => Err(self.invalid_type(&visitor)),
+            }
+        }
+
+        fn deserialize_string<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            self.deserialize_str(visitor)
+        }
+
+        fn deserialize_bytes<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            match *self.content {
+                Content::String(ref v) => visitor.visit_str(v),
+                Content::Str(v) => visitor.visit_borrowed_str(v),
+                Content::ByteBuf(ref v) => visitor.visit_bytes(v),
+                Content::Bytes(v) => visitor.visit_borrowed_bytes(v),
+                Content::Seq(ref v) => visit_content_seq_ref(v, visitor),
+                _ => Err(self.invalid_type(&visitor)),
+            }
+        }
+
+        fn deserialize_byte_buf<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            self.deserialize_bytes(visitor)
+        }
+
+        fn deserialize_option<V>(self, visitor: V) -> Result<V::Value, E>
+        where
+            V: Visitor<'de>,
+        {
+            match *self.content {
+                Content::None => visitor.visit_none(),
+                Content::Some(ref v) => visitor.visit_some(ContentRefDeserializer::new(v)),
+                Content::Unit => visitor.visit_unit(),
+                _ => visitor.visit_some(self),
+            }
+        }
+
+        fn deserialize_unit<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            match *self.content {
+                Content::Unit => visitor.visit_unit(),
+                _ => Err(self.invalid_type(&visitor)),
+            }
+        }
+
+        fn deserialize_unit_struct<V>(
+            self,
+            _name: &'static str,
+            visitor: V,
+        ) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            self.deserialize_unit(visitor)
+        }
+
+        fn deserialize_newtype_struct<V>(self, _name: &str, visitor: V) -> Result<V::Value, E>
+        where
+            V: Visitor<'de>,
+        {
+            match *self.content {
+                Content::Newtype(ref v) => {
+                    visitor.visit_newtype_struct(ContentRefDeserializer::new(v))
+                }
+                _ => visitor.visit_newtype_struct(self),
+            }
+        }
+
+        fn deserialize_seq<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            match *self.content {
+                Content::Seq(ref v) => visit_content_seq_ref(v, visitor),
+                _ => Err(self.invalid_type(&visitor)),
+            }
+        }
+
+        fn deserialize_tuple<V>(self, _len: usize, visitor: V) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            self.deserialize_seq(visitor)
+        }
+
+        fn deserialize_tuple_struct<V>(
+            self,
+            _name: &'static str,
+            _len: usize,
+            visitor: V,
+        ) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            self.deserialize_seq(visitor)
+        }
+
+        fn deserialize_map<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            match *self.content {
+                Content::Map(ref v) => visit_content_map_ref(v, visitor),
+                _ => Err(self.invalid_type(&visitor)),
+            }
+        }
+
+        fn deserialize_struct<V>(
+            self,
+            _name: &'static str,
+            _fields: &'static [&'static str],
+            visitor: V,
+        ) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            match *self.content {
+                Content::Seq(ref v) => visit_content_seq_ref(v, visitor),
+                Content::Map(ref v) => visit_content_map_ref(v, visitor),
+                _ => Err(self.invalid_type(&visitor)),
+            }
+        }
+
+        fn deserialize_enum<V>(
+            self,
+            _name: &str,
+            _variants: &'static [&'static str],
+            visitor: V,
+        ) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            let (variant, value) = match *self.content {
+                Content::Map(ref value) => {
+                    let mut iter = value.iter();
+                    let &(ref variant, ref value) = match iter.next() {
+                        Some(v) => v,
+                        None => {
+                            return Err(de::Error::invalid_value(
+                                de::Unexpected::Map,
+                                &"map with a single key",
+                            ));
+                        }
+                    };
+                    // enums are encoded in json as maps with a single key:value pair
+                    if iter.next().is_some() {
+                        return Err(de::Error::invalid_value(
+                            de::Unexpected::Map,
+                            &"map with a single key",
+                        ));
+                    }
+                    (variant, Some(value))
+                }
+                ref s @ Content::String(_) | ref s @ Content::Str(_) => (s, None),
+                ref other => {
+                    return Err(de::Error::invalid_type(
+                        other.unexpected(),
+                        &"string or map",
+                    ));
+                }
+            };
+
+            visitor.visit_enum(EnumRefDeserializer {
+                variant: variant,
+                value: value,
+                err: PhantomData,
+            })
+        }
+
+        fn deserialize_identifier<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            match *self.content {
+                Content::String(ref v) => visitor.visit_str(v),
+                Content::Str(v) => visitor.visit_borrowed_str(v),
+                Content::ByteBuf(ref v) => visitor.visit_bytes(v),
+                Content::Bytes(v) => visitor.visit_borrowed_bytes(v),
+                Content::U8(v) => visitor.visit_u8(v),
+                Content::U64(v) => visitor.visit_u64(v),
+                _ => Err(self.invalid_type(&visitor)),
+            }
+        }
+
+        fn deserialize_ignored_any<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            visitor.visit_unit()
+        }
+    }
+
+    impl<'a, 'de, E> ContentRefDeserializer<'a, 'de, E> {
+        /// private API, don't use
+        pub fn new(content: &'a Content<'de>) -> Self {
+            ContentRefDeserializer {
+                content: content,
+                err: PhantomData,
+            }
+        }
+    }
+
+    struct EnumRefDeserializer<'a, 'de: 'a, E>
+    where
+        E: de::Error,
+    {
+        variant: &'a Content<'de>,
+        value: Option<&'a Content<'de>>,
+        err: PhantomData<E>,
+    }
+
+    impl<'de, 'a, E> de::EnumAccess<'de> for EnumRefDeserializer<'a, 'de, E>
+    where
+        E: de::Error,
+    {
+        type Error = E;
+        type Variant = VariantRefDeserializer<'a, 'de, Self::Error>;
+
+        fn variant_seed<V>(self, seed: V) -> Result<(V::Value, Self::Variant), Self::Error>
+        where
+            V: de::DeserializeSeed<'de>,
+        {
+            let visitor = VariantRefDeserializer {
+                value: self.value,
+                err: PhantomData,
+            };
+            seed.deserialize(ContentRefDeserializer::new(self.variant))
+                .map(|v| (v, visitor))
+        }
+    }
+
+    struct VariantRefDeserializer<'a, 'de: 'a, E>
+    where
+        E: de::Error,
+    {
+        value: Option<&'a Content<'de>>,
+        err: PhantomData<E>,
+    }
+
+    impl<'de, 'a, E> de::VariantAccess<'de> for VariantRefDeserializer<'a, 'de, E>
+    where
+        E: de::Error,
+    {
+        type Error = E;
+
+        fn unit_variant(self) -> Result<(), E> {
+            match self.value {
+                Some(value) => de::Deserialize::deserialize(ContentRefDeserializer::new(value)),
+                None => Ok(()),
+            }
+        }
+
+        fn newtype_variant_seed<T>(self, seed: T) -> Result<T::Value, E>
+        where
+            T: de::DeserializeSeed<'de>,
+        {
+            match self.value {
+                Some(value) => seed.deserialize(ContentRefDeserializer::new(value)),
+                None => Err(de::Error::invalid_type(
+                    de::Unexpected::UnitVariant,
+                    &"newtype variant",
+                )),
+            }
+        }
+
+        fn tuple_variant<V>(self, _len: usize, visitor: V) -> Result<V::Value, Self::Error>
+        where
+            V: de::Visitor<'de>,
+        {
+            match self.value {
+                Some(&Content::Seq(ref v)) => {
+                    de::Deserializer::deserialize_any(SeqRefDeserializer::new(v), visitor)
+                }
+                Some(other) => Err(de::Error::invalid_type(
+                    other.unexpected(),
+                    &"tuple variant",
+                )),
+                None => Err(de::Error::invalid_type(
+                    de::Unexpected::UnitVariant,
+                    &"tuple variant",
+                )),
+            }
+        }
+
+        fn struct_variant<V>(
+            self,
+            _fields: &'static [&'static str],
+            visitor: V,
+        ) -> Result<V::Value, Self::Error>
+        where
+            V: de::Visitor<'de>,
+        {
+            match self.value {
+                Some(&Content::Map(ref v)) => {
+                    de::Deserializer::deserialize_any(MapRefDeserializer::new(v), visitor)
+                }
+                Some(&Content::Seq(ref v)) => {
+                    de::Deserializer::deserialize_any(SeqRefDeserializer::new(v), visitor)
+                }
+                Some(other) => Err(de::Error::invalid_type(
+                    other.unexpected(),
+                    &"struct variant",
+                )),
+                None => Err(de::Error::invalid_type(
+                    de::Unexpected::UnitVariant,
+                    &"struct variant",
+                )),
+            }
+        }
+    }
+
+    struct SeqRefDeserializer<'a, 'de: 'a, E>
+    where
+        E: de::Error,
+    {
+        iter: <&'a [Content<'de>] as IntoIterator>::IntoIter,
+        err: PhantomData<E>,
+    }
+
+    impl<'a, 'de, E> SeqRefDeserializer<'a, 'de, E>
+    where
+        E: de::Error,
+    {
+        fn new(slice: &'a [Content<'de>]) -> Self {
+            SeqRefDeserializer {
+                iter: slice.iter(),
+                err: PhantomData,
+            }
+        }
+    }
+
+    impl<'de, 'a, E> de::Deserializer<'de> for SeqRefDeserializer<'a, 'de, E>
+    where
+        E: de::Error,
+    {
+        type Error = E;
+
+        #[inline]
+        fn deserialize_any<V>(mut self, visitor: V) -> Result<V::Value, Self::Error>
+        where
+            V: de::Visitor<'de>,
+        {
+            let len = self.iter.len();
+            if len == 0 {
+                visitor.visit_unit()
+            } else {
+                let ret = try!(visitor.visit_seq(&mut self));
+                let remaining = self.iter.len();
+                if remaining == 0 {
+                    Ok(ret)
+                } else {
+                    Err(de::Error::invalid_length(len, &"fewer elements in array"))
+                }
+            }
+        }
+
+        forward_to_deserialize_any! {
+            bool i8 i16 i32 i64 i128 u8 u16 u32 u64 u128 f32 f64 char str string
+            bytes byte_buf option unit unit_struct newtype_struct seq tuple
+            tuple_struct map struct enum identifier ignored_any
+        }
+    }
+
+    impl<'de, 'a, E> de::SeqAccess<'de> for SeqRefDeserializer<'a, 'de, E>
+    where
+        E: de::Error,
+    {
+        type Error = E;
+
+        fn next_element_seed<T>(&mut self, seed: T) -> Result<Option<T::Value>, Self::Error>
+        where
+            T: de::DeserializeSeed<'de>,
+        {
+            match self.iter.next() {
+                Some(value) => seed
+                    .deserialize(ContentRefDeserializer::new(value))
+                    .map(Some),
+                None => Ok(None),
+            }
+        }
+
+        fn size_hint(&self) -> Option<usize> {
+            size_hint::from_bounds(&self.iter)
+        }
+    }
+
+    struct MapRefDeserializer<'a, 'de: 'a, E>
+    where
+        E: de::Error,
+    {
+        iter: <&'a [(Content<'de>, Content<'de>)] as IntoIterator>::IntoIter,
+        value: Option<&'a Content<'de>>,
+        err: PhantomData<E>,
+    }
+
+    impl<'a, 'de, E> MapRefDeserializer<'a, 'de, E>
+    where
+        E: de::Error,
+    {
+        fn new(map: &'a [(Content<'de>, Content<'de>)]) -> Self {
+            MapRefDeserializer {
+                iter: map.iter(),
+                value: None,
+                err: PhantomData,
+            }
+        }
+    }
+
+    impl<'de, 'a, E> de::MapAccess<'de> for MapRefDeserializer<'a, 'de, E>
+    where
+        E: de::Error,
+    {
+        type Error = E;
+
+        fn next_key_seed<T>(&mut self, seed: T) -> Result<Option<T::Value>, Self::Error>
+        where
+            T: de::DeserializeSeed<'de>,
+        {
+            match self.iter.next() {
+                Some(&(ref key, ref value)) => {
+                    self.value = Some(value);
+                    seed.deserialize(ContentRefDeserializer::new(key)).map(Some)
+                }
+                None => Ok(None),
+            }
+        }
+
+        fn next_value_seed<T>(&mut self, seed: T) -> Result<T::Value, Self::Error>
+        where
+            T: de::DeserializeSeed<'de>,
+        {
+            match self.value.take() {
+                Some(value) => seed.deserialize(ContentRefDeserializer::new(value)),
+                None => Err(de::Error::custom("value is missing")),
+            }
+        }
+
+        fn size_hint(&self) -> Option<usize> {
+            size_hint::from_bounds(&self.iter)
+        }
+    }
+
+    impl<'de, 'a, E> de::Deserializer<'de> for MapRefDeserializer<'a, 'de, E>
+    where
+        E: de::Error,
+    {
+        type Error = E;
+
+        #[inline]
+        fn deserialize_any<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+        where
+            V: de::Visitor<'de>,
+        {
+            visitor.visit_map(self)
+        }
+
+        forward_to_deserialize_any! {
+            bool i8 i16 i32 i64 i128 u8 u16 u32 u64 u128 f32 f64 char str string
+            bytes byte_buf option unit unit_struct newtype_struct seq tuple
+            tuple_struct map struct enum identifier ignored_any
+        }
+    }
+
+    impl<'de, E> de::IntoDeserializer<'de, E> for ContentDeserializer<'de, E>
+    where
+        E: de::Error,
+    {
+        type Deserializer = Self;
+
+        fn into_deserializer(self) -> Self {
+            self
+        }
+    }
+
+    impl<'de, 'a, E> de::IntoDeserializer<'de, E> for ContentRefDeserializer<'a, 'de, E>
+    where
+        E: de::Error,
+    {
+        type Deserializer = Self;
+
+        fn into_deserializer(self) -> Self {
+            self
+        }
+    }
+
+    /// Visitor for deserializing an internally tagged unit variant.
+    ///
+    /// Not public API.
+    pub struct InternallyTaggedUnitVisitor<'a> {
+        type_name: &'a str,
+        variant_name: &'a str,
+    }
+
+    impl<'a> InternallyTaggedUnitVisitor<'a> {
+        /// Not public API.
+        pub fn new(type_name: &'a str, variant_name: &'a str) -> Self {
+            InternallyTaggedUnitVisitor {
+                type_name: type_name,
+                variant_name: variant_name,
+            }
+        }
+    }
+
+    impl<'de, 'a> Visitor<'de> for InternallyTaggedUnitVisitor<'a> {
+        type Value = ();
+
+        fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+            write!(
+                formatter,
+                "unit variant {}::{}",
+                self.type_name, self.variant_name
+            )
+        }
+
+        fn visit_seq<S>(self, _: S) -> Result<(), S::Error>
+        where
+            S: SeqAccess<'de>,
+        {
+            Ok(())
+        }
+
+        fn visit_map<M>(self, mut access: M) -> Result<(), M::Error>
+        where
+            M: MapAccess<'de>,
+        {
+            while try!(access.next_entry::<IgnoredAny, IgnoredAny>()).is_some() {}
+            Ok(())
+        }
+    }
+
+    /// Visitor for deserializing an untagged unit variant.
+    ///
+    /// Not public API.
+    pub struct UntaggedUnitVisitor<'a> {
+        type_name: &'a str,
+        variant_name: &'a str,
+    }
+
+    impl<'a> UntaggedUnitVisitor<'a> {
+        /// Not public API.
+        pub fn new(type_name: &'a str, variant_name: &'a str) -> Self {
+            UntaggedUnitVisitor {
+                type_name: type_name,
+                variant_name: variant_name,
+            }
+        }
+    }
+
+    impl<'de, 'a> Visitor<'de> for UntaggedUnitVisitor<'a> {
+        type Value = ();
+
+        fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+            write!(
+                formatter,
+                "unit variant {}::{}",
+                self.type_name, self.variant_name
+            )
+        }
+
+        fn visit_unit<E>(self) -> Result<(), E>
+        where
+            E: de::Error,
+        {
+            Ok(())
+        }
+
+        fn visit_none<E>(self) -> Result<(), E>
+        where
+            E: de::Error,
+        {
+            Ok(())
+        }
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+// Like `IntoDeserializer` but also implemented for `&[u8]`. This is used for
+// the newtype fallthrough case of `field_identifier`.
+//
+//    #[derive(Deserialize)]
+//    #[serde(field_identifier)]
+//    enum F {
+//        A,
+//        B,
+//        Other(String), // deserialized using IdentifierDeserializer
+//    }
+pub trait IdentifierDeserializer<'de, E: Error> {
+    type Deserializer: Deserializer<'de, Error = E>;
+
+    fn from(self) -> Self::Deserializer;
+}
+
+impl<'de, E> IdentifierDeserializer<'de, E> for u32
+where
+    E: Error,
+{
+    type Deserializer = <u32 as IntoDeserializer<'de, E>>::Deserializer;
+
+    fn from(self) -> Self::Deserializer {
+        self.into_deserializer()
+    }
+}
+
+pub struct StrDeserializer<'a, E> {
+    value: &'a str,
+    marker: PhantomData<E>,
+}
+
+impl<'a, E> IdentifierDeserializer<'a, E> for &'a str
+where
+    E: Error,
+{
+    type Deserializer = StrDeserializer<'a, E>;
+
+    fn from(self) -> Self::Deserializer {
+        StrDeserializer {
+            value: self,
+            marker: PhantomData,
+        }
+    }
+}
+
+impl<'de, 'a, E> Deserializer<'de> for StrDeserializer<'a, E>
+where
+    E: Error,
+{
+    type Error = E;
+
+    fn deserialize_any<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+    where
+        V: Visitor<'de>,
+    {
+        visitor.visit_str(self.value)
+    }
+
+    forward_to_deserialize_any! {
+        bool i8 i16 i32 i64 i128 u8 u16 u32 u64 u128 f32 f64 char str string
+        bytes byte_buf option unit unit_struct newtype_struct seq tuple
+        tuple_struct map struct enum identifier ignored_any
+    }
+}
+
+pub struct BytesDeserializer<'a, E> {
+    value: &'a [u8],
+    marker: PhantomData<E>,
+}
+
+impl<'a, E> IdentifierDeserializer<'a, E> for &'a [u8]
+where
+    E: Error,
+{
+    type Deserializer = BytesDeserializer<'a, E>;
+
+    fn from(self) -> Self::Deserializer {
+        BytesDeserializer {
+            value: self,
+            marker: PhantomData,
+        }
+    }
+}
+
+impl<'de, 'a, E> Deserializer<'de> for BytesDeserializer<'a, E>
+where
+    E: Error,
+{
+    type Error = E;
+
+    fn deserialize_any<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+    where
+        V: Visitor<'de>,
+    {
+        visitor.visit_bytes(self.value)
+    }
+
+    forward_to_deserialize_any! {
+        bool i8 i16 i32 i64 i128 u8 u16 u32 u64 u128 f32 f64 char str string
+        bytes byte_buf option unit unit_struct newtype_struct seq tuple
+        tuple_struct map struct enum identifier ignored_any
+    }
+}
+
+/// A DeserializeSeed helper for implementing deserialize_in_place Visitors.
+///
+/// Wraps a mutable reference and calls deserialize_in_place on it.
+pub struct InPlaceSeed<'a, T: 'a>(pub &'a mut T);
+
+impl<'a, 'de, T> DeserializeSeed<'de> for InPlaceSeed<'a, T>
+where
+    T: Deserialize<'de>,
+{
+    type Value = ();
+    fn deserialize<D>(self, deserializer: D) -> Result<Self::Value, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        T::deserialize_in_place(deserializer, self.0)
+    }
+}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+pub struct FlatMapDeserializer<'a, 'de: 'a, E>(
+    pub &'a mut Vec<Option<(Content<'de>, Content<'de>)>>,
+    pub PhantomData<E>,
+);
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+impl<'a, 'de, E> FlatMapDeserializer<'a, 'de, E>
+where
+    E: Error,
+{
+    fn deserialize_other<V>() -> Result<V, E> {
+        Err(Error::custom("can only flatten structs and maps"))
+    }
+}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+macro_rules! forward_to_deserialize_other {
+    ($($func:ident ( $($arg:ty),* ))*) => {
+        $(
+            fn $func<V>(self, $(_: $arg,)* _visitor: V) -> Result<V::Value, Self::Error>
+            where
+                V: Visitor<'de>,
+            {
+                Self::deserialize_other()
+            }
+        )*
+    }
+}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+impl<'a, 'de, E> Deserializer<'de> for FlatMapDeserializer<'a, 'de, E>
+where
+    E: Error,
+{
+    type Error = E;
+
+    fn deserialize_any<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+    where
+        V: Visitor<'de>,
+    {
+        visitor.visit_map(FlatInternallyTaggedAccess {
+            iter: self.0.iter_mut(),
+            pending: None,
+            _marker: PhantomData,
+        })
+    }
+
+    fn deserialize_enum<V>(
+        self,
+        name: &'static str,
+        variants: &'static [&'static str],
+        visitor: V,
+    ) -> Result<V::Value, Self::Error>
+    where
+        V: Visitor<'de>,
+    {
+        for item in self.0.iter_mut() {
+            // items in the vector are nulled out when used.  So we can only use
+            // an item if it's still filled in and if the field is one we care
+            // about.
+            let use_item = match *item {
+                None => false,
+                Some((ref c, _)) => c.as_str().map_or(false, |x| variants.contains(&x)),
+            };
+
+            if use_item {
+                let (key, value) = item.take().unwrap();
+                return visitor.visit_enum(EnumDeserializer::new(key, Some(value)));
+            }
+        }
+
+        Err(Error::custom(format_args!(
+            "no variant of enum {} found in flattened data",
+            name
+        )))
+    }
+
+    fn deserialize_map<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+    where
+        V: Visitor<'de>,
+    {
+        visitor.visit_map(FlatMapAccess::new(self.0.iter()))
+    }
+
+    fn deserialize_struct<V>(
+        self,
+        _: &'static str,
+        fields: &'static [&'static str],
+        visitor: V,
+    ) -> Result<V::Value, Self::Error>
+    where
+        V: Visitor<'de>,
+    {
+        visitor.visit_map(FlatStructAccess::new(self.0.iter_mut(), fields))
+    }
+
+    fn deserialize_newtype_struct<V>(self, _name: &str, visitor: V) -> Result<V::Value, Self::Error>
+    where
+        V: Visitor<'de>,
+    {
+        visitor.visit_newtype_struct(self)
+    }
+
+    fn deserialize_option<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+    where
+        V: Visitor<'de>,
+    {
+        match visitor.__private_visit_untagged_option(self) {
+            Ok(value) => Ok(value),
+            Err(()) => Self::deserialize_other(),
+        }
+    }
+
+    fn deserialize_unit<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+    where
+        V: Visitor<'de>,
+    {
+        visitor.visit_unit()
+    }
+
+    forward_to_deserialize_other! {
+        deserialize_bool()
+        deserialize_i8()
+        deserialize_i16()
+        deserialize_i32()
+        deserialize_i64()
+        deserialize_u8()
+        deserialize_u16()
+        deserialize_u32()
+        deserialize_u64()
+        deserialize_f32()
+        deserialize_f64()
+        deserialize_char()
+        deserialize_str()
+        deserialize_string()
+        deserialize_bytes()
+        deserialize_byte_buf()
+        deserialize_unit_struct(&'static str)
+        deserialize_seq()
+        deserialize_tuple(usize)
+        deserialize_tuple_struct(&'static str, usize)
+        deserialize_identifier()
+        deserialize_ignored_any()
+    }
+}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+pub struct FlatMapAccess<'a, 'de: 'a, E> {
+    iter: slice::Iter<'a, Option<(Content<'de>, Content<'de>)>>,
+    pending_content: Option<&'a Content<'de>>,
+    _marker: PhantomData<E>,
+}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+impl<'a, 'de, E> FlatMapAccess<'a, 'de, E> {
+    fn new(
+        iter: slice::Iter<'a, Option<(Content<'de>, Content<'de>)>>,
+    ) -> FlatMapAccess<'a, 'de, E> {
+        FlatMapAccess {
+            iter: iter,
+            pending_content: None,
+            _marker: PhantomData,
+        }
+    }
+}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+impl<'a, 'de, E> MapAccess<'de> for FlatMapAccess<'a, 'de, E>
+where
+    E: Error,
+{
+    type Error = E;
+
+    fn next_key_seed<T>(&mut self, seed: T) -> Result<Option<T::Value>, Self::Error>
+    where
+        T: DeserializeSeed<'de>,
+    {
+        while let Some(item) = self.iter.next() {
+            // Items in the vector are nulled out when used by a struct.
+            if let Some((ref key, ref content)) = *item {
+                self.pending_content = Some(content);
+                return seed.deserialize(ContentRefDeserializer::new(key)).map(Some);
+            }
+        }
+        Ok(None)
+    }
+
+    fn next_value_seed<T>(&mut self, seed: T) -> Result<T::Value, Self::Error>
+    where
+        T: DeserializeSeed<'de>,
+    {
+        match self.pending_content.take() {
+            Some(value) => seed.deserialize(ContentRefDeserializer::new(value)),
+            None => Err(Error::custom("value is missing")),
+        }
+    }
+}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+pub struct FlatStructAccess<'a, 'de: 'a, E> {
+    iter: slice::IterMut<'a, Option<(Content<'de>, Content<'de>)>>,
+    pending_content: Option<Content<'de>>,
+    fields: &'static [&'static str],
+    _marker: PhantomData<E>,
+}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+impl<'a, 'de, E> FlatStructAccess<'a, 'de, E> {
+    fn new(
+        iter: slice::IterMut<'a, Option<(Content<'de>, Content<'de>)>>,
+        fields: &'static [&'static str],
+    ) -> FlatStructAccess<'a, 'de, E> {
+        FlatStructAccess {
+            iter: iter,
+            pending_content: None,
+            fields: fields,
+            _marker: PhantomData,
+        }
+    }
+}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+impl<'a, 'de, E> MapAccess<'de> for FlatStructAccess<'a, 'de, E>
+where
+    E: Error,
+{
+    type Error = E;
+
+    fn next_key_seed<T>(&mut self, seed: T) -> Result<Option<T::Value>, Self::Error>
+    where
+        T: DeserializeSeed<'de>,
+    {
+        while let Some(item) = self.iter.next() {
+            // items in the vector are nulled out when used.  So we can only use
+            // an item if it's still filled in and if the field is one we care
+            // about.  In case we do not know which fields we want, we take them all.
+            let use_item = match *item {
+                None => false,
+                Some((ref c, _)) => c.as_str().map_or(false, |key| self.fields.contains(&key)),
+            };
+
+            if use_item {
+                let (key, content) = item.take().unwrap();
+                self.pending_content = Some(content);
+                return seed.deserialize(ContentDeserializer::new(key)).map(Some);
+            }
+        }
+        Ok(None)
+    }
+
+    fn next_value_seed<T>(&mut self, seed: T) -> Result<T::Value, Self::Error>
+    where
+        T: DeserializeSeed<'de>,
+    {
+        match self.pending_content.take() {
+            Some(value) => seed.deserialize(ContentDeserializer::new(value)),
+            None => Err(Error::custom("value is missing")),
+        }
+    }
+}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+pub struct FlatInternallyTaggedAccess<'a, 'de: 'a, E> {
+    iter: slice::IterMut<'a, Option<(Content<'de>, Content<'de>)>>,
+    pending: Option<&'a Content<'de>>,
+    _marker: PhantomData<E>,
+}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+impl<'a, 'de, E> MapAccess<'de> for FlatInternallyTaggedAccess<'a, 'de, E>
+where
+    E: Error,
+{
+    type Error = E;
+
+    fn next_key_seed<T>(&mut self, seed: T) -> Result<Option<T::Value>, Self::Error>
+    where
+        T: DeserializeSeed<'de>,
+    {
+        while let Some(item) = self.iter.next() {
+            if let Some((ref key, ref content)) = *item {
+                // Do not take(), instead borrow this entry. The internally tagged
+                // enum does its own buffering so we can't tell whether this entry
+                // is going to be consumed. Borrowing here leaves the entry
+                // available for later flattened fields.
+                self.pending = Some(content);
+                return seed.deserialize(ContentRefDeserializer::new(key)).map(Some);
+            }
+        }
+        Ok(None)
+    }
+
+    fn next_value_seed<T>(&mut self, seed: T) -> Result<T::Value, Self::Error>
+    where
+        T: DeserializeSeed<'de>,
+    {
+        match self.pending.take() {
+            Some(value) => seed.deserialize(ContentRefDeserializer::new(value)),
+            None => panic!("value is missing"),
+        }
+    }
+}
diff --git a/src/private/macros.rs b/src/private/macros.rs
new file mode 100644
index 0000000..4f7054f
--- /dev/null
+++ b/src/private/macros.rs
@@ -0,0 +1,140 @@
+#[doc(hidden)]
+#[macro_export]
+macro_rules! __private_serialize {
+    () => {
+        trait Serialize {
+            fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+            where
+                S: $crate::Serializer;
+        }
+    };
+}
+
+#[doc(hidden)]
+#[macro_export]
+macro_rules! __private_deserialize {
+    () => {
+        trait Deserialize<'de>: Sized {
+            fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+            where
+                D: $crate::Deserializer<'de>;
+        }
+    };
+}
+
+/// Used only by Serde doc tests. Not public API.
+#[doc(hidden)]
+#[macro_export(local_inner_macros)]
+macro_rules! __serialize_unimplemented {
+    ($($func:ident)*) => {
+        $(
+            __serialize_unimplemented_helper!($func);
+        )*
+    };
+}
+
+#[doc(hidden)]
+#[macro_export]
+macro_rules! __serialize_unimplemented_method {
+    ($func:ident $(<$t:ident>)* ($($arg:ty),*) -> $ret:ident) => {
+        fn $func $(<$t: ?Sized + $crate::Serialize>)* (self $(, _: $arg)*) -> $crate::export::Result<Self::$ret, Self::Error> {
+            unimplemented!()
+        }
+    };
+}
+
+#[doc(hidden)]
+#[macro_export(local_inner_macros)]
+macro_rules! __serialize_unimplemented_helper {
+    (bool) => {
+        __serialize_unimplemented_method!(serialize_bool(bool) -> Ok);
+    };
+    (i8) => {
+        __serialize_unimplemented_method!(serialize_i8(i8) -> Ok);
+    };
+    (i16) => {
+        __serialize_unimplemented_method!(serialize_i16(i16) -> Ok);
+    };
+    (i32) => {
+        __serialize_unimplemented_method!(serialize_i32(i32) -> Ok);
+    };
+    (i64) => {
+        __serialize_unimplemented_method!(serialize_i64(i64) -> Ok);
+    };
+    (u8) => {
+        __serialize_unimplemented_method!(serialize_u8(u8) -> Ok);
+    };
+    (u16) => {
+        __serialize_unimplemented_method!(serialize_u16(u16) -> Ok);
+    };
+    (u32) => {
+        __serialize_unimplemented_method!(serialize_u32(u32) -> Ok);
+    };
+    (u64) => {
+        __serialize_unimplemented_method!(serialize_u64(u64) -> Ok);
+    };
+    (f32) => {
+        __serialize_unimplemented_method!(serialize_f32(f32) -> Ok);
+    };
+    (f64) => {
+        __serialize_unimplemented_method!(serialize_f64(f64) -> Ok);
+    };
+    (char) => {
+        __serialize_unimplemented_method!(serialize_char(char) -> Ok);
+    };
+    (str) => {
+        __serialize_unimplemented_method!(serialize_str(&str) -> Ok);
+    };
+    (bytes) => {
+        __serialize_unimplemented_method!(serialize_bytes(&[u8]) -> Ok);
+    };
+    (none) => {
+        __serialize_unimplemented_method!(serialize_none() -> Ok);
+    };
+    (some) => {
+        __serialize_unimplemented_method!(serialize_some<T>(&T) -> Ok);
+    };
+    (unit) => {
+        __serialize_unimplemented_method!(serialize_unit() -> Ok);
+    };
+    (unit_struct) => {
+        __serialize_unimplemented_method!(serialize_unit_struct(&str) -> Ok);
+    };
+    (unit_variant) => {
+        __serialize_unimplemented_method!(serialize_unit_variant(&str, u32, &str) -> Ok);
+    };
+    (newtype_struct) => {
+        __serialize_unimplemented_method!(serialize_newtype_struct<T>(&str, &T) -> Ok);
+    };
+    (newtype_variant) => {
+        __serialize_unimplemented_method!(serialize_newtype_variant<T>(&str, u32, &str, &T) -> Ok);
+    };
+    (seq) => {
+        type SerializeSeq = $crate::ser::Impossible<Self::Ok, Self::Error>;
+        __serialize_unimplemented_method!(serialize_seq(Option<usize>) -> SerializeSeq);
+    };
+    (tuple) => {
+        type SerializeTuple = $crate::ser::Impossible<Self::Ok, Self::Error>;
+        __serialize_unimplemented_method!(serialize_tuple(usize) -> SerializeTuple);
+    };
+    (tuple_struct) => {
+        type SerializeTupleStruct = $crate::ser::Impossible<Self::Ok, Self::Error>;
+        __serialize_unimplemented_method!(serialize_tuple_struct(&str, usize) -> SerializeTupleStruct);
+    };
+    (tuple_variant) => {
+        type SerializeTupleVariant = $crate::ser::Impossible<Self::Ok, Self::Error>;
+        __serialize_unimplemented_method!(serialize_tuple_variant(&str, u32, &str, usize) -> SerializeTupleVariant);
+    };
+    (map) => {
+        type SerializeMap = $crate::ser::Impossible<Self::Ok, Self::Error>;
+        __serialize_unimplemented_method!(serialize_map(Option<usize>) -> SerializeMap);
+    };
+    (struct) => {
+        type SerializeStruct = $crate::ser::Impossible<Self::Ok, Self::Error>;
+        __serialize_unimplemented_method!(serialize_struct(&str, usize) -> SerializeStruct);
+    };
+    (struct_variant) => {
+        type SerializeStructVariant = $crate::ser::Impossible<Self::Ok, Self::Error>;
+        __serialize_unimplemented_method!(serialize_struct_variant(&str, u32, &str, usize) -> SerializeStructVariant);
+    };
+}
diff --git a/src/private/mod.rs b/src/private/mod.rs
new file mode 100644
index 0000000..79e0a7d
--- /dev/null
+++ b/src/private/mod.rs
@@ -0,0 +1,4 @@
+mod macros;
+
+pub mod de;
+pub mod ser;
diff --git a/src/private/ser.rs b/src/private/ser.rs
new file mode 100644
index 0000000..eb8cfc9
--- /dev/null
+++ b/src/private/ser.rs
@@ -0,0 +1,1338 @@
+use lib::*;
+
+use ser::{self, Impossible, Serialize, SerializeMap, SerializeStruct, Serializer};
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+use self::content::{
+    Content, ContentSerializer, SerializeStructVariantAsMapValue, SerializeTupleVariantAsMapValue,
+};
+
+/// Used to check that serde(getter) attributes return the expected type.
+/// Not public API.
+pub fn constrain<T: ?Sized>(t: &T) -> &T {
+    t
+}
+
+/// Not public API.
+pub fn serialize_tagged_newtype<S, T>(
+    serializer: S,
+    type_ident: &'static str,
+    variant_ident: &'static str,
+    tag: &'static str,
+    variant_name: &'static str,
+    value: &T,
+) -> Result<S::Ok, S::Error>
+where
+    S: Serializer,
+    T: Serialize,
+{
+    value.serialize(TaggedSerializer {
+        type_ident: type_ident,
+        variant_ident: variant_ident,
+        tag: tag,
+        variant_name: variant_name,
+        delegate: serializer,
+    })
+}
+
+struct TaggedSerializer<S> {
+    type_ident: &'static str,
+    variant_ident: &'static str,
+    tag: &'static str,
+    variant_name: &'static str,
+    delegate: S,
+}
+
+enum Unsupported {
+    Boolean,
+    Integer,
+    Float,
+    Char,
+    String,
+    ByteArray,
+    Optional,
+    Unit,
+    #[cfg(any(feature = "std", feature = "alloc"))]
+    UnitStruct,
+    Sequence,
+    Tuple,
+    TupleStruct,
+    Enum,
+}
+
+impl Display for Unsupported {
+    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        match *self {
+            Unsupported::Boolean => formatter.write_str("a boolean"),
+            Unsupported::Integer => formatter.write_str("an integer"),
+            Unsupported::Float => formatter.write_str("a float"),
+            Unsupported::Char => formatter.write_str("a char"),
+            Unsupported::String => formatter.write_str("a string"),
+            Unsupported::ByteArray => formatter.write_str("a byte array"),
+            Unsupported::Optional => formatter.write_str("an optional"),
+            Unsupported::Unit => formatter.write_str("unit"),
+            #[cfg(any(feature = "std", feature = "alloc"))]
+            Unsupported::UnitStruct => formatter.write_str("unit struct"),
+            Unsupported::Sequence => formatter.write_str("a sequence"),
+            Unsupported::Tuple => formatter.write_str("a tuple"),
+            Unsupported::TupleStruct => formatter.write_str("a tuple struct"),
+            Unsupported::Enum => formatter.write_str("an enum"),
+        }
+    }
+}
+
+impl<S> TaggedSerializer<S>
+where
+    S: Serializer,
+{
+    fn bad_type(self, what: Unsupported) -> S::Error {
+        ser::Error::custom(format_args!(
+            "cannot serialize tagged newtype variant {}::{} containing {}",
+            self.type_ident, self.variant_ident, what
+        ))
+    }
+}
+
+impl<S> Serializer for TaggedSerializer<S>
+where
+    S: Serializer,
+{
+    type Ok = S::Ok;
+    type Error = S::Error;
+
+    type SerializeSeq = Impossible<S::Ok, S::Error>;
+    type SerializeTuple = Impossible<S::Ok, S::Error>;
+    type SerializeTupleStruct = Impossible<S::Ok, S::Error>;
+    type SerializeMap = S::SerializeMap;
+    type SerializeStruct = S::SerializeStruct;
+
+    #[cfg(not(any(feature = "std", feature = "alloc")))]
+    type SerializeTupleVariant = Impossible<S::Ok, S::Error>;
+    #[cfg(any(feature = "std", feature = "alloc"))]
+    type SerializeTupleVariant = SerializeTupleVariantAsMapValue<S::SerializeMap>;
+
+    #[cfg(not(any(feature = "std", feature = "alloc")))]
+    type SerializeStructVariant = Impossible<S::Ok, S::Error>;
+    #[cfg(any(feature = "std", feature = "alloc"))]
+    type SerializeStructVariant = SerializeStructVariantAsMapValue<S::SerializeMap>;
+
+    fn serialize_bool(self, _: bool) -> Result<Self::Ok, Self::Error> {
+        Err(self.bad_type(Unsupported::Boolean))
+    }
+
+    fn serialize_i8(self, _: i8) -> Result<Self::Ok, Self::Error> {
+        Err(self.bad_type(Unsupported::Integer))
+    }
+
+    fn serialize_i16(self, _: i16) -> Result<Self::Ok, Self::Error> {
+        Err(self.bad_type(Unsupported::Integer))
+    }
+
+    fn serialize_i32(self, _: i32) -> Result<Self::Ok, Self::Error> {
+        Err(self.bad_type(Unsupported::Integer))
+    }
+
+    fn serialize_i64(self, _: i64) -> Result<Self::Ok, Self::Error> {
+        Err(self.bad_type(Unsupported::Integer))
+    }
+
+    fn serialize_u8(self, _: u8) -> Result<Self::Ok, Self::Error> {
+        Err(self.bad_type(Unsupported::Integer))
+    }
+
+    fn serialize_u16(self, _: u16) -> Result<Self::Ok, Self::Error> {
+        Err(self.bad_type(Unsupported::Integer))
+    }
+
+    fn serialize_u32(self, _: u32) -> Result<Self::Ok, Self::Error> {
+        Err(self.bad_type(Unsupported::Integer))
+    }
+
+    fn serialize_u64(self, _: u64) -> Result<Self::Ok, Self::Error> {
+        Err(self.bad_type(Unsupported::Integer))
+    }
+
+    fn serialize_f32(self, _: f32) -> Result<Self::Ok, Self::Error> {
+        Err(self.bad_type(Unsupported::Float))
+    }
+
+    fn serialize_f64(self, _: f64) -> Result<Self::Ok, Self::Error> {
+        Err(self.bad_type(Unsupported::Float))
+    }
+
+    fn serialize_char(self, _: char) -> Result<Self::Ok, Self::Error> {
+        Err(self.bad_type(Unsupported::Char))
+    }
+
+    fn serialize_str(self, _: &str) -> Result<Self::Ok, Self::Error> {
+        Err(self.bad_type(Unsupported::String))
+    }
+
+    fn serialize_bytes(self, _: &[u8]) -> Result<Self::Ok, Self::Error> {
+        Err(self.bad_type(Unsupported::ByteArray))
+    }
+
+    fn serialize_none(self) -> Result<Self::Ok, Self::Error> {
+        Err(self.bad_type(Unsupported::Optional))
+    }
+
+    fn serialize_some<T: ?Sized>(self, _: &T) -> Result<Self::Ok, Self::Error>
+    where
+        T: Serialize,
+    {
+        Err(self.bad_type(Unsupported::Optional))
+    }
+
+    fn serialize_unit(self) -> Result<Self::Ok, Self::Error> {
+        Err(self.bad_type(Unsupported::Unit))
+    }
+
+    fn serialize_unit_struct(self, _: &'static str) -> Result<Self::Ok, Self::Error> {
+        let mut map = try!(self.delegate.serialize_map(Some(1)));
+        try!(map.serialize_entry(self.tag, self.variant_name));
+        map.end()
+    }
+
+    fn serialize_unit_variant(
+        self,
+        _: &'static str,
+        _: u32,
+        inner_variant: &'static str,
+    ) -> Result<Self::Ok, Self::Error> {
+        let mut map = try!(self.delegate.serialize_map(Some(2)));
+        try!(map.serialize_entry(self.tag, self.variant_name));
+        try!(map.serialize_entry(inner_variant, &()));
+        map.end()
+    }
+
+    fn serialize_newtype_struct<T: ?Sized>(
+        self,
+        _: &'static str,
+        value: &T,
+    ) -> Result<Self::Ok, Self::Error>
+    where
+        T: Serialize,
+    {
+        value.serialize(self)
+    }
+
+    fn serialize_newtype_variant<T: ?Sized>(
+        self,
+        _: &'static str,
+        _: u32,
+        inner_variant: &'static str,
+        inner_value: &T,
+    ) -> Result<Self::Ok, Self::Error>
+    where
+        T: Serialize,
+    {
+        let mut map = try!(self.delegate.serialize_map(Some(2)));
+        try!(map.serialize_entry(self.tag, self.variant_name));
+        try!(map.serialize_entry(inner_variant, inner_value));
+        map.end()
+    }
+
+    fn serialize_seq(self, _: Option<usize>) -> Result<Self::SerializeSeq, Self::Error> {
+        Err(self.bad_type(Unsupported::Sequence))
+    }
+
+    fn serialize_tuple(self, _: usize) -> Result<Self::SerializeTuple, Self::Error> {
+        Err(self.bad_type(Unsupported::Tuple))
+    }
+
+    fn serialize_tuple_struct(
+        self,
+        _: &'static str,
+        _: usize,
+    ) -> Result<Self::SerializeTupleStruct, Self::Error> {
+        Err(self.bad_type(Unsupported::TupleStruct))
+    }
+
+    #[cfg(not(any(feature = "std", feature = "alloc")))]
+    fn serialize_tuple_variant(
+        self,
+        _: &'static str,
+        _: u32,
+        _: &'static str,
+        _: usize,
+    ) -> Result<Self::SerializeTupleVariant, Self::Error> {
+        // Lack of push-based serialization means we need to buffer the content
+        // of the tuple variant, so it requires std.
+        Err(self.bad_type(Unsupported::Enum))
+    }
+
+    #[cfg(any(feature = "std", feature = "alloc"))]
+    fn serialize_tuple_variant(
+        self,
+        _: &'static str,
+        _: u32,
+        inner_variant: &'static str,
+        len: usize,
+    ) -> Result<Self::SerializeTupleVariant, Self::Error> {
+        let mut map = try!(self.delegate.serialize_map(Some(2)));
+        try!(map.serialize_entry(self.tag, self.variant_name));
+        try!(map.serialize_key(inner_variant));
+        Ok(SerializeTupleVariantAsMapValue::new(
+            map,
+            inner_variant,
+            len,
+        ))
+    }
+
+    fn serialize_map(self, len: Option<usize>) -> Result<Self::SerializeMap, Self::Error> {
+        let mut map = try!(self.delegate.serialize_map(len.map(|len| len + 1)));
+        try!(map.serialize_entry(self.tag, self.variant_name));
+        Ok(map)
+    }
+
+    fn serialize_struct(
+        self,
+        name: &'static str,
+        len: usize,
+    ) -> Result<Self::SerializeStruct, Self::Error> {
+        let mut state = try!(self.delegate.serialize_struct(name, len + 1));
+        try!(state.serialize_field(self.tag, self.variant_name));
+        Ok(state)
+    }
+
+    #[cfg(not(any(feature = "std", feature = "alloc")))]
+    fn serialize_struct_variant(
+        self,
+        _: &'static str,
+        _: u32,
+        _: &'static str,
+        _: usize,
+    ) -> Result<Self::SerializeStructVariant, Self::Error> {
+        // Lack of push-based serialization means we need to buffer the content
+        // of the struct variant, so it requires std.
+        Err(self.bad_type(Unsupported::Enum))
+    }
+
+    #[cfg(any(feature = "std", feature = "alloc"))]
+    fn serialize_struct_variant(
+        self,
+        _: &'static str,
+        _: u32,
+        inner_variant: &'static str,
+        len: usize,
+    ) -> Result<Self::SerializeStructVariant, Self::Error> {
+        let mut map = try!(self.delegate.serialize_map(Some(2)));
+        try!(map.serialize_entry(self.tag, self.variant_name));
+        try!(map.serialize_key(inner_variant));
+        Ok(SerializeStructVariantAsMapValue::new(
+            map,
+            inner_variant,
+            len,
+        ))
+    }
+
+    #[cfg(not(any(feature = "std", feature = "alloc")))]
+    fn collect_str<T: ?Sized>(self, _: &T) -> Result<Self::Ok, Self::Error>
+    where
+        T: Display,
+    {
+        Err(self.bad_type(Unsupported::String))
+    }
+}
+
+/// Used only by Serde doc tests. Not public API.
+#[doc(hidden)]
+#[derive(Debug)]
+pub struct Error;
+
+impl ser::Error for Error {
+    fn custom<T>(_: T) -> Self
+    where
+        T: Display,
+    {
+        unimplemented!()
+    }
+}
+
+#[cfg(feature = "std")]
+impl error::Error for Error {
+    fn description(&self) -> &str {
+        unimplemented!()
+    }
+}
+
+impl Display for Error {
+    fn fmt(&self, _: &mut fmt::Formatter) -> fmt::Result {
+        unimplemented!()
+    }
+}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+mod content {
+    use lib::*;
+
+    use ser::{self, Serialize, Serializer};
+
+    pub struct SerializeTupleVariantAsMapValue<M> {
+        map: M,
+        name: &'static str,
+        fields: Vec<Content>,
+    }
+
+    impl<M> SerializeTupleVariantAsMapValue<M> {
+        pub fn new(map: M, name: &'static str, len: usize) -> Self {
+            SerializeTupleVariantAsMapValue {
+                map: map,
+                name: name,
+                fields: Vec::with_capacity(len),
+            }
+        }
+    }
+
+    impl<M> ser::SerializeTupleVariant for SerializeTupleVariantAsMapValue<M>
+    where
+        M: ser::SerializeMap,
+    {
+        type Ok = M::Ok;
+        type Error = M::Error;
+
+        fn serialize_field<T: ?Sized>(&mut self, value: &T) -> Result<(), M::Error>
+        where
+            T: Serialize,
+        {
+            let value = try!(value.serialize(ContentSerializer::<M::Error>::new()));
+            self.fields.push(value);
+            Ok(())
+        }
+
+        fn end(mut self) -> Result<M::Ok, M::Error> {
+            try!(self
+                .map
+                .serialize_value(&Content::TupleStruct(self.name, self.fields)));
+            self.map.end()
+        }
+    }
+
+    pub struct SerializeStructVariantAsMapValue<M> {
+        map: M,
+        name: &'static str,
+        fields: Vec<(&'static str, Content)>,
+    }
+
+    impl<M> SerializeStructVariantAsMapValue<M> {
+        pub fn new(map: M, name: &'static str, len: usize) -> Self {
+            SerializeStructVariantAsMapValue {
+                map: map,
+                name: name,
+                fields: Vec::with_capacity(len),
+            }
+        }
+    }
+
+    impl<M> ser::SerializeStructVariant for SerializeStructVariantAsMapValue<M>
+    where
+        M: ser::SerializeMap,
+    {
+        type Ok = M::Ok;
+        type Error = M::Error;
+
+        fn serialize_field<T: ?Sized>(
+            &mut self,
+            key: &'static str,
+            value: &T,
+        ) -> Result<(), M::Error>
+        where
+            T: Serialize,
+        {
+            let value = try!(value.serialize(ContentSerializer::<M::Error>::new()));
+            self.fields.push((key, value));
+            Ok(())
+        }
+
+        fn end(mut self) -> Result<M::Ok, M::Error> {
+            try!(self
+                .map
+                .serialize_value(&Content::Struct(self.name, self.fields)));
+            self.map.end()
+        }
+    }
+
+    #[derive(Debug)]
+    pub enum Content {
+        Bool(bool),
+
+        U8(u8),
+        U16(u16),
+        U32(u32),
+        U64(u64),
+
+        I8(i8),
+        I16(i16),
+        I32(i32),
+        I64(i64),
+
+        F32(f32),
+        F64(f64),
+
+        Char(char),
+        String(String),
+        Bytes(Vec<u8>),
+
+        None,
+        Some(Box<Content>),
+
+        Unit,
+        UnitStruct(&'static str),
+        UnitVariant(&'static str, u32, &'static str),
+        NewtypeStruct(&'static str, Box<Content>),
+        NewtypeVariant(&'static str, u32, &'static str, Box<Content>),
+
+        Seq(Vec<Content>),
+        Tuple(Vec<Content>),
+        TupleStruct(&'static str, Vec<Content>),
+        TupleVariant(&'static str, u32, &'static str, Vec<Content>),
+        Map(Vec<(Content, Content)>),
+        Struct(&'static str, Vec<(&'static str, Content)>),
+        StructVariant(
+            &'static str,
+            u32,
+            &'static str,
+            Vec<(&'static str, Content)>,
+        ),
+    }
+
+    impl Serialize for Content {
+        fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+        where
+            S: Serializer,
+        {
+            match *self {
+                Content::Bool(b) => serializer.serialize_bool(b),
+                Content::U8(u) => serializer.serialize_u8(u),
+                Content::U16(u) => serializer.serialize_u16(u),
+                Content::U32(u) => serializer.serialize_u32(u),
+                Content::U64(u) => serializer.serialize_u64(u),
+                Content::I8(i) => serializer.serialize_i8(i),
+                Content::I16(i) => serializer.serialize_i16(i),
+                Content::I32(i) => serializer.serialize_i32(i),
+                Content::I64(i) => serializer.serialize_i64(i),
+                Content::F32(f) => serializer.serialize_f32(f),
+                Content::F64(f) => serializer.serialize_f64(f),
+                Content::Char(c) => serializer.serialize_char(c),
+                Content::String(ref s) => serializer.serialize_str(s),
+                Content::Bytes(ref b) => serializer.serialize_bytes(b),
+                Content::None => serializer.serialize_none(),
+                Content::Some(ref c) => serializer.serialize_some(&**c),
+                Content::Unit => serializer.serialize_unit(),
+                Content::UnitStruct(n) => serializer.serialize_unit_struct(n),
+                Content::UnitVariant(n, i, v) => serializer.serialize_unit_variant(n, i, v),
+                Content::NewtypeStruct(n, ref c) => serializer.serialize_newtype_struct(n, &**c),
+                Content::NewtypeVariant(n, i, v, ref c) => {
+                    serializer.serialize_newtype_variant(n, i, v, &**c)
+                }
+                Content::Seq(ref elements) => elements.serialize(serializer),
+                Content::Tuple(ref elements) => {
+                    use ser::SerializeTuple;
+                    let mut tuple = try!(serializer.serialize_tuple(elements.len()));
+                    for e in elements {
+                        try!(tuple.serialize_element(e));
+                    }
+                    tuple.end()
+                }
+                Content::TupleStruct(n, ref fields) => {
+                    use ser::SerializeTupleStruct;
+                    let mut ts = try!(serializer.serialize_tuple_struct(n, fields.len()));
+                    for f in fields {
+                        try!(ts.serialize_field(f));
+                    }
+                    ts.end()
+                }
+                Content::TupleVariant(n, i, v, ref fields) => {
+                    use ser::SerializeTupleVariant;
+                    let mut tv = try!(serializer.serialize_tuple_variant(n, i, v, fields.len()));
+                    for f in fields {
+                        try!(tv.serialize_field(f));
+                    }
+                    tv.end()
+                }
+                Content::Map(ref entries) => {
+                    use ser::SerializeMap;
+                    let mut map = try!(serializer.serialize_map(Some(entries.len())));
+                    for &(ref k, ref v) in entries {
+                        try!(map.serialize_entry(k, v));
+                    }
+                    map.end()
+                }
+                Content::Struct(n, ref fields) => {
+                    use ser::SerializeStruct;
+                    let mut s = try!(serializer.serialize_struct(n, fields.len()));
+                    for &(k, ref v) in fields {
+                        try!(s.serialize_field(k, v));
+                    }
+                    s.end()
+                }
+                Content::StructVariant(n, i, v, ref fields) => {
+                    use ser::SerializeStructVariant;
+                    let mut sv = try!(serializer.serialize_struct_variant(n, i, v, fields.len()));
+                    for &(k, ref v) in fields {
+                        try!(sv.serialize_field(k, v));
+                    }
+                    sv.end()
+                }
+            }
+        }
+    }
+
+    pub struct ContentSerializer<E> {
+        error: PhantomData<E>,
+    }
+
+    impl<E> ContentSerializer<E> {
+        pub fn new() -> Self {
+            ContentSerializer { error: PhantomData }
+        }
+    }
+
+    impl<E> Serializer for ContentSerializer<E>
+    where
+        E: ser::Error,
+    {
+        type Ok = Content;
+        type Error = E;
+
+        type SerializeSeq = SerializeSeq<E>;
+        type SerializeTuple = SerializeTuple<E>;
+        type SerializeTupleStruct = SerializeTupleStruct<E>;
+        type SerializeTupleVariant = SerializeTupleVariant<E>;
+        type SerializeMap = SerializeMap<E>;
+        type SerializeStruct = SerializeStruct<E>;
+        type SerializeStructVariant = SerializeStructVariant<E>;
+
+        fn serialize_bool(self, v: bool) -> Result<Content, E> {
+            Ok(Content::Bool(v))
+        }
+
+        fn serialize_i8(self, v: i8) -> Result<Content, E> {
+            Ok(Content::I8(v))
+        }
+
+        fn serialize_i16(self, v: i16) -> Result<Content, E> {
+            Ok(Content::I16(v))
+        }
+
+        fn serialize_i32(self, v: i32) -> Result<Content, E> {
+            Ok(Content::I32(v))
+        }
+
+        fn serialize_i64(self, v: i64) -> Result<Content, E> {
+            Ok(Content::I64(v))
+        }
+
+        fn serialize_u8(self, v: u8) -> Result<Content, E> {
+            Ok(Content::U8(v))
+        }
+
+        fn serialize_u16(self, v: u16) -> Result<Content, E> {
+            Ok(Content::U16(v))
+        }
+
+        fn serialize_u32(self, v: u32) -> Result<Content, E> {
+            Ok(Content::U32(v))
+        }
+
+        fn serialize_u64(self, v: u64) -> Result<Content, E> {
+            Ok(Content::U64(v))
+        }
+
+        fn serialize_f32(self, v: f32) -> Result<Content, E> {
+            Ok(Content::F32(v))
+        }
+
+        fn serialize_f64(self, v: f64) -> Result<Content, E> {
+            Ok(Content::F64(v))
+        }
+
+        fn serialize_char(self, v: char) -> Result<Content, E> {
+            Ok(Content::Char(v))
+        }
+
+        fn serialize_str(self, value: &str) -> Result<Content, E> {
+            Ok(Content::String(value.to_owned()))
+        }
+
+        fn serialize_bytes(self, value: &[u8]) -> Result<Content, E> {
+            Ok(Content::Bytes(value.to_owned()))
+        }
+
+        fn serialize_none(self) -> Result<Content, E> {
+            Ok(Content::None)
+        }
+
+        fn serialize_some<T: ?Sized>(self, value: &T) -> Result<Content, E>
+        where
+            T: Serialize,
+        {
+            Ok(Content::Some(Box::new(try!(value.serialize(self)))))
+        }
+
+        fn serialize_unit(self) -> Result<Content, E> {
+            Ok(Content::Unit)
+        }
+
+        fn serialize_unit_struct(self, name: &'static str) -> Result<Content, E> {
+            Ok(Content::UnitStruct(name))
+        }
+
+        fn serialize_unit_variant(
+            self,
+            name: &'static str,
+            variant_index: u32,
+            variant: &'static str,
+        ) -> Result<Content, E> {
+            Ok(Content::UnitVariant(name, variant_index, variant))
+        }
+
+        fn serialize_newtype_struct<T: ?Sized>(
+            self,
+            name: &'static str,
+            value: &T,
+        ) -> Result<Content, E>
+        where
+            T: Serialize,
+        {
+            Ok(Content::NewtypeStruct(
+                name,
+                Box::new(try!(value.serialize(self))),
+            ))
+        }
+
+        fn serialize_newtype_variant<T: ?Sized>(
+            self,
+            name: &'static str,
+            variant_index: u32,
+            variant: &'static str,
+            value: &T,
+        ) -> Result<Content, E>
+        where
+            T: Serialize,
+        {
+            Ok(Content::NewtypeVariant(
+                name,
+                variant_index,
+                variant,
+                Box::new(try!(value.serialize(self))),
+            ))
+        }
+
+        fn serialize_seq(self, len: Option<usize>) -> Result<Self::SerializeSeq, E> {
+            Ok(SerializeSeq {
+                elements: Vec::with_capacity(len.unwrap_or(0)),
+                error: PhantomData,
+            })
+        }
+
+        fn serialize_tuple(self, len: usize) -> Result<Self::SerializeTuple, E> {
+            Ok(SerializeTuple {
+                elements: Vec::with_capacity(len),
+                error: PhantomData,
+            })
+        }
+
+        fn serialize_tuple_struct(
+            self,
+            name: &'static str,
+            len: usize,
+        ) -> Result<Self::SerializeTupleStruct, E> {
+            Ok(SerializeTupleStruct {
+                name: name,
+                fields: Vec::with_capacity(len),
+                error: PhantomData,
+            })
+        }
+
+        fn serialize_tuple_variant(
+            self,
+            name: &'static str,
+            variant_index: u32,
+            variant: &'static str,
+            len: usize,
+        ) -> Result<Self::SerializeTupleVariant, E> {
+            Ok(SerializeTupleVariant {
+                name: name,
+                variant_index: variant_index,
+                variant: variant,
+                fields: Vec::with_capacity(len),
+                error: PhantomData,
+            })
+        }
+
+        fn serialize_map(self, len: Option<usize>) -> Result<Self::SerializeMap, E> {
+            Ok(SerializeMap {
+                entries: Vec::with_capacity(len.unwrap_or(0)),
+                key: None,
+                error: PhantomData,
+            })
+        }
+
+        fn serialize_struct(
+            self,
+            name: &'static str,
+            len: usize,
+        ) -> Result<Self::SerializeStruct, E> {
+            Ok(SerializeStruct {
+                name: name,
+                fields: Vec::with_capacity(len),
+                error: PhantomData,
+            })
+        }
+
+        fn serialize_struct_variant(
+            self,
+            name: &'static str,
+            variant_index: u32,
+            variant: &'static str,
+            len: usize,
+        ) -> Result<Self::SerializeStructVariant, E> {
+            Ok(SerializeStructVariant {
+                name: name,
+                variant_index: variant_index,
+                variant: variant,
+                fields: Vec::with_capacity(len),
+                error: PhantomData,
+            })
+        }
+    }
+
+    pub struct SerializeSeq<E> {
+        elements: Vec<Content>,
+        error: PhantomData<E>,
+    }
+
+    impl<E> ser::SerializeSeq for SerializeSeq<E>
+    where
+        E: ser::Error,
+    {
+        type Ok = Content;
+        type Error = E;
+
+        fn serialize_element<T: ?Sized>(&mut self, value: &T) -> Result<(), E>
+        where
+            T: Serialize,
+        {
+            let value = try!(value.serialize(ContentSerializer::<E>::new()));
+            self.elements.push(value);
+            Ok(())
+        }
+
+        fn end(self) -> Result<Content, E> {
+            Ok(Content::Seq(self.elements))
+        }
+    }
+
+    pub struct SerializeTuple<E> {
+        elements: Vec<Content>,
+        error: PhantomData<E>,
+    }
+
+    impl<E> ser::SerializeTuple for SerializeTuple<E>
+    where
+        E: ser::Error,
+    {
+        type Ok = Content;
+        type Error = E;
+
+        fn serialize_element<T: ?Sized>(&mut self, value: &T) -> Result<(), E>
+        where
+            T: Serialize,
+        {
+            let value = try!(value.serialize(ContentSerializer::<E>::new()));
+            self.elements.push(value);
+            Ok(())
+        }
+
+        fn end(self) -> Result<Content, E> {
+            Ok(Content::Tuple(self.elements))
+        }
+    }
+
+    pub struct SerializeTupleStruct<E> {
+        name: &'static str,
+        fields: Vec<Content>,
+        error: PhantomData<E>,
+    }
+
+    impl<E> ser::SerializeTupleStruct for SerializeTupleStruct<E>
+    where
+        E: ser::Error,
+    {
+        type Ok = Content;
+        type Error = E;
+
+        fn serialize_field<T: ?Sized>(&mut self, value: &T) -> Result<(), E>
+        where
+            T: Serialize,
+        {
+            let value = try!(value.serialize(ContentSerializer::<E>::new()));
+            self.fields.push(value);
+            Ok(())
+        }
+
+        fn end(self) -> Result<Content, E> {
+            Ok(Content::TupleStruct(self.name, self.fields))
+        }
+    }
+
+    pub struct SerializeTupleVariant<E> {
+        name: &'static str,
+        variant_index: u32,
+        variant: &'static str,
+        fields: Vec<Content>,
+        error: PhantomData<E>,
+    }
+
+    impl<E> ser::SerializeTupleVariant for SerializeTupleVariant<E>
+    where
+        E: ser::Error,
+    {
+        type Ok = Content;
+        type Error = E;
+
+        fn serialize_field<T: ?Sized>(&mut self, value: &T) -> Result<(), E>
+        where
+            T: Serialize,
+        {
+            let value = try!(value.serialize(ContentSerializer::<E>::new()));
+            self.fields.push(value);
+            Ok(())
+        }
+
+        fn end(self) -> Result<Content, E> {
+            Ok(Content::TupleVariant(
+                self.name,
+                self.variant_index,
+                self.variant,
+                self.fields,
+            ))
+        }
+    }
+
+    pub struct SerializeMap<E> {
+        entries: Vec<(Content, Content)>,
+        key: Option<Content>,
+        error: PhantomData<E>,
+    }
+
+    impl<E> ser::SerializeMap for SerializeMap<E>
+    where
+        E: ser::Error,
+    {
+        type Ok = Content;
+        type Error = E;
+
+        fn serialize_key<T: ?Sized>(&mut self, key: &T) -> Result<(), E>
+        where
+            T: Serialize,
+        {
+            let key = try!(key.serialize(ContentSerializer::<E>::new()));
+            self.key = Some(key);
+            Ok(())
+        }
+
+        fn serialize_value<T: ?Sized>(&mut self, value: &T) -> Result<(), E>
+        where
+            T: Serialize,
+        {
+            let key = self
+                .key
+                .take()
+                .expect("serialize_value called before serialize_key");
+            let value = try!(value.serialize(ContentSerializer::<E>::new()));
+            self.entries.push((key, value));
+            Ok(())
+        }
+
+        fn end(self) -> Result<Content, E> {
+            Ok(Content::Map(self.entries))
+        }
+
+        fn serialize_entry<K: ?Sized, V: ?Sized>(&mut self, key: &K, value: &V) -> Result<(), E>
+        where
+            K: Serialize,
+            V: Serialize,
+        {
+            let key = try!(key.serialize(ContentSerializer::<E>::new()));
+            let value = try!(value.serialize(ContentSerializer::<E>::new()));
+            self.entries.push((key, value));
+            Ok(())
+        }
+    }
+
+    pub struct SerializeStruct<E> {
+        name: &'static str,
+        fields: Vec<(&'static str, Content)>,
+        error: PhantomData<E>,
+    }
+
+    impl<E> ser::SerializeStruct for SerializeStruct<E>
+    where
+        E: ser::Error,
+    {
+        type Ok = Content;
+        type Error = E;
+
+        fn serialize_field<T: ?Sized>(&mut self, key: &'static str, value: &T) -> Result<(), E>
+        where
+            T: Serialize,
+        {
+            let value = try!(value.serialize(ContentSerializer::<E>::new()));
+            self.fields.push((key, value));
+            Ok(())
+        }
+
+        fn end(self) -> Result<Content, E> {
+            Ok(Content::Struct(self.name, self.fields))
+        }
+    }
+
+    pub struct SerializeStructVariant<E> {
+        name: &'static str,
+        variant_index: u32,
+        variant: &'static str,
+        fields: Vec<(&'static str, Content)>,
+        error: PhantomData<E>,
+    }
+
+    impl<E> ser::SerializeStructVariant for SerializeStructVariant<E>
+    where
+        E: ser::Error,
+    {
+        type Ok = Content;
+        type Error = E;
+
+        fn serialize_field<T: ?Sized>(&mut self, key: &'static str, value: &T) -> Result<(), E>
+        where
+            T: Serialize,
+        {
+            let value = try!(value.serialize(ContentSerializer::<E>::new()));
+            self.fields.push((key, value));
+            Ok(())
+        }
+
+        fn end(self) -> Result<Content, E> {
+            Ok(Content::StructVariant(
+                self.name,
+                self.variant_index,
+                self.variant,
+                self.fields,
+            ))
+        }
+    }
+}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+pub struct FlatMapSerializer<'a, M: 'a>(pub &'a mut M);
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+impl<'a, M> FlatMapSerializer<'a, M>
+where
+    M: SerializeMap + 'a,
+{
+    fn bad_type(what: Unsupported) -> M::Error {
+        ser::Error::custom(format_args!(
+            "can only flatten structs and maps (got {})",
+            what
+        ))
+    }
+}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+impl<'a, M> Serializer for FlatMapSerializer<'a, M>
+where
+    M: SerializeMap + 'a,
+{
+    type Ok = ();
+    type Error = M::Error;
+
+    type SerializeSeq = Impossible<Self::Ok, M::Error>;
+    type SerializeTuple = Impossible<Self::Ok, M::Error>;
+    type SerializeTupleStruct = Impossible<Self::Ok, M::Error>;
+    type SerializeMap = FlatMapSerializeMap<'a, M>;
+    type SerializeStruct = FlatMapSerializeStruct<'a, M>;
+    type SerializeTupleVariant = Impossible<Self::Ok, M::Error>;
+    type SerializeStructVariant = FlatMapSerializeStructVariantAsMapValue<'a, M>;
+
+    fn serialize_bool(self, _: bool) -> Result<Self::Ok, Self::Error> {
+        Err(Self::bad_type(Unsupported::Boolean))
+    }
+
+    fn serialize_i8(self, _: i8) -> Result<Self::Ok, Self::Error> {
+        Err(Self::bad_type(Unsupported::Integer))
+    }
+
+    fn serialize_i16(self, _: i16) -> Result<Self::Ok, Self::Error> {
+        Err(Self::bad_type(Unsupported::Integer))
+    }
+
+    fn serialize_i32(self, _: i32) -> Result<Self::Ok, Self::Error> {
+        Err(Self::bad_type(Unsupported::Integer))
+    }
+
+    fn serialize_i64(self, _: i64) -> Result<Self::Ok, Self::Error> {
+        Err(Self::bad_type(Unsupported::Integer))
+    }
+
+    fn serialize_u8(self, _: u8) -> Result<Self::Ok, Self::Error> {
+        Err(Self::bad_type(Unsupported::Integer))
+    }
+
+    fn serialize_u16(self, _: u16) -> Result<Self::Ok, Self::Error> {
+        Err(Self::bad_type(Unsupported::Integer))
+    }
+
+    fn serialize_u32(self, _: u32) -> Result<Self::Ok, Self::Error> {
+        Err(Self::bad_type(Unsupported::Integer))
+    }
+
+    fn serialize_u64(self, _: u64) -> Result<Self::Ok, Self::Error> {
+        Err(Self::bad_type(Unsupported::Integer))
+    }
+
+    fn serialize_f32(self, _: f32) -> Result<Self::Ok, Self::Error> {
+        Err(Self::bad_type(Unsupported::Float))
+    }
+
+    fn serialize_f64(self, _: f64) -> Result<Self::Ok, Self::Error> {
+        Err(Self::bad_type(Unsupported::Float))
+    }
+
+    fn serialize_char(self, _: char) -> Result<Self::Ok, Self::Error> {
+        Err(Self::bad_type(Unsupported::Char))
+    }
+
+    fn serialize_str(self, _: &str) -> Result<Self::Ok, Self::Error> {
+        Err(Self::bad_type(Unsupported::String))
+    }
+
+    fn serialize_bytes(self, _: &[u8]) -> Result<Self::Ok, Self::Error> {
+        Err(Self::bad_type(Unsupported::ByteArray))
+    }
+
+    fn serialize_none(self) -> Result<Self::Ok, Self::Error> {
+        Ok(())
+    }
+
+    fn serialize_some<T: ?Sized>(self, value: &T) -> Result<Self::Ok, Self::Error>
+    where
+        T: Serialize,
+    {
+        value.serialize(self)
+    }
+
+    fn serialize_unit(self) -> Result<Self::Ok, Self::Error> {
+        Ok(())
+    }
+
+    fn serialize_unit_struct(self, _: &'static str) -> Result<Self::Ok, Self::Error> {
+        Err(Self::bad_type(Unsupported::UnitStruct))
+    }
+
+    fn serialize_unit_variant(
+        self,
+        _: &'static str,
+        _: u32,
+        _: &'static str,
+    ) -> Result<Self::Ok, Self::Error> {
+        Err(Self::bad_type(Unsupported::Enum))
+    }
+
+    fn serialize_newtype_struct<T: ?Sized>(
+        self,
+        _: &'static str,
+        value: &T,
+    ) -> Result<Self::Ok, Self::Error>
+    where
+        T: Serialize,
+    {
+        value.serialize(self)
+    }
+
+    fn serialize_newtype_variant<T: ?Sized>(
+        self,
+        _: &'static str,
+        _: u32,
+        variant: &'static str,
+        value: &T,
+    ) -> Result<Self::Ok, Self::Error>
+    where
+        T: Serialize,
+    {
+        try!(self.0.serialize_key(variant));
+        self.0.serialize_value(value)
+    }
+
+    fn serialize_seq(self, _: Option<usize>) -> Result<Self::SerializeSeq, Self::Error> {
+        Err(Self::bad_type(Unsupported::Sequence))
+    }
+
+    fn serialize_tuple(self, _: usize) -> Result<Self::SerializeTuple, Self::Error> {
+        Err(Self::bad_type(Unsupported::Tuple))
+    }
+
+    fn serialize_tuple_struct(
+        self,
+        _: &'static str,
+        _: usize,
+    ) -> Result<Self::SerializeTupleStruct, Self::Error> {
+        Err(Self::bad_type(Unsupported::TupleStruct))
+    }
+
+    fn serialize_tuple_variant(
+        self,
+        _: &'static str,
+        _: u32,
+        _: &'static str,
+        _: usize,
+    ) -> Result<Self::SerializeTupleVariant, Self::Error> {
+        Err(Self::bad_type(Unsupported::Enum))
+    }
+
+    fn serialize_map(self, _: Option<usize>) -> Result<Self::SerializeMap, Self::Error> {
+        Ok(FlatMapSerializeMap(self.0))
+    }
+
+    fn serialize_struct(
+        self,
+        _: &'static str,
+        _: usize,
+    ) -> Result<Self::SerializeStruct, Self::Error> {
+        Ok(FlatMapSerializeStruct(self.0))
+    }
+
+    fn serialize_struct_variant(
+        self,
+        _: &'static str,
+        _: u32,
+        inner_variant: &'static str,
+        _: usize,
+    ) -> Result<Self::SerializeStructVariant, Self::Error> {
+        try!(self.0.serialize_key(inner_variant));
+        Ok(FlatMapSerializeStructVariantAsMapValue::new(
+            self.0,
+            inner_variant,
+        ))
+    }
+}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+pub struct FlatMapSerializeMap<'a, M: 'a>(&'a mut M);
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+impl<'a, M> ser::SerializeMap for FlatMapSerializeMap<'a, M>
+where
+    M: SerializeMap + 'a,
+{
+    type Ok = ();
+    type Error = M::Error;
+
+    fn serialize_key<T: ?Sized>(&mut self, key: &T) -> Result<(), Self::Error>
+    where
+        T: Serialize,
+    {
+        self.0.serialize_key(key)
+    }
+
+    fn serialize_value<T: ?Sized>(&mut self, value: &T) -> Result<(), Self::Error>
+    where
+        T: Serialize,
+    {
+        self.0.serialize_value(value)
+    }
+
+    fn serialize_entry<K: ?Sized, V: ?Sized>(
+        &mut self,
+        key: &K,
+        value: &V,
+    ) -> Result<(), Self::Error>
+    where
+        K: Serialize,
+        V: Serialize,
+    {
+        self.0.serialize_entry(key, value)
+    }
+
+    fn end(self) -> Result<(), Self::Error> {
+        Ok(())
+    }
+}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+pub struct FlatMapSerializeStruct<'a, M: 'a>(&'a mut M);
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+impl<'a, M> ser::SerializeStruct for FlatMapSerializeStruct<'a, M>
+where
+    M: SerializeMap + 'a,
+{
+    type Ok = ();
+    type Error = M::Error;
+
+    fn serialize_field<T: ?Sized>(
+        &mut self,
+        key: &'static str,
+        value: &T,
+    ) -> Result<(), Self::Error>
+    where
+        T: Serialize,
+    {
+        self.0.serialize_entry(key, value)
+    }
+
+    fn end(self) -> Result<(), Self::Error> {
+        Ok(())
+    }
+}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+pub struct FlatMapSerializeStructVariantAsMapValue<'a, M: 'a> {
+    map: &'a mut M,
+    name: &'static str,
+    fields: Vec<(&'static str, Content)>,
+}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+impl<'a, M> FlatMapSerializeStructVariantAsMapValue<'a, M>
+where
+    M: SerializeMap + 'a,
+{
+    fn new(map: &'a mut M, name: &'static str) -> FlatMapSerializeStructVariantAsMapValue<'a, M> {
+        FlatMapSerializeStructVariantAsMapValue {
+            map: map,
+            name: name,
+            fields: Vec::new(),
+        }
+    }
+}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+impl<'a, M> ser::SerializeStructVariant for FlatMapSerializeStructVariantAsMapValue<'a, M>
+where
+    M: SerializeMap + 'a,
+{
+    type Ok = ();
+    type Error = M::Error;
+
+    fn serialize_field<T: ?Sized>(
+        &mut self,
+        key: &'static str,
+        value: &T,
+    ) -> Result<(), Self::Error>
+    where
+        T: Serialize,
+    {
+        let value = try!(value.serialize(ContentSerializer::<M::Error>::new()));
+        self.fields.push((key, value));
+        Ok(())
+    }
+
+    fn end(self) -> Result<(), Self::Error> {
+        try!(self
+            .map
+            .serialize_value(&Content::Struct(self.name, self.fields)));
+        Ok(())
+    }
+}
diff --git a/src/ser/fmt.rs b/src/ser/fmt.rs
new file mode 100644
index 0000000..e7e09a1
--- /dev/null
+++ b/src/ser/fmt.rs
@@ -0,0 +1,174 @@
+use lib::*;
+use ser::{Error, Impossible, Serialize, Serializer};
+
+impl Error for fmt::Error {
+    fn custom<T: Display>(_msg: T) -> Self {
+        fmt::Error
+    }
+}
+
+macro_rules! fmt_primitives {
+    ($($f:ident: $t:ty,)*) => {
+        $(
+            fn $f(self, v: $t) -> fmt::Result {
+                Display::fmt(&v, self)
+            }
+        )*
+    };
+}
+
+/// ```edition2018
+/// use serde::Serialize;
+/// use std::fmt::{self, Display};
+///
+/// #[derive(Serialize)]
+/// #[serde(rename_all = "kebab-case")]
+/// pub enum MessageType {
+///     StartRequest,
+///     EndRequest,
+/// }
+///
+/// impl Display for MessageType {
+///     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+///         self.serialize(f)
+///     }
+/// }
+/// ```
+impl<'a, 'b> Serializer for &'a mut fmt::Formatter<'b> {
+    type Ok = ();
+    type Error = fmt::Error;
+    type SerializeSeq = Impossible<(), fmt::Error>;
+    type SerializeTuple = Impossible<(), fmt::Error>;
+    type SerializeTupleStruct = Impossible<(), fmt::Error>;
+    type SerializeTupleVariant = Impossible<(), fmt::Error>;
+    type SerializeMap = Impossible<(), fmt::Error>;
+    type SerializeStruct = Impossible<(), fmt::Error>;
+    type SerializeStructVariant = Impossible<(), fmt::Error>;
+
+    fmt_primitives! {
+        serialize_bool: bool,
+        serialize_i8: i8,
+        serialize_i16: i16,
+        serialize_i32: i32,
+        serialize_i64: i64,
+        serialize_u8: u8,
+        serialize_u16: u16,
+        serialize_u32: u32,
+        serialize_u64: u64,
+        serialize_f32: f32,
+        serialize_f64: f64,
+        serialize_char: char,
+        serialize_str: &str,
+        serialize_unit_struct: &'static str,
+    }
+
+    serde_if_integer128! {
+        fmt_primitives! {
+            serialize_i128: i128,
+            serialize_u128: u128,
+        }
+    }
+
+    fn serialize_unit_variant(
+        self,
+        _name: &'static str,
+        _variant_index: u32,
+        variant: &'static str,
+    ) -> fmt::Result {
+        Display::fmt(variant, self)
+    }
+
+    fn serialize_newtype_struct<T: ?Sized>(self, _name: &'static str, value: &T) -> fmt::Result
+    where
+        T: Serialize,
+    {
+        Serialize::serialize(value, self)
+    }
+
+    fn serialize_bytes(self, _v: &[u8]) -> fmt::Result {
+        Err(fmt::Error)
+    }
+
+    fn serialize_none(self) -> fmt::Result {
+        Err(fmt::Error)
+    }
+
+    fn serialize_some<T: ?Sized>(self, _value: &T) -> fmt::Result
+    where
+        T: Serialize,
+    {
+        Err(fmt::Error)
+    }
+
+    fn serialize_unit(self) -> fmt::Result {
+        Err(fmt::Error)
+    }
+
+    fn serialize_newtype_variant<T: ?Sized>(
+        self,
+        _name: &'static str,
+        _variant_index: u32,
+        _variant: &'static str,
+        _value: &T,
+    ) -> fmt::Result
+    where
+        T: Serialize,
+    {
+        Err(fmt::Error)
+    }
+
+    fn serialize_seq(self, _len: Option<usize>) -> Result<Self::SerializeSeq, fmt::Error> {
+        Err(fmt::Error)
+    }
+
+    fn serialize_tuple(self, _len: usize) -> Result<Self::SerializeTuple, fmt::Error> {
+        Err(fmt::Error)
+    }
+
+    fn serialize_tuple_struct(
+        self,
+        _name: &'static str,
+        _len: usize,
+    ) -> Result<Self::SerializeTupleStruct, fmt::Error> {
+        Err(fmt::Error)
+    }
+
+    fn serialize_tuple_variant(
+        self,
+        _name: &'static str,
+        _variant_index: u32,
+        _variant: &'static str,
+        _len: usize,
+    ) -> Result<Self::SerializeTupleVariant, fmt::Error> {
+        Err(fmt::Error)
+    }
+
+    fn serialize_map(self, _len: Option<usize>) -> Result<Self::SerializeMap, fmt::Error> {
+        Err(fmt::Error)
+    }
+
+    fn serialize_struct(
+        self,
+        _name: &'static str,
+        _len: usize,
+    ) -> Result<Self::SerializeStruct, fmt::Error> {
+        Err(fmt::Error)
+    }
+
+    fn serialize_struct_variant(
+        self,
+        _name: &'static str,
+        _variant_index: u32,
+        _variant: &'static str,
+        _len: usize,
+    ) -> Result<Self::SerializeStructVariant, fmt::Error> {
+        Err(fmt::Error)
+    }
+
+    fn collect_str<T: ?Sized>(self, value: &T) -> fmt::Result
+    where
+        T: Display,
+    {
+        Display::fmt(value, self)
+    }
+}
diff --git a/src/ser/impls.rs b/src/ser/impls.rs
new file mode 100644
index 0000000..0dc8085
--- /dev/null
+++ b/src/ser/impls.rs
@@ -0,0 +1,883 @@
+use lib::*;
+
+use ser::{Error, Serialize, SerializeTuple, Serializer};
+
+////////////////////////////////////////////////////////////////////////////////
+
+macro_rules! primitive_impl {
+    ($ty:ident, $method:ident $($cast:tt)*) => {
+        impl Serialize for $ty {
+            #[inline]
+            fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+            where
+                S: Serializer,
+            {
+                serializer.$method(*self $($cast)*)
+            }
+        }
+    }
+}
+
+primitive_impl!(bool, serialize_bool);
+primitive_impl!(isize, serialize_i64 as i64);
+primitive_impl!(i8, serialize_i8);
+primitive_impl!(i16, serialize_i16);
+primitive_impl!(i32, serialize_i32);
+primitive_impl!(i64, serialize_i64);
+primitive_impl!(usize, serialize_u64 as u64);
+primitive_impl!(u8, serialize_u8);
+primitive_impl!(u16, serialize_u16);
+primitive_impl!(u32, serialize_u32);
+primitive_impl!(u64, serialize_u64);
+primitive_impl!(f32, serialize_f32);
+primitive_impl!(f64, serialize_f64);
+primitive_impl!(char, serialize_char);
+
+serde_if_integer128! {
+    primitive_impl!(i128, serialize_i128);
+    primitive_impl!(u128, serialize_u128);
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+impl Serialize for str {
+    #[inline]
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        serializer.serialize_str(self)
+    }
+}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+impl Serialize for String {
+    #[inline]
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        serializer.serialize_str(self)
+    }
+}
+
+impl<'a> Serialize for fmt::Arguments<'a> {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        serializer.collect_str(self)
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+#[cfg(feature = "std")]
+impl Serialize for CStr {
+    #[inline]
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        serializer.serialize_bytes(self.to_bytes())
+    }
+}
+
+#[cfg(feature = "std")]
+impl Serialize for CString {
+    #[inline]
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        serializer.serialize_bytes(self.to_bytes())
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+impl<T> Serialize for Option<T>
+where
+    T: Serialize,
+{
+    #[inline]
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        match *self {
+            Some(ref value) => serializer.serialize_some(value),
+            None => serializer.serialize_none(),
+        }
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+impl<T: ?Sized> Serialize for PhantomData<T> {
+    #[inline]
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        serializer.serialize_unit_struct("PhantomData")
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+// Does not require T: Serialize.
+impl<T> Serialize for [T; 0] {
+    #[inline]
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        try!(serializer.serialize_tuple(0)).end()
+    }
+}
+
+macro_rules! array_impls {
+    ($($len:tt)+) => {
+        $(
+            impl<T> Serialize for [T; $len]
+            where
+                T: Serialize,
+            {
+                #[inline]
+                fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+                where
+                    S: Serializer,
+                {
+                    let mut seq = try!(serializer.serialize_tuple($len));
+                    for e in self {
+                        try!(seq.serialize_element(e));
+                    }
+                    seq.end()
+                }
+            }
+        )+
+    }
+}
+
+array_impls! {
+    01 02 03 04 05 06 07 08 09 10
+    11 12 13 14 15 16 17 18 19 20
+    21 22 23 24 25 26 27 28 29 30
+    31 32
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+impl<T> Serialize for [T]
+where
+    T: Serialize,
+{
+    #[inline]
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        serializer.collect_seq(self)
+    }
+}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+macro_rules! seq_impl {
+    ($ty:ident < T $(: $tbound1:ident $(+ $tbound2:ident)*)* $(, $typaram:ident : $bound:ident)* >) => {
+        impl<T $(, $typaram)*> Serialize for $ty<T $(, $typaram)*>
+        where
+            T: Serialize $(+ $tbound1 $(+ $tbound2)*)*,
+            $($typaram: $bound,)*
+        {
+            #[inline]
+            fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+            where
+                S: Serializer,
+            {
+                serializer.collect_seq(self)
+            }
+        }
+    }
+}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+seq_impl!(BinaryHeap<T: Ord>);
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+seq_impl!(BTreeSet<T: Ord>);
+
+#[cfg(feature = "std")]
+seq_impl!(HashSet<T: Eq + Hash, H: BuildHasher>);
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+seq_impl!(LinkedList<T>);
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+seq_impl!(Vec<T>);
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+seq_impl!(VecDeque<T>);
+
+////////////////////////////////////////////////////////////////////////////////
+
+impl<Idx> Serialize for Range<Idx>
+where
+    Idx: Serialize,
+{
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        use super::SerializeStruct;
+        let mut state = try!(serializer.serialize_struct("Range", 2));
+        try!(state.serialize_field("start", &self.start));
+        try!(state.serialize_field("end", &self.end));
+        state.end()
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+#[cfg(range_inclusive)]
+impl<Idx> Serialize for RangeInclusive<Idx>
+where
+    Idx: Serialize,
+{
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        use super::SerializeStruct;
+        let mut state = try!(serializer.serialize_struct("RangeInclusive", 2));
+        try!(state.serialize_field("start", &self.start()));
+        try!(state.serialize_field("end", &self.end()));
+        state.end()
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+#[cfg(any(ops_bound, collections_bound))]
+impl<T> Serialize for Bound<T>
+where
+    T: Serialize,
+{
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        match *self {
+            Bound::Unbounded => serializer.serialize_unit_variant("Bound", 0, "Unbounded"),
+            Bound::Included(ref value) => {
+                serializer.serialize_newtype_variant("Bound", 1, "Included", value)
+            }
+            Bound::Excluded(ref value) => {
+                serializer.serialize_newtype_variant("Bound", 2, "Excluded", value)
+            }
+        }
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+impl Serialize for () {
+    #[inline]
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        serializer.serialize_unit()
+    }
+}
+
+#[cfg(feature = "unstable")]
+impl Serialize for ! {
+    fn serialize<S>(&self, _serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        *self
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+macro_rules! tuple_impls {
+    ($($len:expr => ($($n:tt $name:ident)+))+) => {
+        $(
+            impl<$($name),+> Serialize for ($($name,)+)
+            where
+                $($name: Serialize,)+
+            {
+                #[inline]
+                fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+                where
+                    S: Serializer,
+                {
+                    let mut tuple = try!(serializer.serialize_tuple($len));
+                    $(
+                        try!(tuple.serialize_element(&self.$n));
+                    )+
+                    tuple.end()
+                }
+            }
+        )+
+    }
+}
+
+tuple_impls! {
+    1 => (0 T0)
+    2 => (0 T0 1 T1)
+    3 => (0 T0 1 T1 2 T2)
+    4 => (0 T0 1 T1 2 T2 3 T3)
+    5 => (0 T0 1 T1 2 T2 3 T3 4 T4)
+    6 => (0 T0 1 T1 2 T2 3 T3 4 T4 5 T5)
+    7 => (0 T0 1 T1 2 T2 3 T3 4 T4 5 T5 6 T6)
+    8 => (0 T0 1 T1 2 T2 3 T3 4 T4 5 T5 6 T6 7 T7)
+    9 => (0 T0 1 T1 2 T2 3 T3 4 T4 5 T5 6 T6 7 T7 8 T8)
+    10 => (0 T0 1 T1 2 T2 3 T3 4 T4 5 T5 6 T6 7 T7 8 T8 9 T9)
+    11 => (0 T0 1 T1 2 T2 3 T3 4 T4 5 T5 6 T6 7 T7 8 T8 9 T9 10 T10)
+    12 => (0 T0 1 T1 2 T2 3 T3 4 T4 5 T5 6 T6 7 T7 8 T8 9 T9 10 T10 11 T11)
+    13 => (0 T0 1 T1 2 T2 3 T3 4 T4 5 T5 6 T6 7 T7 8 T8 9 T9 10 T10 11 T11 12 T12)
+    14 => (0 T0 1 T1 2 T2 3 T3 4 T4 5 T5 6 T6 7 T7 8 T8 9 T9 10 T10 11 T11 12 T12 13 T13)
+    15 => (0 T0 1 T1 2 T2 3 T3 4 T4 5 T5 6 T6 7 T7 8 T8 9 T9 10 T10 11 T11 12 T12 13 T13 14 T14)
+    16 => (0 T0 1 T1 2 T2 3 T3 4 T4 5 T5 6 T6 7 T7 8 T8 9 T9 10 T10 11 T11 12 T12 13 T13 14 T14 15 T15)
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+macro_rules! map_impl {
+    ($ty:ident < K $(: $kbound1:ident $(+ $kbound2:ident)*)*, V $(, $typaram:ident : $bound:ident)* >) => {
+        impl<K, V $(, $typaram)*> Serialize for $ty<K, V $(, $typaram)*>
+        where
+            K: Serialize $(+ $kbound1 $(+ $kbound2)*)*,
+            V: Serialize,
+            $($typaram: $bound,)*
+        {
+            #[inline]
+            fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+            where
+                S: Serializer,
+            {
+                serializer.collect_map(self)
+            }
+        }
+    }
+}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+map_impl!(BTreeMap<K: Ord, V>);
+
+#[cfg(feature = "std")]
+map_impl!(HashMap<K: Eq + Hash, V, H: BuildHasher>);
+
+////////////////////////////////////////////////////////////////////////////////
+
+macro_rules! deref_impl {
+    (
+        $(#[doc = $doc:tt])*
+        <$($desc:tt)+
+    ) => {
+        $(#[doc = $doc])*
+        impl <$($desc)+ {
+            #[inline]
+            fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+            where
+                S: Serializer,
+            {
+                (**self).serialize(serializer)
+            }
+        }
+    };
+}
+
+deref_impl!(<'a, T: ?Sized> Serialize for &'a T where T: Serialize);
+deref_impl!(<'a, T: ?Sized> Serialize for &'a mut T where T: Serialize);
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+deref_impl!(<T: ?Sized> Serialize for Box<T> where T: Serialize);
+
+#[cfg(all(feature = "rc", any(feature = "std", feature = "alloc")))]
+deref_impl! {
+    /// This impl requires the [`"rc"`] Cargo feature of Serde.
+    ///
+    /// Serializing a data structure containing `Rc` will serialize a copy of
+    /// the contents of the `Rc` each time the `Rc` is referenced within the
+    /// data structure. Serialization will not attempt to deduplicate these
+    /// repeated data.
+    ///
+    /// [`"rc"`]: https://serde.rs/feature-flags.html#-features-rc
+    <T: ?Sized> Serialize for Rc<T> where T: Serialize
+}
+
+#[cfg(all(feature = "rc", any(feature = "std", feature = "alloc")))]
+deref_impl! {
+    /// This impl requires the [`"rc"`] Cargo feature of Serde.
+    ///
+    /// Serializing a data structure containing `Arc` will serialize a copy of
+    /// the contents of the `Arc` each time the `Arc` is referenced within the
+    /// data structure. Serialization will not attempt to deduplicate these
+    /// repeated data.
+    ///
+    /// [`"rc"`]: https://serde.rs/feature-flags.html#-features-rc
+    <T: ?Sized> Serialize for Arc<T> where T: Serialize
+}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+deref_impl!(<'a, T: ?Sized> Serialize for Cow<'a, T> where T: Serialize + ToOwned);
+
+////////////////////////////////////////////////////////////////////////////////
+
+/// This impl requires the [`"rc"`] Cargo feature of Serde.
+///
+/// [`"rc"`]: https://serde.rs/feature-flags.html#-features-rc
+#[cfg(all(feature = "rc", any(feature = "std", feature = "alloc")))]
+impl<T: ?Sized> Serialize for RcWeak<T>
+where
+    T: Serialize,
+{
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        self.upgrade().serialize(serializer)
+    }
+}
+
+/// This impl requires the [`"rc"`] Cargo feature of Serde.
+///
+/// [`"rc"`]: https://serde.rs/feature-flags.html#-features-rc
+#[cfg(all(feature = "rc", any(feature = "std", feature = "alloc")))]
+impl<T: ?Sized> Serialize for ArcWeak<T>
+where
+    T: Serialize,
+{
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        self.upgrade().serialize(serializer)
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+macro_rules! nonzero_integers {
+    ( $( $T: ident, )+ ) => {
+        $(
+            #[cfg(num_nonzero)]
+            impl Serialize for num::$T {
+                fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+                where
+                    S: Serializer,
+                {
+                    self.get().serialize(serializer)
+                }
+            }
+        )+
+    }
+}
+
+nonzero_integers! {
+    NonZeroU8,
+    NonZeroU16,
+    NonZeroU32,
+    NonZeroU64,
+    NonZeroUsize,
+}
+
+#[cfg(num_nonzero_signed)]
+nonzero_integers! {
+    NonZeroI8,
+    NonZeroI16,
+    NonZeroI32,
+    NonZeroI64,
+    NonZeroIsize,
+}
+
+// Currently 128-bit integers do not work on Emscripten targets so we need an
+// additional `#[cfg]`
+serde_if_integer128! {
+    nonzero_integers! {
+        NonZeroU128,
+    }
+
+    #[cfg(num_nonzero_signed)]
+    nonzero_integers! {
+        NonZeroI128,
+    }
+}
+
+impl<T> Serialize for Cell<T>
+where
+    T: Serialize + Copy,
+{
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        self.get().serialize(serializer)
+    }
+}
+
+impl<T> Serialize for RefCell<T>
+where
+    T: Serialize,
+{
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        match self.try_borrow() {
+            Ok(value) => value.serialize(serializer),
+            Err(_) => Err(S::Error::custom("already mutably borrowed")),
+        }
+    }
+}
+
+#[cfg(feature = "std")]
+impl<T> Serialize for Mutex<T>
+where
+    T: Serialize,
+{
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        match self.lock() {
+            Ok(locked) => locked.serialize(serializer),
+            Err(_) => Err(S::Error::custom("lock poison error while serializing")),
+        }
+    }
+}
+
+#[cfg(feature = "std")]
+impl<T> Serialize for RwLock<T>
+where
+    T: Serialize,
+{
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        match self.read() {
+            Ok(locked) => locked.serialize(serializer),
+            Err(_) => Err(S::Error::custom("lock poison error while serializing")),
+        }
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+impl<T, E> Serialize for Result<T, E>
+where
+    T: Serialize,
+    E: Serialize,
+{
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        match *self {
+            Result::Ok(ref value) => serializer.serialize_newtype_variant("Result", 0, "Ok", value),
+            Result::Err(ref value) => {
+                serializer.serialize_newtype_variant("Result", 1, "Err", value)
+            }
+        }
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+#[cfg(any(core_duration, feature = "std"))]
+impl Serialize for Duration {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        use super::SerializeStruct;
+        let mut state = try!(serializer.serialize_struct("Duration", 2));
+        try!(state.serialize_field("secs", &self.as_secs()));
+        try!(state.serialize_field("nanos", &self.subsec_nanos()));
+        state.end()
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+#[cfg(feature = "std")]
+impl Serialize for SystemTime {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        use super::SerializeStruct;
+        let duration_since_epoch = self
+            .duration_since(UNIX_EPOCH)
+            .map_err(|_| S::Error::custom("SystemTime must be later than UNIX_EPOCH"))?;
+        let mut state = try!(serializer.serialize_struct("SystemTime", 2));
+        try!(state.serialize_field("secs_since_epoch", &duration_since_epoch.as_secs()));
+        try!(state.serialize_field("nanos_since_epoch", &duration_since_epoch.subsec_nanos()));
+        state.end()
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+/// Serialize a value that implements `Display` as a string, when that string is
+/// statically known to never have more than a constant `MAX_LEN` bytes.
+///
+/// Panics if the `Display` impl tries to write more than `MAX_LEN` bytes.
+#[cfg(feature = "std")]
+macro_rules! serialize_display_bounded_length {
+    ($value:expr, $max:expr, $serializer:expr) => {{
+        let mut buffer = [0u8; $max];
+        let remaining_len = {
+            let mut remaining = &mut buffer[..];
+            write!(remaining, "{}", $value).unwrap();
+            remaining.len()
+        };
+        let written_len = buffer.len() - remaining_len;
+        let written = &buffer[..written_len];
+
+        // write! only provides fmt::Formatter to Display implementations, which
+        // has methods write_str and write_char but no method to write arbitrary
+        // bytes. Therefore `written` must be valid UTF-8.
+        let written_str = str::from_utf8(written).expect("must be valid UTF-8");
+        $serializer.serialize_str(written_str)
+    }};
+}
+
+#[cfg(feature = "std")]
+impl Serialize for net::IpAddr {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        if serializer.is_human_readable() {
+            match *self {
+                net::IpAddr::V4(ref a) => a.serialize(serializer),
+                net::IpAddr::V6(ref a) => a.serialize(serializer),
+            }
+        } else {
+            match *self {
+                net::IpAddr::V4(ref a) => {
+                    serializer.serialize_newtype_variant("IpAddr", 0, "V4", a)
+                }
+                net::IpAddr::V6(ref a) => {
+                    serializer.serialize_newtype_variant("IpAddr", 1, "V6", a)
+                }
+            }
+        }
+    }
+}
+
+#[cfg(feature = "std")]
+impl Serialize for net::Ipv4Addr {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        if serializer.is_human_readable() {
+            const MAX_LEN: usize = 15;
+            debug_assert_eq!(MAX_LEN, "101.102.103.104".len());
+            serialize_display_bounded_length!(self, MAX_LEN, serializer)
+        } else {
+            self.octets().serialize(serializer)
+        }
+    }
+}
+
+#[cfg(feature = "std")]
+impl Serialize for net::Ipv6Addr {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        if serializer.is_human_readable() {
+            const MAX_LEN: usize = 39;
+            debug_assert_eq!(MAX_LEN, "1001:1002:1003:1004:1005:1006:1007:1008".len());
+            serialize_display_bounded_length!(self, MAX_LEN, serializer)
+        } else {
+            self.octets().serialize(serializer)
+        }
+    }
+}
+
+#[cfg(feature = "std")]
+impl Serialize for net::SocketAddr {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        if serializer.is_human_readable() {
+            match *self {
+                net::SocketAddr::V4(ref addr) => addr.serialize(serializer),
+                net::SocketAddr::V6(ref addr) => addr.serialize(serializer),
+            }
+        } else {
+            match *self {
+                net::SocketAddr::V4(ref addr) => {
+                    serializer.serialize_newtype_variant("SocketAddr", 0, "V4", addr)
+                }
+                net::SocketAddr::V6(ref addr) => {
+                    serializer.serialize_newtype_variant("SocketAddr", 1, "V6", addr)
+                }
+            }
+        }
+    }
+}
+
+#[cfg(feature = "std")]
+impl Serialize for net::SocketAddrV4 {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        if serializer.is_human_readable() {
+            const MAX_LEN: usize = 21;
+            debug_assert_eq!(MAX_LEN, "101.102.103.104:65000".len());
+            serialize_display_bounded_length!(self, MAX_LEN, serializer)
+        } else {
+            (self.ip(), self.port()).serialize(serializer)
+        }
+    }
+}
+
+#[cfg(feature = "std")]
+impl Serialize for net::SocketAddrV6 {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        if serializer.is_human_readable() {
+            const MAX_LEN: usize = 47;
+            debug_assert_eq!(
+                MAX_LEN,
+                "[1001:1002:1003:1004:1005:1006:1007:1008]:65000".len()
+            );
+            serialize_display_bounded_length!(self, MAX_LEN, serializer)
+        } else {
+            (self.ip(), self.port()).serialize(serializer)
+        }
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+#[cfg(feature = "std")]
+impl Serialize for Path {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        match self.to_str() {
+            Some(s) => s.serialize(serializer),
+            None => Err(Error::custom("path contains invalid UTF-8 characters")),
+        }
+    }
+}
+
+#[cfg(feature = "std")]
+impl Serialize for PathBuf {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        self.as_path().serialize(serializer)
+    }
+}
+
+#[cfg(all(feature = "std", any(unix, windows)))]
+impl Serialize for OsStr {
+    #[cfg(unix)]
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        use std::os::unix::ffi::OsStrExt;
+        serializer.serialize_newtype_variant("OsString", 0, "Unix", self.as_bytes())
+    }
+
+    #[cfg(windows)]
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        use std::os::windows::ffi::OsStrExt;
+        let val = self.encode_wide().collect::<Vec<_>>();
+        serializer.serialize_newtype_variant("OsString", 1, "Windows", &val)
+    }
+}
+
+#[cfg(all(feature = "std", any(unix, windows)))]
+impl Serialize for OsString {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        self.as_os_str().serialize(serializer)
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+#[cfg(feature = "std")]
+impl<T> Serialize for Wrapping<T>
+where
+    T: Serialize,
+{
+    #[inline]
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        self.0.serialize(serializer)
+    }
+}
+
+#[cfg(core_reverse)]
+impl<T> Serialize for Reverse<T>
+where
+    T: Serialize,
+{
+    #[inline]
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        self.0.serialize(serializer)
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+#[cfg(all(feature = "std", std_atomic))]
+macro_rules! atomic_impl {
+    ($($ty:ident)*) => {
+        $(
+            impl Serialize for $ty {
+                fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+                where
+                    S: Serializer,
+                {
+                    self.load(Ordering::SeqCst).serialize(serializer)
+                }
+            }
+        )*
+    }
+}
+
+#[cfg(all(feature = "std", std_atomic))]
+atomic_impl! {
+    AtomicBool
+    AtomicI8 AtomicI16 AtomicI32 AtomicIsize
+    AtomicU8 AtomicU16 AtomicU32 AtomicUsize
+}
+
+#[cfg(all(feature = "std", std_atomic64))]
+atomic_impl! {
+    AtomicI64 AtomicU64
+}
diff --git a/src/ser/impossible.rs b/src/ser/impossible.rs
new file mode 100644
index 0000000..90bab29
--- /dev/null
+++ b/src/ser/impossible.rs
@@ -0,0 +1,216 @@
+//! This module contains `Impossible` serializer and its implementations.
+
+use lib::*;
+
+use ser::{
+    self, Serialize, SerializeMap, SerializeSeq, SerializeStruct, SerializeStructVariant,
+    SerializeTuple, SerializeTupleStruct, SerializeTupleVariant,
+};
+
+/// Helper type for implementing a `Serializer` that does not support
+/// serializing one of the compound types.
+///
+/// This type cannot be instantiated, but implements every one of the traits
+/// corresponding to the [`Serializer`] compound types: [`SerializeSeq`],
+/// [`SerializeTuple`], [`SerializeTupleStruct`], [`SerializeTupleVariant`],
+/// [`SerializeMap`], [`SerializeStruct`], and [`SerializeStructVariant`].
+///
+/// ```edition2018
+/// # use serde::ser::{Serializer, Impossible};
+/// # use serde::private::ser::Error;
+/// #
+/// # struct MySerializer;
+/// #
+/// impl Serializer for MySerializer {
+///     type Ok = ();
+///     type Error = Error;
+///
+///     type SerializeSeq = Impossible<(), Error>;
+///     /* other associated types */
+///
+///     /// This data format does not support serializing sequences.
+///     fn serialize_seq(self,
+///                      len: Option<usize>)
+///                      -> Result<Self::SerializeSeq, Error> {
+///         // Given Impossible cannot be instantiated, the only
+///         // thing we can do here is to return an error.
+/// #         stringify! {
+///         Err(...)
+/// #         };
+/// #         unimplemented!()
+///     }
+///
+///     /* other Serializer methods */
+/// #     serde::__serialize_unimplemented! {
+/// #         bool i8 i16 i32 i64 u8 u16 u32 u64 f32 f64 char str bytes none some
+/// #         unit unit_struct unit_variant newtype_struct newtype_variant
+/// #         tuple tuple_struct tuple_variant map struct struct_variant
+/// #     }
+/// }
+/// ```
+///
+/// [`Serializer`]: trait.Serializer.html
+/// [`SerializeSeq`]: trait.SerializeSeq.html
+/// [`SerializeTuple`]: trait.SerializeTuple.html
+/// [`SerializeTupleStruct`]: trait.SerializeTupleStruct.html
+/// [`SerializeTupleVariant`]: trait.SerializeTupleVariant.html
+/// [`SerializeMap`]: trait.SerializeMap.html
+/// [`SerializeStruct`]: trait.SerializeStruct.html
+/// [`SerializeStructVariant`]: trait.SerializeStructVariant.html
+pub struct Impossible<Ok, Error> {
+    void: Void,
+    ok: PhantomData<Ok>,
+    error: PhantomData<Error>,
+}
+
+enum Void {}
+
+impl<Ok, Error> SerializeSeq for Impossible<Ok, Error>
+where
+    Error: ser::Error,
+{
+    type Ok = Ok;
+    type Error = Error;
+
+    fn serialize_element<T: ?Sized>(&mut self, value: &T) -> Result<(), Error>
+    where
+        T: Serialize,
+    {
+        let _ = value;
+        match self.void {}
+    }
+
+    fn end(self) -> Result<Ok, Error> {
+        match self.void {}
+    }
+}
+
+impl<Ok, Error> SerializeTuple for Impossible<Ok, Error>
+where
+    Error: ser::Error,
+{
+    type Ok = Ok;
+    type Error = Error;
+
+    fn serialize_element<T: ?Sized>(&mut self, value: &T) -> Result<(), Error>
+    where
+        T: Serialize,
+    {
+        let _ = value;
+        match self.void {}
+    }
+
+    fn end(self) -> Result<Ok, Error> {
+        match self.void {}
+    }
+}
+
+impl<Ok, Error> SerializeTupleStruct for Impossible<Ok, Error>
+where
+    Error: ser::Error,
+{
+    type Ok = Ok;
+    type Error = Error;
+
+    fn serialize_field<T: ?Sized>(&mut self, value: &T) -> Result<(), Error>
+    where
+        T: Serialize,
+    {
+        let _ = value;
+        match self.void {}
+    }
+
+    fn end(self) -> Result<Ok, Error> {
+        match self.void {}
+    }
+}
+
+impl<Ok, Error> SerializeTupleVariant for Impossible<Ok, Error>
+where
+    Error: ser::Error,
+{
+    type Ok = Ok;
+    type Error = Error;
+
+    fn serialize_field<T: ?Sized>(&mut self, value: &T) -> Result<(), Error>
+    where
+        T: Serialize,
+    {
+        let _ = value;
+        match self.void {}
+    }
+
+    fn end(self) -> Result<Ok, Error> {
+        match self.void {}
+    }
+}
+
+impl<Ok, Error> SerializeMap for Impossible<Ok, Error>
+where
+    Error: ser::Error,
+{
+    type Ok = Ok;
+    type Error = Error;
+
+    fn serialize_key<T: ?Sized>(&mut self, key: &T) -> Result<(), Error>
+    where
+        T: Serialize,
+    {
+        let _ = key;
+        match self.void {}
+    }
+
+    fn serialize_value<T: ?Sized>(&mut self, value: &T) -> Result<(), Error>
+    where
+        T: Serialize,
+    {
+        let _ = value;
+        match self.void {}
+    }
+
+    fn end(self) -> Result<Ok, Error> {
+        match self.void {}
+    }
+}
+
+impl<Ok, Error> SerializeStruct for Impossible<Ok, Error>
+where
+    Error: ser::Error,
+{
+    type Ok = Ok;
+    type Error = Error;
+
+    fn serialize_field<T: ?Sized>(&mut self, key: &'static str, value: &T) -> Result<(), Error>
+    where
+        T: Serialize,
+    {
+        let _ = key;
+        let _ = value;
+        match self.void {}
+    }
+
+    fn end(self) -> Result<Ok, Error> {
+        match self.void {}
+    }
+}
+
+impl<Ok, Error> SerializeStructVariant for Impossible<Ok, Error>
+where
+    Error: ser::Error,
+{
+    type Ok = Ok;
+    type Error = Error;
+
+    fn serialize_field<T: ?Sized>(&mut self, key: &'static str, value: &T) -> Result<(), Error>
+    where
+        T: Serialize,
+    {
+        let _ = key;
+        let _ = value;
+        match self.void {}
+    }
+
+    fn end(self) -> Result<Ok, Error> {
+        match self.void {}
+    }
+}
diff --git a/src/ser/mod.rs b/src/ser/mod.rs
new file mode 100644
index 0000000..676b0e2
--- /dev/null
+++ b/src/ser/mod.rs
@@ -0,0 +1,1964 @@
+//! Generic data structure serialization framework.
+//!
+//! The two most important traits in this module are [`Serialize`] and
+//! [`Serializer`].
+//!
+//!  - **A type that implements `Serialize` is a data structure** that can be
+//!    serialized to any data format supported by Serde, and conversely
+//!  - **A type that implements `Serializer` is a data format** that can
+//!    serialize any data structure supported by Serde.
+//!
+//! # The Serialize trait
+//!
+//! Serde provides [`Serialize`] implementations for many Rust primitive and
+//! standard library types. The complete list is below. All of these can be
+//! serialized using Serde out of the box.
+//!
+//! Additionally, Serde provides a procedural macro called [`serde_derive`] to
+//! automatically generate [`Serialize`] implementations for structs and enums
+//! in your program. See the [derive section of the manual] for how to use this.
+//!
+//! In rare cases it may be necessary to implement [`Serialize`] manually for
+//! some type in your program. See the [Implementing `Serialize`] section of the
+//! manual for more about this.
+//!
+//! Third-party crates may provide [`Serialize`] implementations for types that
+//! they expose. For example the [`linked-hash-map`] crate provides a
+//! [`LinkedHashMap<K, V>`] type that is serializable by Serde because the crate
+//! provides an implementation of [`Serialize`] for it.
+//!
+//! # The Serializer trait
+//!
+//! [`Serializer`] implementations are provided by third-party crates, for
+//! example [`serde_json`], [`serde_yaml`] and [`bincode`].
+//!
+//! A partial list of well-maintained formats is given on the [Serde
+//! website][data formats].
+//!
+//! # Implementations of Serialize provided by Serde
+//!
+//!  - **Primitive types**:
+//!    - bool
+//!    - i8, i16, i32, i64, i128, isize
+//!    - u8, u16, u32, u64, u128, usize
+//!    - f32, f64
+//!    - char
+//!    - str
+//!    - &T and &mut T
+//!  - **Compound types**:
+//!    - \[T\]
+//!    - \[T; 0\] through \[T; 32\]
+//!    - tuples up to size 16
+//!  - **Common standard library types**:
+//!    - String
+//!    - Option\<T\>
+//!    - Result\<T, E\>
+//!    - PhantomData\<T\>
+//!  - **Wrapper types**:
+//!    - Box\<T\>
+//!    - Cow\<'a, T\>
+//!    - Cell\<T\>
+//!    - RefCell\<T\>
+//!    - Mutex\<T\>
+//!    - RwLock\<T\>
+//!    - Rc\<T\>&emsp;*(if* features = ["rc"] *is enabled)*
+//!    - Arc\<T\>&emsp;*(if* features = ["rc"] *is enabled)*
+//!  - **Collection types**:
+//!    - BTreeMap\<K, V\>
+//!    - BTreeSet\<T\>
+//!    - BinaryHeap\<T\>
+//!    - HashMap\<K, V, H\>
+//!    - HashSet\<T, H\>
+//!    - LinkedList\<T\>
+//!    - VecDeque\<T\>
+//!    - Vec\<T\>
+//!  - **FFI types**:
+//!    - CStr
+//!    - CString
+//!    - OsStr
+//!    - OsString
+//!  - **Miscellaneous standard library types**:
+//!    - Duration
+//!    - SystemTime
+//!    - Path
+//!    - PathBuf
+//!    - Range\<T\>
+//!    - RangeInclusive\<T\>
+//!    - Bound\<T\>
+//!    - num::NonZero*
+//!    - `!` *(unstable)*
+//!  - **Net types**:
+//!    - IpAddr
+//!    - Ipv4Addr
+//!    - Ipv6Addr
+//!    - SocketAddr
+//!    - SocketAddrV4
+//!    - SocketAddrV6
+//!
+//! [Implementing `Serialize`]: https://serde.rs/impl-serialize.html
+//! [`LinkedHashMap<K, V>`]: https://docs.rs/linked-hash-map/*/linked_hash_map/struct.LinkedHashMap.html
+//! [`Serialize`]: ../trait.Serialize.html
+//! [`Serializer`]: ../trait.Serializer.html
+//! [`bincode`]: https://github.com/TyOverby/bincode
+//! [`linked-hash-map`]: https://crates.io/crates/linked-hash-map
+//! [`serde_derive`]: https://crates.io/crates/serde_derive
+//! [`serde_json`]: https://github.com/serde-rs/json
+//! [`serde_yaml`]: https://github.com/dtolnay/serde-yaml
+//! [derive section of the manual]: https://serde.rs/derive.html
+//! [data formats]: https://serde.rs/#data-formats
+
+use lib::*;
+
+mod fmt;
+mod impls;
+mod impossible;
+
+pub use self::impossible::Impossible;
+
+#[cfg(feature = "std")]
+#[doc(no_inline)]
+pub use std::error::Error as StdError;
+#[cfg(not(feature = "std"))]
+#[doc(no_inline)]
+pub use std_error::Error as StdError;
+
+////////////////////////////////////////////////////////////////////////////////
+
+macro_rules! declare_error_trait {
+    (Error: Sized $(+ $($supertrait:ident)::+)*) => {
+        /// Trait used by `Serialize` implementations to generically construct
+        /// errors belonging to the `Serializer` against which they are
+        /// currently running.
+        ///
+        /// # Example implementation
+        ///
+        /// The [example data format] presented on the website shows an error
+        /// type appropriate for a basic JSON data format.
+        ///
+        /// [example data format]: https://serde.rs/data-format.html
+        pub trait Error: Sized $(+ $($supertrait)::+)* {
+            /// Used when a [`Serialize`] implementation encounters any error
+            /// while serializing a type.
+            ///
+            /// The message should not be capitalized and should not end with a
+            /// period.
+            ///
+            /// For example, a filesystem [`Path`] may refuse to serialize
+            /// itself if it contains invalid UTF-8 data.
+            ///
+            /// ```edition2018
+            /// # struct Path;
+            /// #
+            /// # impl Path {
+            /// #     fn to_str(&self) -> Option<&str> {
+            /// #         unimplemented!()
+            /// #     }
+            /// # }
+            /// #
+            /// use serde::ser::{self, Serialize, Serializer};
+            ///
+            /// impl Serialize for Path {
+            ///     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+            ///     where
+            ///         S: Serializer,
+            ///     {
+            ///         match self.to_str() {
+            ///             Some(s) => serializer.serialize_str(s),
+            ///             None => Err(ser::Error::custom("path contains invalid UTF-8 characters")),
+            ///         }
+            ///     }
+            /// }
+            /// ```
+            ///
+            /// [`Path`]: https://doc.rust-lang.org/std/path/struct.Path.html
+            /// [`Serialize`]: ../trait.Serialize.html
+            fn custom<T>(msg: T) -> Self
+            where
+                T: Display;
+        }
+    }
+}
+
+#[cfg(feature = "std")]
+declare_error_trait!(Error: Sized + StdError);
+
+#[cfg(not(feature = "std"))]
+declare_error_trait!(Error: Sized + Debug + Display);
+
+////////////////////////////////////////////////////////////////////////////////
+
+/// A **data structure** that can be serialized into any data format supported
+/// by Serde.
+///
+/// Serde provides `Serialize` implementations for many Rust primitive and
+/// standard library types. The complete list is [here][ser]. All of these can
+/// be serialized using Serde out of the box.
+///
+/// Additionally, Serde provides a procedural macro called [`serde_derive`] to
+/// automatically generate `Serialize` implementations for structs and enums in
+/// your program. See the [derive section of the manual] for how to use this.
+///
+/// In rare cases it may be necessary to implement `Serialize` manually for some
+/// type in your program. See the [Implementing `Serialize`] section of the
+/// manual for more about this.
+///
+/// Third-party crates may provide `Serialize` implementations for types that
+/// they expose. For example the [`linked-hash-map`] crate provides a
+/// [`LinkedHashMap<K, V>`] type that is serializable by Serde because the crate
+/// provides an implementation of `Serialize` for it.
+///
+/// [Implementing `Serialize`]: https://serde.rs/impl-serialize.html
+/// [`LinkedHashMap<K, V>`]: https://docs.rs/linked-hash-map/*/linked_hash_map/struct.LinkedHashMap.html
+/// [`linked-hash-map`]: https://crates.io/crates/linked-hash-map
+/// [`serde_derive`]: https://crates.io/crates/serde_derive
+/// [derive section of the manual]: https://serde.rs/derive.html
+/// [ser]: https://docs.serde.rs/serde/ser/index.html
+pub trait Serialize {
+    /// Serialize this value into the given Serde serializer.
+    ///
+    /// See the [Implementing `Serialize`] section of the manual for more
+    /// information about how to implement this method.
+    ///
+    /// ```edition2018
+    /// use serde::ser::{Serialize, SerializeStruct, Serializer};
+    ///
+    /// struct Person {
+    ///     name: String,
+    ///     age: u8,
+    ///     phones: Vec<String>,
+    /// }
+    ///
+    /// // This is what #[derive(Serialize)] would generate.
+    /// impl Serialize for Person {
+    ///     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    ///     where
+    ///         S: Serializer,
+    ///     {
+    ///         let mut s = serializer.serialize_struct("Person", 3)?;
+    ///         s.serialize_field("name", &self.name)?;
+    ///         s.serialize_field("age", &self.age)?;
+    ///         s.serialize_field("phones", &self.phones)?;
+    ///         s.end()
+    ///     }
+    /// }
+    /// ```
+    ///
+    /// [Implementing `Serialize`]: https://serde.rs/impl-serialize.html
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer;
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+/// A **data format** that can serialize any data structure supported by Serde.
+///
+/// The role of this trait is to define the serialization half of the [Serde
+/// data model], which is a way to categorize every Rust data structure into one
+/// of 29 possible types. Each method of the `Serializer` trait corresponds to
+/// one of the types of the data model.
+///
+/// Implementations of `Serialize` map themselves into this data model by
+/// invoking exactly one of the `Serializer` methods.
+///
+/// The types that make up the Serde data model are:
+///
+///  - **14 primitive types**
+///    - bool
+///    - i8, i16, i32, i64, i128
+///    - u8, u16, u32, u64, u128
+///    - f32, f64
+///    - char
+///  - **string**
+///    - UTF-8 bytes with a length and no null terminator.
+///    - When serializing, all strings are handled equally. When deserializing,
+///      there are three flavors of strings: transient, owned, and borrowed.
+///  - **byte array** - \[u8\]
+///    - Similar to strings, during deserialization byte arrays can be
+///      transient, owned, or borrowed.
+///  - **option**
+///    - Either none or some value.
+///  - **unit**
+///    - The type of `()` in Rust. It represents an anonymous value containing
+///      no data.
+///  - **unit_struct**
+///    - For example `struct Unit` or `PhantomData<T>`. It represents a named
+///      value containing no data.
+///  - **unit_variant**
+///    - For example the `E::A` and `E::B` in `enum E { A, B }`.
+///  - **newtype_struct**
+///    - For example `struct Millimeters(u8)`.
+///  - **newtype_variant**
+///    - For example the `E::N` in `enum E { N(u8) }`.
+///  - **seq**
+///    - A variably sized heterogeneous sequence of values, for example
+///      `Vec<T>` or `HashSet<T>`. When serializing, the length may or may not
+///      be known before iterating through all the data. When deserializing,
+///      the length is determined by looking at the serialized data.
+///  - **tuple**
+///    - A statically sized heterogeneous sequence of values for which the
+///      length will be known at deserialization time without looking at the
+///      serialized data, for example `(u8,)` or `(String, u64, Vec<T>)` or
+///      `[u64; 10]`.
+///  - **tuple_struct**
+///    - A named tuple, for example `struct Rgb(u8, u8, u8)`.
+///  - **tuple_variant**
+///    - For example the `E::T` in `enum E { T(u8, u8) }`.
+///  - **map**
+///    - A heterogeneous key-value pairing, for example `BTreeMap<K, V>`.
+///  - **struct**
+///    - A heterogeneous key-value pairing in which the keys are strings and
+///      will be known at deserialization time without looking at the
+///      serialized data, for example `struct S { r: u8, g: u8, b: u8 }`.
+///  - **struct_variant**
+///    - For example the `E::S` in `enum E { S { r: u8, g: u8, b: u8 } }`.
+///
+/// Many Serde serializers produce text or binary data as output, for example
+/// JSON or Bincode. This is not a requirement of the `Serializer` trait, and
+/// there are serializers that do not produce text or binary output. One example
+/// is the `serde_json::value::Serializer` (distinct from the main `serde_json`
+/// serializer) that produces a `serde_json::Value` data structure in memory as
+/// output.
+///
+/// [Serde data model]: https://serde.rs/data-model.html
+///
+/// # Example implementation
+///
+/// The [example data format] presented on the website contains example code for
+/// a basic JSON `Serializer`.
+///
+/// [example data format]: https://serde.rs/data-format.html
+pub trait Serializer: Sized {
+    /// The output type produced by this `Serializer` during successful
+    /// serialization. Most serializers that produce text or binary output
+    /// should set `Ok = ()` and serialize into an [`io::Write`] or buffer
+    /// contained within the `Serializer` instance. Serializers that build
+    /// in-memory data structures may be simplified by using `Ok` to propagate
+    /// the data structure around.
+    ///
+    /// [`io::Write`]: https://doc.rust-lang.org/std/io/trait.Write.html
+    type Ok;
+
+    /// The error type when some error occurs during serialization.
+    type Error: Error;
+
+    /// Type returned from [`serialize_seq`] for serializing the content of the
+    /// sequence.
+    ///
+    /// [`serialize_seq`]: #tymethod.serialize_seq
+    type SerializeSeq: SerializeSeq<Ok = Self::Ok, Error = Self::Error>;
+
+    /// Type returned from [`serialize_tuple`] for serializing the content of
+    /// the tuple.
+    ///
+    /// [`serialize_tuple`]: #tymethod.serialize_tuple
+    type SerializeTuple: SerializeTuple<Ok = Self::Ok, Error = Self::Error>;
+
+    /// Type returned from [`serialize_tuple_struct`] for serializing the
+    /// content of the tuple struct.
+    ///
+    /// [`serialize_tuple_struct`]: #tymethod.serialize_tuple_struct
+    type SerializeTupleStruct: SerializeTupleStruct<Ok = Self::Ok, Error = Self::Error>;
+
+    /// Type returned from [`serialize_tuple_variant`] for serializing the
+    /// content of the tuple variant.
+    ///
+    /// [`serialize_tuple_variant`]: #tymethod.serialize_tuple_variant
+    type SerializeTupleVariant: SerializeTupleVariant<Ok = Self::Ok, Error = Self::Error>;
+
+    /// Type returned from [`serialize_map`] for serializing the content of the
+    /// map.
+    ///
+    /// [`serialize_map`]: #tymethod.serialize_map
+    type SerializeMap: SerializeMap<Ok = Self::Ok, Error = Self::Error>;
+
+    /// Type returned from [`serialize_struct`] for serializing the content of
+    /// the struct.
+    ///
+    /// [`serialize_struct`]: #tymethod.serialize_struct
+    type SerializeStruct: SerializeStruct<Ok = Self::Ok, Error = Self::Error>;
+
+    /// Type returned from [`serialize_struct_variant`] for serializing the
+    /// content of the struct variant.
+    ///
+    /// [`serialize_struct_variant`]: #tymethod.serialize_struct_variant
+    type SerializeStructVariant: SerializeStructVariant<Ok = Self::Ok, Error = Self::Error>;
+
+    /// Serialize a `bool` value.
+    ///
+    /// ```edition2018
+    /// # use serde::Serializer;
+    /// #
+    /// # serde::__private_serialize!();
+    /// #
+    /// impl Serialize for bool {
+    ///     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    ///     where
+    ///         S: Serializer,
+    ///     {
+    ///         serializer.serialize_bool(*self)
+    ///     }
+    /// }
+    /// ```
+    fn serialize_bool(self, v: bool) -> Result<Self::Ok, Self::Error>;
+
+    /// Serialize an `i8` value.
+    ///
+    /// If the format does not differentiate between `i8` and `i64`, a
+    /// reasonable implementation would be to cast the value to `i64` and
+    /// forward to `serialize_i64`.
+    ///
+    /// ```edition2018
+    /// # use serde::Serializer;
+    /// #
+    /// # serde::__private_serialize!();
+    /// #
+    /// impl Serialize for i8 {
+    ///     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    ///     where
+    ///         S: Serializer,
+    ///     {
+    ///         serializer.serialize_i8(*self)
+    ///     }
+    /// }
+    /// ```
+    fn serialize_i8(self, v: i8) -> Result<Self::Ok, Self::Error>;
+
+    /// Serialize an `i16` value.
+    ///
+    /// If the format does not differentiate between `i16` and `i64`, a
+    /// reasonable implementation would be to cast the value to `i64` and
+    /// forward to `serialize_i64`.
+    ///
+    /// ```edition2018
+    /// # use serde::Serializer;
+    /// #
+    /// # serde::__private_serialize!();
+    /// #
+    /// impl Serialize for i16 {
+    ///     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    ///     where
+    ///         S: Serializer,
+    ///     {
+    ///         serializer.serialize_i16(*self)
+    ///     }
+    /// }
+    /// ```
+    fn serialize_i16(self, v: i16) -> Result<Self::Ok, Self::Error>;
+
+    /// Serialize an `i32` value.
+    ///
+    /// If the format does not differentiate between `i32` and `i64`, a
+    /// reasonable implementation would be to cast the value to `i64` and
+    /// forward to `serialize_i64`.
+    ///
+    /// ```edition2018
+    /// # use serde::Serializer;
+    /// #
+    /// # serde::__private_serialize!();
+    /// #
+    /// impl Serialize for i32 {
+    ///     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    ///     where
+    ///         S: Serializer,
+    ///     {
+    ///         serializer.serialize_i32(*self)
+    ///     }
+    /// }
+    /// ```
+    fn serialize_i32(self, v: i32) -> Result<Self::Ok, Self::Error>;
+
+    /// Serialize an `i64` value.
+    ///
+    /// ```edition2018
+    /// # use serde::Serializer;
+    /// #
+    /// # serde::__private_serialize!();
+    /// #
+    /// impl Serialize for i64 {
+    ///     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    ///     where
+    ///         S: Serializer,
+    ///     {
+    ///         serializer.serialize_i64(*self)
+    ///     }
+    /// }
+    /// ```
+    fn serialize_i64(self, v: i64) -> Result<Self::Ok, Self::Error>;
+
+    serde_if_integer128! {
+        /// Serialize an `i128` value.
+        ///
+        /// ```edition2018
+        /// # use serde::Serializer;
+        /// #
+        /// # serde::__private_serialize!();
+        /// #
+        /// impl Serialize for i128 {
+        ///     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+        ///     where
+        ///         S: Serializer,
+        ///     {
+        ///         serializer.serialize_i128(*self)
+        ///     }
+        /// }
+        /// ```
+        ///
+        /// This method is available only on Rust compiler versions >=1.26. The
+        /// default behavior unconditionally returns an error.
+        fn serialize_i128(self, v: i128) -> Result<Self::Ok, Self::Error> {
+            let _ = v;
+            Err(Error::custom("i128 is not supported"))
+        }
+    }
+
+    /// Serialize a `u8` value.
+    ///
+    /// If the format does not differentiate between `u8` and `u64`, a
+    /// reasonable implementation would be to cast the value to `u64` and
+    /// forward to `serialize_u64`.
+    ///
+    /// ```edition2018
+    /// # use serde::Serializer;
+    /// #
+    /// # serde::__private_serialize!();
+    /// #
+    /// impl Serialize for u8 {
+    ///     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    ///     where
+    ///         S: Serializer,
+    ///     {
+    ///         serializer.serialize_u8(*self)
+    ///     }
+    /// }
+    /// ```
+    fn serialize_u8(self, v: u8) -> Result<Self::Ok, Self::Error>;
+
+    /// Serialize a `u16` value.
+    ///
+    /// If the format does not differentiate between `u16` and `u64`, a
+    /// reasonable implementation would be to cast the value to `u64` and
+    /// forward to `serialize_u64`.
+    ///
+    /// ```edition2018
+    /// # use serde::Serializer;
+    /// #
+    /// # serde::__private_serialize!();
+    /// #
+    /// impl Serialize for u16 {
+    ///     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    ///     where
+    ///         S: Serializer,
+    ///     {
+    ///         serializer.serialize_u16(*self)
+    ///     }
+    /// }
+    /// ```
+    fn serialize_u16(self, v: u16) -> Result<Self::Ok, Self::Error>;
+
+    /// Serialize a `u32` value.
+    ///
+    /// If the format does not differentiate between `u32` and `u64`, a
+    /// reasonable implementation would be to cast the value to `u64` and
+    /// forward to `serialize_u64`.
+    ///
+    /// ```edition2018
+    /// # use serde::Serializer;
+    /// #
+    /// # serde::__private_serialize!();
+    /// #
+    /// impl Serialize for u32 {
+    ///     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    ///     where
+    ///         S: Serializer,
+    ///     {
+    ///         serializer.serialize_u32(*self)
+    ///     }
+    /// }
+    /// ```
+    fn serialize_u32(self, v: u32) -> Result<Self::Ok, Self::Error>;
+
+    /// Serialize a `u64` value.
+    ///
+    /// ```edition2018
+    /// # use serde::Serializer;
+    /// #
+    /// # serde::__private_serialize!();
+    /// #
+    /// impl Serialize for u64 {
+    ///     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    ///     where
+    ///         S: Serializer,
+    ///     {
+    ///         serializer.serialize_u64(*self)
+    ///     }
+    /// }
+    /// ```
+    fn serialize_u64(self, v: u64) -> Result<Self::Ok, Self::Error>;
+
+    serde_if_integer128! {
+        /// Serialize a `u128` value.
+        ///
+        /// ```edition2018
+        /// # use serde::Serializer;
+        /// #
+        /// # serde::__private_serialize!();
+        /// #
+        /// impl Serialize for u128 {
+        ///     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+        ///     where
+        ///         S: Serializer,
+        ///     {
+        ///         serializer.serialize_u128(*self)
+        ///     }
+        /// }
+        /// ```
+        ///
+        /// This method is available only on Rust compiler versions >=1.26. The
+        /// default behavior unconditionally returns an error.
+        fn serialize_u128(self, v: u128) -> Result<Self::Ok, Self::Error> {
+            let _ = v;
+            Err(Error::custom("u128 is not supported"))
+        }
+    }
+
+    /// Serialize an `f32` value.
+    ///
+    /// If the format does not differentiate between `f32` and `f64`, a
+    /// reasonable implementation would be to cast the value to `f64` and
+    /// forward to `serialize_f64`.
+    ///
+    /// ```edition2018
+    /// # use serde::Serializer;
+    /// #
+    /// # serde::__private_serialize!();
+    /// #
+    /// impl Serialize for f32 {
+    ///     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    ///     where
+    ///         S: Serializer,
+    ///     {
+    ///         serializer.serialize_f32(*self)
+    ///     }
+    /// }
+    /// ```
+    fn serialize_f32(self, v: f32) -> Result<Self::Ok, Self::Error>;
+
+    /// Serialize an `f64` value.
+    ///
+    /// ```edition2018
+    /// # use serde::Serializer;
+    /// #
+    /// # serde::__private_serialize!();
+    /// #
+    /// impl Serialize for f64 {
+    ///     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    ///     where
+    ///         S: Serializer,
+    ///     {
+    ///         serializer.serialize_f64(*self)
+    ///     }
+    /// }
+    /// ```
+    fn serialize_f64(self, v: f64) -> Result<Self::Ok, Self::Error>;
+
+    /// Serialize a character.
+    ///
+    /// If the format does not support characters, it is reasonable to serialize
+    /// it as a single element `str` or a `u32`.
+    ///
+    /// ```edition2018
+    /// # use serde::Serializer;
+    /// #
+    /// # serde::__private_serialize!();
+    /// #
+    /// impl Serialize for char {
+    ///     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    ///     where
+    ///         S: Serializer,
+    ///     {
+    ///         serializer.serialize_char(*self)
+    ///     }
+    /// }
+    /// ```
+    fn serialize_char(self, v: char) -> Result<Self::Ok, Self::Error>;
+
+    /// Serialize a `&str`.
+    ///
+    /// ```edition2018
+    /// # use serde::Serializer;
+    /// #
+    /// # serde::__private_serialize!();
+    /// #
+    /// impl Serialize for str {
+    ///     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    ///     where
+    ///         S: Serializer,
+    ///     {
+    ///         serializer.serialize_str(self)
+    ///     }
+    /// }
+    /// ```
+    fn serialize_str(self, v: &str) -> Result<Self::Ok, Self::Error>;
+
+    /// Serialize a chunk of raw byte data.
+    ///
+    /// Enables serializers to serialize byte slices more compactly or more
+    /// efficiently than other types of slices. If no efficient implementation
+    /// is available, a reasonable implementation would be to forward to
+    /// `serialize_seq`. If forwarded, the implementation looks usually just
+    /// like this:
+    ///
+    /// ```edition2018
+    /// # use serde::ser::{Serializer, SerializeSeq};
+    /// # use serde::private::ser::Error;
+    /// #
+    /// # struct MySerializer;
+    /// #
+    /// # impl Serializer for MySerializer {
+    /// #     type Ok = ();
+    /// #     type Error = Error;
+    /// #
+    /// fn serialize_bytes(self, v: &[u8]) -> Result<Self::Ok, Self::Error> {
+    ///     let mut seq = self.serialize_seq(Some(v.len()))?;
+    ///     for b in v {
+    ///         seq.serialize_element(b)?;
+    ///     }
+    ///     seq.end()
+    /// }
+    /// #
+    /// #     serde::__serialize_unimplemented! {
+    /// #         bool i8 i16 i32 i64 u8 u16 u32 u64 f32 f64 char str none some
+    /// #         unit unit_struct unit_variant newtype_struct newtype_variant
+    /// #         seq tuple tuple_struct tuple_variant map struct struct_variant
+    /// #     }
+    /// # }
+    /// ```
+    fn serialize_bytes(self, v: &[u8]) -> Result<Self::Ok, Self::Error>;
+
+    /// Serialize a [`None`] value.
+    ///
+    /// ```edition2018
+    /// # use serde::{Serialize, Serializer};
+    /// #
+    /// # enum Option<T> {
+    /// #     Some(T),
+    /// #     None,
+    /// # }
+    /// #
+    /// # use self::Option::{Some, None};
+    /// #
+    /// impl<T> Serialize for Option<T>
+    /// where
+    ///     T: Serialize,
+    /// {
+    ///     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    ///     where
+    ///         S: Serializer,
+    ///     {
+    ///         match *self {
+    ///             Some(ref value) => serializer.serialize_some(value),
+    ///             None => serializer.serialize_none(),
+    ///         }
+    ///     }
+    /// }
+    /// #
+    /// # fn main() {}
+    /// ```
+    ///
+    /// [`None`]: https://doc.rust-lang.org/std/option/enum.Option.html#variant.None
+    fn serialize_none(self) -> Result<Self::Ok, Self::Error>;
+
+    /// Serialize a [`Some(T)`] value.
+    ///
+    /// ```edition2018
+    /// # use serde::{Serialize, Serializer};
+    /// #
+    /// # enum Option<T> {
+    /// #     Some(T),
+    /// #     None,
+    /// # }
+    /// #
+    /// # use self::Option::{Some, None};
+    /// #
+    /// impl<T> Serialize for Option<T>
+    /// where
+    ///     T: Serialize,
+    /// {
+    ///     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    ///     where
+    ///         S: Serializer,
+    ///     {
+    ///         match *self {
+    ///             Some(ref value) => serializer.serialize_some(value),
+    ///             None => serializer.serialize_none(),
+    ///         }
+    ///     }
+    /// }
+    /// #
+    /// # fn main() {}
+    /// ```
+    ///
+    /// [`Some(T)`]: https://doc.rust-lang.org/std/option/enum.Option.html#variant.Some
+    fn serialize_some<T: ?Sized>(self, value: &T) -> Result<Self::Ok, Self::Error>
+    where
+        T: Serialize;
+
+    /// Serialize a `()` value.
+    ///
+    /// ```edition2018
+    /// # use serde::Serializer;
+    /// #
+    /// # serde::__private_serialize!();
+    /// #
+    /// impl Serialize for () {
+    ///     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    ///     where
+    ///         S: Serializer,
+    ///     {
+    ///         serializer.serialize_unit()
+    ///     }
+    /// }
+    /// ```
+    fn serialize_unit(self) -> Result<Self::Ok, Self::Error>;
+
+    /// Serialize a unit struct like `struct Unit` or `PhantomData<T>`.
+    ///
+    /// A reasonable implementation would be to forward to `serialize_unit`.
+    ///
+    /// ```edition2018
+    /// use serde::{Serialize, Serializer};
+    ///
+    /// struct Nothing;
+    ///
+    /// impl Serialize for Nothing {
+    ///     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    ///     where
+    ///         S: Serializer,
+    ///     {
+    ///         serializer.serialize_unit_struct("Nothing")
+    ///     }
+    /// }
+    /// ```
+    fn serialize_unit_struct(self, name: &'static str) -> Result<Self::Ok, Self::Error>;
+
+    /// Serialize a unit variant like `E::A` in `enum E { A, B }`.
+    ///
+    /// The `name` is the name of the enum, the `variant_index` is the index of
+    /// this variant within the enum, and the `variant` is the name of the
+    /// variant.
+    ///
+    /// ```edition2018
+    /// use serde::{Serialize, Serializer};
+    ///
+    /// enum E {
+    ///     A,
+    ///     B,
+    /// }
+    ///
+    /// impl Serialize for E {
+    ///     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    ///     where
+    ///         S: Serializer,
+    ///     {
+    ///         match *self {
+    ///             E::A => serializer.serialize_unit_variant("E", 0, "A"),
+    ///             E::B => serializer.serialize_unit_variant("E", 1, "B"),
+    ///         }
+    ///     }
+    /// }
+    /// ```
+    fn serialize_unit_variant(
+        self,
+        name: &'static str,
+        variant_index: u32,
+        variant: &'static str,
+    ) -> Result<Self::Ok, Self::Error>;
+
+    /// Serialize a newtype struct like `struct Millimeters(u8)`.
+    ///
+    /// Serializers are encouraged to treat newtype structs as insignificant
+    /// wrappers around the data they contain. A reasonable implementation would
+    /// be to forward to `value.serialize(self)`.
+    ///
+    /// ```edition2018
+    /// use serde::{Serialize, Serializer};
+    ///
+    /// struct Millimeters(u8);
+    ///
+    /// impl Serialize for Millimeters {
+    ///     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    ///     where
+    ///         S: Serializer,
+    ///     {
+    ///         serializer.serialize_newtype_struct("Millimeters", &self.0)
+    ///     }
+    /// }
+    /// ```
+    fn serialize_newtype_struct<T: ?Sized>(
+        self,
+        name: &'static str,
+        value: &T,
+    ) -> Result<Self::Ok, Self::Error>
+    where
+        T: Serialize;
+
+    /// Serialize a newtype variant like `E::N` in `enum E { N(u8) }`.
+    ///
+    /// The `name` is the name of the enum, the `variant_index` is the index of
+    /// this variant within the enum, and the `variant` is the name of the
+    /// variant. The `value` is the data contained within this newtype variant.
+    ///
+    /// ```edition2018
+    /// use serde::{Serialize, Serializer};
+    ///
+    /// enum E {
+    ///     M(String),
+    ///     N(u8),
+    /// }
+    ///
+    /// impl Serialize for E {
+    ///     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    ///     where
+    ///         S: Serializer,
+    ///     {
+    ///         match *self {
+    ///             E::M(ref s) => serializer.serialize_newtype_variant("E", 0, "M", s),
+    ///             E::N(n) => serializer.serialize_newtype_variant("E", 1, "N", &n),
+    ///         }
+    ///     }
+    /// }
+    /// ```
+    fn serialize_newtype_variant<T: ?Sized>(
+        self,
+        name: &'static str,
+        variant_index: u32,
+        variant: &'static str,
+        value: &T,
+    ) -> Result<Self::Ok, Self::Error>
+    where
+        T: Serialize;
+
+    /// Begin to serialize a variably sized sequence. This call must be
+    /// followed by zero or more calls to `serialize_element`, then a call to
+    /// `end`.
+    ///
+    /// The argument is the number of elements in the sequence, which may or may
+    /// not be computable before the sequence is iterated. Some serializers only
+    /// support sequences whose length is known up front.
+    ///
+    /// ```edition2018
+    /// # use std::marker::PhantomData;
+    /// #
+    /// # struct Vec<T>(PhantomData<T>);
+    /// #
+    /// # impl<T> Vec<T> {
+    /// #     fn len(&self) -> usize {
+    /// #         unimplemented!()
+    /// #     }
+    /// # }
+    /// #
+    /// # impl<'a, T> IntoIterator for &'a Vec<T> {
+    /// #     type Item = &'a T;
+    /// #     type IntoIter = Box<Iterator<Item = &'a T>>;
+    /// #
+    /// #     fn into_iter(self) -> Self::IntoIter {
+    /// #         unimplemented!()
+    /// #     }
+    /// # }
+    /// #
+    /// use serde::ser::{Serialize, Serializer, SerializeSeq};
+    ///
+    /// impl<T> Serialize for Vec<T>
+    /// where
+    ///     T: Serialize,
+    /// {
+    ///     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    ///     where
+    ///         S: Serializer,
+    ///     {
+    ///         let mut seq = serializer.serialize_seq(Some(self.len()))?;
+    ///         for element in self {
+    ///             seq.serialize_element(element)?;
+    ///         }
+    ///         seq.end()
+    ///     }
+    /// }
+    /// ```
+    fn serialize_seq(self, len: Option<usize>) -> Result<Self::SerializeSeq, Self::Error>;
+
+    /// Begin to serialize a statically sized sequence whose length will be
+    /// known at deserialization time without looking at the serialized data.
+    /// This call must be followed by zero or more calls to `serialize_element`,
+    /// then a call to `end`.
+    ///
+    /// ```edition2018
+    /// use serde::ser::{Serialize, Serializer, SerializeTuple};
+    ///
+    /// # mod fool {
+    /// #     trait Serialize {}
+    /// impl<A, B, C> Serialize for (A, B, C)
+    /// #     {}
+    /// # }
+    /// #
+    /// # struct Tuple3<A, B, C>(A, B, C);
+    /// #
+    /// # impl<A, B, C> Serialize for Tuple3<A, B, C>
+    /// where
+    ///     A: Serialize,
+    ///     B: Serialize,
+    ///     C: Serialize,
+    /// {
+    ///     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    ///     where
+    ///         S: Serializer,
+    ///     {
+    ///         let mut tup = serializer.serialize_tuple(3)?;
+    ///         tup.serialize_element(&self.0)?;
+    ///         tup.serialize_element(&self.1)?;
+    ///         tup.serialize_element(&self.2)?;
+    ///         tup.end()
+    ///     }
+    /// }
+    /// ```
+    ///
+    /// ```edition2018
+    /// use serde::ser::{Serialize, SerializeTuple, Serializer};
+    ///
+    /// const VRAM_SIZE: usize = 386;
+    /// struct Vram([u16; VRAM_SIZE]);
+    ///
+    /// impl Serialize for Vram {
+    ///     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    ///     where
+    ///         S: Serializer,
+    ///     {
+    ///         let mut seq = serializer.serialize_tuple(VRAM_SIZE)?;
+    ///         for element in &self.0[..] {
+    ///             seq.serialize_element(element)?;
+    ///         }
+    ///         seq.end()
+    ///     }
+    /// }
+    /// ```
+    fn serialize_tuple(self, len: usize) -> Result<Self::SerializeTuple, Self::Error>;
+
+    /// Begin to serialize a tuple struct like `struct Rgb(u8, u8, u8)`. This
+    /// call must be followed by zero or more calls to `serialize_field`, then a
+    /// call to `end`.
+    ///
+    /// The `name` is the name of the tuple struct and the `len` is the number
+    /// of data fields that will be serialized.
+    ///
+    /// ```edition2018
+    /// use serde::ser::{Serialize, SerializeTupleStruct, Serializer};
+    ///
+    /// struct Rgb(u8, u8, u8);
+    ///
+    /// impl Serialize for Rgb {
+    ///     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    ///     where
+    ///         S: Serializer,
+    ///     {
+    ///         let mut ts = serializer.serialize_tuple_struct("Rgb", 3)?;
+    ///         ts.serialize_field(&self.0)?;
+    ///         ts.serialize_field(&self.1)?;
+    ///         ts.serialize_field(&self.2)?;
+    ///         ts.end()
+    ///     }
+    /// }
+    /// ```
+    fn serialize_tuple_struct(
+        self,
+        name: &'static str,
+        len: usize,
+    ) -> Result<Self::SerializeTupleStruct, Self::Error>;
+
+    /// Begin to serialize a tuple variant like `E::T` in `enum E { T(u8, u8)
+    /// }`. This call must be followed by zero or more calls to
+    /// `serialize_field`, then a call to `end`.
+    ///
+    /// The `name` is the name of the enum, the `variant_index` is the index of
+    /// this variant within the enum, the `variant` is the name of the variant,
+    /// and the `len` is the number of data fields that will be serialized.
+    ///
+    /// ```edition2018
+    /// use serde::ser::{Serialize, SerializeTupleVariant, Serializer};
+    ///
+    /// enum E {
+    ///     T(u8, u8),
+    ///     U(String, u32, u32),
+    /// }
+    ///
+    /// impl Serialize for E {
+    ///     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    ///     where
+    ///         S: Serializer,
+    ///     {
+    ///         match *self {
+    ///             E::T(ref a, ref b) => {
+    ///                 let mut tv = serializer.serialize_tuple_variant("E", 0, "T", 2)?;
+    ///                 tv.serialize_field(a)?;
+    ///                 tv.serialize_field(b)?;
+    ///                 tv.end()
+    ///             }
+    ///             E::U(ref a, ref b, ref c) => {
+    ///                 let mut tv = serializer.serialize_tuple_variant("E", 1, "U", 3)?;
+    ///                 tv.serialize_field(a)?;
+    ///                 tv.serialize_field(b)?;
+    ///                 tv.serialize_field(c)?;
+    ///                 tv.end()
+    ///             }
+    ///         }
+    ///     }
+    /// }
+    /// ```
+    fn serialize_tuple_variant(
+        self,
+        name: &'static str,
+        variant_index: u32,
+        variant: &'static str,
+        len: usize,
+    ) -> Result<Self::SerializeTupleVariant, Self::Error>;
+
+    /// Begin to serialize a map. This call must be followed by zero or more
+    /// calls to `serialize_key` and `serialize_value`, then a call to `end`.
+    ///
+    /// The argument is the number of elements in the map, which may or may not
+    /// be computable before the map is iterated. Some serializers only support
+    /// maps whose length is known up front.
+    ///
+    /// ```edition2018
+    /// # use std::marker::PhantomData;
+    /// #
+    /// # struct HashMap<K, V>(PhantomData<K>, PhantomData<V>);
+    /// #
+    /// # impl<K, V> HashMap<K, V> {
+    /// #     fn len(&self) -> usize {
+    /// #         unimplemented!()
+    /// #     }
+    /// # }
+    /// #
+    /// # impl<'a, K, V> IntoIterator for &'a HashMap<K, V> {
+    /// #     type Item = (&'a K, &'a V);
+    /// #     type IntoIter = Box<Iterator<Item = (&'a K, &'a V)>>;
+    /// #
+    /// #     fn into_iter(self) -> Self::IntoIter {
+    /// #         unimplemented!()
+    /// #     }
+    /// # }
+    /// #
+    /// use serde::ser::{Serialize, Serializer, SerializeMap};
+    ///
+    /// impl<K, V> Serialize for HashMap<K, V>
+    /// where
+    ///     K: Serialize,
+    ///     V: Serialize,
+    /// {
+    ///     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    ///     where
+    ///         S: Serializer,
+    ///     {
+    ///         let mut map = serializer.serialize_map(Some(self.len()))?;
+    ///         for (k, v) in self {
+    ///             map.serialize_entry(k, v)?;
+    ///         }
+    ///         map.end()
+    ///     }
+    /// }
+    /// ```
+    fn serialize_map(self, len: Option<usize>) -> Result<Self::SerializeMap, Self::Error>;
+
+    /// Begin to serialize a struct like `struct Rgb { r: u8, g: u8, b: u8 }`.
+    /// This call must be followed by zero or more calls to `serialize_field`,
+    /// then a call to `end`.
+    ///
+    /// The `name` is the name of the struct and the `len` is the number of
+    /// data fields that will be serialized.
+    ///
+    /// ```edition2018
+    /// use serde::ser::{Serialize, SerializeStruct, Serializer};
+    ///
+    /// struct Rgb {
+    ///     r: u8,
+    ///     g: u8,
+    ///     b: u8,
+    /// }
+    ///
+    /// impl Serialize for Rgb {
+    ///     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    ///     where
+    ///         S: Serializer,
+    ///     {
+    ///         let mut rgb = serializer.serialize_struct("Rgb", 3)?;
+    ///         rgb.serialize_field("r", &self.r)?;
+    ///         rgb.serialize_field("g", &self.g)?;
+    ///         rgb.serialize_field("b", &self.b)?;
+    ///         rgb.end()
+    ///     }
+    /// }
+    /// ```
+    fn serialize_struct(
+        self,
+        name: &'static str,
+        len: usize,
+    ) -> Result<Self::SerializeStruct, Self::Error>;
+
+    /// Begin to serialize a struct variant like `E::S` in `enum E { S { r: u8,
+    /// g: u8, b: u8 } }`. This call must be followed by zero or more calls to
+    /// `serialize_field`, then a call to `end`.
+    ///
+    /// The `name` is the name of the enum, the `variant_index` is the index of
+    /// this variant within the enum, the `variant` is the name of the variant,
+    /// and the `len` is the number of data fields that will be serialized.
+    ///
+    /// ```edition2018
+    /// use serde::ser::{Serialize, SerializeStructVariant, Serializer};
+    ///
+    /// enum E {
+    ///     S { r: u8, g: u8, b: u8 },
+    /// }
+    ///
+    /// impl Serialize for E {
+    ///     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    ///     where
+    ///         S: Serializer,
+    ///     {
+    ///         match *self {
+    ///             E::S {
+    ///                 ref r,
+    ///                 ref g,
+    ///                 ref b,
+    ///             } => {
+    ///                 let mut sv = serializer.serialize_struct_variant("E", 0, "S", 3)?;
+    ///                 sv.serialize_field("r", r)?;
+    ///                 sv.serialize_field("g", g)?;
+    ///                 sv.serialize_field("b", b)?;
+    ///                 sv.end()
+    ///             }
+    ///         }
+    ///     }
+    /// }
+    /// ```
+    fn serialize_struct_variant(
+        self,
+        name: &'static str,
+        variant_index: u32,
+        variant: &'static str,
+        len: usize,
+    ) -> Result<Self::SerializeStructVariant, Self::Error>;
+
+    /// Collect an iterator as a sequence.
+    ///
+    /// The default implementation serializes each item yielded by the iterator
+    /// using [`serialize_seq`]. Implementors should not need to override this
+    /// method.
+    ///
+    /// ```edition2018
+    /// use serde::{Serialize, Serializer};
+    ///
+    /// struct SecretlyOneHigher {
+    ///     data: Vec<i32>,
+    /// }
+    ///
+    /// impl Serialize for SecretlyOneHigher {
+    ///     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    ///     where
+    ///         S: Serializer,
+    ///     {
+    ///         serializer.collect_seq(self.data.iter().map(|x| x + 1))
+    ///     }
+    /// }
+    /// ```
+    ///
+    /// [`serialize_seq`]: #tymethod.serialize_seq
+    fn collect_seq<I>(self, iter: I) -> Result<Self::Ok, Self::Error>
+    where
+        I: IntoIterator,
+        <I as IntoIterator>::Item: Serialize,
+    {
+        let iter = iter.into_iter();
+        let mut serializer = try!(self.serialize_seq(iterator_len_hint(&iter)));
+        for item in iter {
+            try!(serializer.serialize_element(&item));
+        }
+        serializer.end()
+    }
+
+    /// Collect an iterator as a map.
+    ///
+    /// The default implementation serializes each pair yielded by the iterator
+    /// using [`serialize_map`]. Implementors should not need to override this
+    /// method.
+    ///
+    /// ```edition2018
+    /// use serde::{Serialize, Serializer};
+    /// use std::collections::BTreeSet;
+    ///
+    /// struct MapToUnit {
+    ///     keys: BTreeSet<i32>,
+    /// }
+    ///
+    /// // Serializes as a map in which the values are all unit.
+    /// impl Serialize for MapToUnit {
+    ///     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    ///     where
+    ///         S: Serializer,
+    ///     {
+    ///         serializer.collect_map(self.keys.iter().map(|k| (k, ())))
+    ///     }
+    /// }
+    /// ```
+    ///
+    /// [`serialize_map`]: #tymethod.serialize_map
+    fn collect_map<K, V, I>(self, iter: I) -> Result<Self::Ok, Self::Error>
+    where
+        K: Serialize,
+        V: Serialize,
+        I: IntoIterator<Item = (K, V)>,
+    {
+        let iter = iter.into_iter();
+        let mut serializer = try!(self.serialize_map(iterator_len_hint(&iter)));
+        for (key, value) in iter {
+            try!(serializer.serialize_entry(&key, &value));
+        }
+        serializer.end()
+    }
+
+    /// Serialize a string produced by an implementation of `Display`.
+    ///
+    /// The default implementation builds a heap-allocated [`String`] and
+    /// delegates to [`serialize_str`]. Serializers are encouraged to provide a
+    /// more efficient implementation if possible.
+    ///
+    /// ```edition2018
+    /// # struct DateTime;
+    /// #
+    /// # impl DateTime {
+    /// #     fn naive_local(&self) -> () { () }
+    /// #     fn offset(&self) -> () { () }
+    /// # }
+    /// #
+    /// use serde::{Serialize, Serializer};
+    ///
+    /// impl Serialize for DateTime {
+    ///     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    ///     where
+    ///         S: Serializer,
+    ///     {
+    ///         serializer.collect_str(&format_args!("{:?}{:?}",
+    ///                                              self.naive_local(),
+    ///                                              self.offset()))
+    ///     }
+    /// }
+    /// ```
+    ///
+    /// [`String`]: https://doc.rust-lang.org/std/string/struct.String.html
+    /// [`serialize_str`]: #tymethod.serialize_str
+    #[cfg(any(feature = "std", feature = "alloc"))]
+    fn collect_str<T: ?Sized>(self, value: &T) -> Result<Self::Ok, Self::Error>
+    where
+        T: Display,
+    {
+        self.serialize_str(&value.to_string())
+    }
+
+    /// Serialize a string produced by an implementation of `Display`.
+    ///
+    /// Serializers that use `no_std` are required to provide an implementation
+    /// of this method. If no more sensible behavior is possible, the
+    /// implementation is expected to return an error.
+    ///
+    /// ```edition2018
+    /// # struct DateTime;
+    /// #
+    /// # impl DateTime {
+    /// #     fn naive_local(&self) -> () { () }
+    /// #     fn offset(&self) -> () { () }
+    /// # }
+    /// #
+    /// use serde::{Serialize, Serializer};
+    ///
+    /// impl Serialize for DateTime {
+    ///     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    ///     where
+    ///         S: Serializer,
+    ///     {
+    ///         serializer.collect_str(&format_args!("{:?}{:?}",
+    ///                                              self.naive_local(),
+    ///                                              self.offset()))
+    ///     }
+    /// }
+    /// ```
+    #[cfg(not(any(feature = "std", feature = "alloc")))]
+    fn collect_str<T: ?Sized>(self, value: &T) -> Result<Self::Ok, Self::Error>
+    where
+        T: Display;
+
+    /// Determine whether `Serialize` implementations should serialize in
+    /// human-readable form.
+    ///
+    /// Some types have a human-readable form that may be somewhat expensive to
+    /// construct, as well as a binary form that is compact and efficient.
+    /// Generally text-based formats like JSON and YAML will prefer to use the
+    /// human-readable one and binary formats like Bincode will prefer the
+    /// compact one.
+    ///
+    /// ```edition2018
+    /// # use std::fmt::{self, Display};
+    /// #
+    /// # struct Timestamp;
+    /// #
+    /// # impl Timestamp {
+    /// #     fn seconds_since_epoch(&self) -> u64 { unimplemented!() }
+    /// # }
+    /// #
+    /// # impl Display for Timestamp {
+    /// #     fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+    /// #         unimplemented!()
+    /// #     }
+    /// # }
+    /// #
+    /// use serde::{Serialize, Serializer};
+    ///
+    /// impl Serialize for Timestamp {
+    ///     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    ///     where
+    ///         S: Serializer,
+    ///     {
+    ///         if serializer.is_human_readable() {
+    ///             // Serialize to a human-readable string "2015-05-15T17:01:00Z".
+    ///             self.to_string().serialize(serializer)
+    ///         } else {
+    ///             // Serialize to a compact binary representation.
+    ///             self.seconds_since_epoch().serialize(serializer)
+    ///         }
+    ///     }
+    /// }
+    /// ```
+    ///
+    /// The default implementation of this method returns `true`. Data formats
+    /// may override this to `false` to request a compact form for types that
+    /// support one. Note that modifying this method to change a format from
+    /// human-readable to compact or vice versa should be regarded as a breaking
+    /// change, as a value serialized in human-readable mode is not required to
+    /// deserialize from the same data in compact mode.
+    #[inline]
+    fn is_human_readable(&self) -> bool {
+        true
+    }
+}
+
+/// Returned from `Serializer::serialize_seq`.
+///
+/// # Example use
+///
+/// ```edition2018
+/// # use std::marker::PhantomData;
+/// #
+/// # struct Vec<T>(PhantomData<T>);
+/// #
+/// # impl<T> Vec<T> {
+/// #     fn len(&self) -> usize {
+/// #         unimplemented!()
+/// #     }
+/// # }
+/// #
+/// # impl<'a, T> IntoIterator for &'a Vec<T> {
+/// #     type Item = &'a T;
+/// #     type IntoIter = Box<Iterator<Item = &'a T>>;
+/// #     fn into_iter(self) -> Self::IntoIter {
+/// #         unimplemented!()
+/// #     }
+/// # }
+/// #
+/// use serde::ser::{Serialize, Serializer, SerializeSeq};
+///
+/// impl<T> Serialize for Vec<T>
+/// where
+///     T: Serialize,
+/// {
+///     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+///     where
+///         S: Serializer,
+///     {
+///         let mut seq = serializer.serialize_seq(Some(self.len()))?;
+///         for element in self {
+///             seq.serialize_element(element)?;
+///         }
+///         seq.end()
+///     }
+/// }
+/// ```
+///
+/// # Example implementation
+///
+/// The [example data format] presented on the website demonstrates an
+/// implementation of `SerializeSeq` for a basic JSON data format.
+///
+/// [example data format]: https://serde.rs/data-format.html
+pub trait SerializeSeq {
+    /// Must match the `Ok` type of our `Serializer`.
+    type Ok;
+
+    /// Must match the `Error` type of our `Serializer`.
+    type Error: Error;
+
+    /// Serialize a sequence element.
+    fn serialize_element<T: ?Sized>(&mut self, value: &T) -> Result<(), Self::Error>
+    where
+        T: Serialize;
+
+    /// Finish serializing a sequence.
+    fn end(self) -> Result<Self::Ok, Self::Error>;
+}
+
+/// Returned from `Serializer::serialize_tuple`.
+///
+/// # Example use
+///
+/// ```edition2018
+/// use serde::ser::{Serialize, Serializer, SerializeTuple};
+///
+/// # mod fool {
+/// #     trait Serialize {}
+/// impl<A, B, C> Serialize for (A, B, C)
+/// #     {}
+/// # }
+/// #
+/// # struct Tuple3<A, B, C>(A, B, C);
+/// #
+/// # impl<A, B, C> Serialize for Tuple3<A, B, C>
+/// where
+///     A: Serialize,
+///     B: Serialize,
+///     C: Serialize,
+/// {
+///     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+///     where
+///         S: Serializer,
+///     {
+///         let mut tup = serializer.serialize_tuple(3)?;
+///         tup.serialize_element(&self.0)?;
+///         tup.serialize_element(&self.1)?;
+///         tup.serialize_element(&self.2)?;
+///         tup.end()
+///     }
+/// }
+/// ```
+///
+/// ```edition2018
+/// # use std::marker::PhantomData;
+/// #
+/// # struct Array<T>(PhantomData<T>);
+/// #
+/// # impl<T> Array<T> {
+/// #     fn len(&self) -> usize {
+/// #         unimplemented!()
+/// #     }
+/// # }
+/// #
+/// # impl<'a, T> IntoIterator for &'a Array<T> {
+/// #     type Item = &'a T;
+/// #     type IntoIter = Box<Iterator<Item = &'a T>>;
+/// #     fn into_iter(self) -> Self::IntoIter {
+/// #         unimplemented!()
+/// #     }
+/// # }
+/// #
+/// use serde::ser::{Serialize, Serializer, SerializeTuple};
+///
+/// # mod fool {
+/// #     trait Serialize {}
+/// impl<T> Serialize for [T; 16]
+/// #     {}
+/// # }
+/// #
+/// # impl<T> Serialize for Array<T>
+/// where
+///     T: Serialize,
+/// {
+///     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+///     where
+///         S: Serializer,
+///     {
+///         let mut seq = serializer.serialize_tuple(16)?;
+///         for element in self {
+///             seq.serialize_element(element)?;
+///         }
+///         seq.end()
+///     }
+/// }
+/// ```
+///
+/// # Example implementation
+///
+/// The [example data format] presented on the website demonstrates an
+/// implementation of `SerializeTuple` for a basic JSON data format.
+///
+/// [example data format]: https://serde.rs/data-format.html
+pub trait SerializeTuple {
+    /// Must match the `Ok` type of our `Serializer`.
+    type Ok;
+
+    /// Must match the `Error` type of our `Serializer`.
+    type Error: Error;
+
+    /// Serialize a tuple element.
+    fn serialize_element<T: ?Sized>(&mut self, value: &T) -> Result<(), Self::Error>
+    where
+        T: Serialize;
+
+    /// Finish serializing a tuple.
+    fn end(self) -> Result<Self::Ok, Self::Error>;
+}
+
+/// Returned from `Serializer::serialize_tuple_struct`.
+///
+/// # Example use
+///
+/// ```edition2018
+/// use serde::ser::{Serialize, SerializeTupleStruct, Serializer};
+///
+/// struct Rgb(u8, u8, u8);
+///
+/// impl Serialize for Rgb {
+///     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+///     where
+///         S: Serializer,
+///     {
+///         let mut ts = serializer.serialize_tuple_struct("Rgb", 3)?;
+///         ts.serialize_field(&self.0)?;
+///         ts.serialize_field(&self.1)?;
+///         ts.serialize_field(&self.2)?;
+///         ts.end()
+///     }
+/// }
+/// ```
+///
+/// # Example implementation
+///
+/// The [example data format] presented on the website demonstrates an
+/// implementation of `SerializeTupleStruct` for a basic JSON data format.
+///
+/// [example data format]: https://serde.rs/data-format.html
+pub trait SerializeTupleStruct {
+    /// Must match the `Ok` type of our `Serializer`.
+    type Ok;
+
+    /// Must match the `Error` type of our `Serializer`.
+    type Error: Error;
+
+    /// Serialize a tuple struct field.
+    fn serialize_field<T: ?Sized>(&mut self, value: &T) -> Result<(), Self::Error>
+    where
+        T: Serialize;
+
+    /// Finish serializing a tuple struct.
+    fn end(self) -> Result<Self::Ok, Self::Error>;
+}
+
+/// Returned from `Serializer::serialize_tuple_variant`.
+///
+/// # Example use
+///
+/// ```edition2018
+/// use serde::ser::{Serialize, SerializeTupleVariant, Serializer};
+///
+/// enum E {
+///     T(u8, u8),
+///     U(String, u32, u32),
+/// }
+///
+/// impl Serialize for E {
+///     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+///     where
+///         S: Serializer,
+///     {
+///         match *self {
+///             E::T(ref a, ref b) => {
+///                 let mut tv = serializer.serialize_tuple_variant("E", 0, "T", 2)?;
+///                 tv.serialize_field(a)?;
+///                 tv.serialize_field(b)?;
+///                 tv.end()
+///             }
+///             E::U(ref a, ref b, ref c) => {
+///                 let mut tv = serializer.serialize_tuple_variant("E", 1, "U", 3)?;
+///                 tv.serialize_field(a)?;
+///                 tv.serialize_field(b)?;
+///                 tv.serialize_field(c)?;
+///                 tv.end()
+///             }
+///         }
+///     }
+/// }
+/// ```
+///
+/// # Example implementation
+///
+/// The [example data format] presented on the website demonstrates an
+/// implementation of `SerializeTupleVariant` for a basic JSON data format.
+///
+/// [example data format]: https://serde.rs/data-format.html
+pub trait SerializeTupleVariant {
+    /// Must match the `Ok` type of our `Serializer`.
+    type Ok;
+
+    /// Must match the `Error` type of our `Serializer`.
+    type Error: Error;
+
+    /// Serialize a tuple variant field.
+    fn serialize_field<T: ?Sized>(&mut self, value: &T) -> Result<(), Self::Error>
+    where
+        T: Serialize;
+
+    /// Finish serializing a tuple variant.
+    fn end(self) -> Result<Self::Ok, Self::Error>;
+}
+
+/// Returned from `Serializer::serialize_map`.
+///
+/// # Example use
+///
+/// ```edition2018
+/// # use std::marker::PhantomData;
+/// #
+/// # struct HashMap<K, V>(PhantomData<K>, PhantomData<V>);
+/// #
+/// # impl<K, V> HashMap<K, V> {
+/// #     fn len(&self) -> usize {
+/// #         unimplemented!()
+/// #     }
+/// # }
+/// #
+/// # impl<'a, K, V> IntoIterator for &'a HashMap<K, V> {
+/// #     type Item = (&'a K, &'a V);
+/// #     type IntoIter = Box<Iterator<Item = (&'a K, &'a V)>>;
+/// #
+/// #     fn into_iter(self) -> Self::IntoIter {
+/// #         unimplemented!()
+/// #     }
+/// # }
+/// #
+/// use serde::ser::{Serialize, Serializer, SerializeMap};
+///
+/// impl<K, V> Serialize for HashMap<K, V>
+/// where
+///     K: Serialize,
+///     V: Serialize,
+/// {
+///     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+///     where
+///         S: Serializer,
+///     {
+///         let mut map = serializer.serialize_map(Some(self.len()))?;
+///         for (k, v) in self {
+///             map.serialize_entry(k, v)?;
+///         }
+///         map.end()
+///     }
+/// }
+/// ```
+///
+/// # Example implementation
+///
+/// The [example data format] presented on the website demonstrates an
+/// implementation of `SerializeMap` for a basic JSON data format.
+///
+/// [example data format]: https://serde.rs/data-format.html
+pub trait SerializeMap {
+    /// Must match the `Ok` type of our `Serializer`.
+    type Ok;
+
+    /// Must match the `Error` type of our `Serializer`.
+    type Error: Error;
+
+    /// Serialize a map key.
+    ///
+    /// If possible, `Serialize` implementations are encouraged to use
+    /// `serialize_entry` instead as it may be implemented more efficiently in
+    /// some formats compared to a pair of calls to `serialize_key` and
+    /// `serialize_value`.
+    fn serialize_key<T: ?Sized>(&mut self, key: &T) -> Result<(), Self::Error>
+    where
+        T: Serialize;
+
+    /// Serialize a map value.
+    ///
+    /// # Panics
+    ///
+    /// Calling `serialize_value` before `serialize_key` is incorrect and is
+    /// allowed to panic or produce bogus results.
+    fn serialize_value<T: ?Sized>(&mut self, value: &T) -> Result<(), Self::Error>
+    where
+        T: Serialize;
+
+    /// Serialize a map entry consisting of a key and a value.
+    ///
+    /// Some [`Serialize`] types are not able to hold a key and value in memory
+    /// at the same time so `SerializeMap` implementations are required to
+    /// support [`serialize_key`] and [`serialize_value`] individually. The
+    /// `serialize_entry` method allows serializers to optimize for the case
+    /// where key and value are both available. [`Serialize`] implementations
+    /// are encouraged to use `serialize_entry` if possible.
+    ///
+    /// The default implementation delegates to [`serialize_key`] and
+    /// [`serialize_value`]. This is appropriate for serializers that do not
+    /// care about performance or are not able to optimize `serialize_entry` any
+    /// better than this.
+    ///
+    /// [`Serialize`]: ../trait.Serialize.html
+    /// [`serialize_key`]: #tymethod.serialize_key
+    /// [`serialize_value`]: #tymethod.serialize_value
+    fn serialize_entry<K: ?Sized, V: ?Sized>(
+        &mut self,
+        key: &K,
+        value: &V,
+    ) -> Result<(), Self::Error>
+    where
+        K: Serialize,
+        V: Serialize,
+    {
+        try!(self.serialize_key(key));
+        self.serialize_value(value)
+    }
+
+    /// Finish serializing a map.
+    fn end(self) -> Result<Self::Ok, Self::Error>;
+}
+
+/// Returned from `Serializer::serialize_struct`.
+///
+/// # Example use
+///
+/// ```edition2018
+/// use serde::ser::{Serialize, SerializeStruct, Serializer};
+///
+/// struct Rgb {
+///     r: u8,
+///     g: u8,
+///     b: u8,
+/// }
+///
+/// impl Serialize for Rgb {
+///     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+///     where
+///         S: Serializer,
+///     {
+///         let mut rgb = serializer.serialize_struct("Rgb", 3)?;
+///         rgb.serialize_field("r", &self.r)?;
+///         rgb.serialize_field("g", &self.g)?;
+///         rgb.serialize_field("b", &self.b)?;
+///         rgb.end()
+///     }
+/// }
+/// ```
+///
+/// # Example implementation
+///
+/// The [example data format] presented on the website demonstrates an
+/// implementation of `SerializeStruct` for a basic JSON data format.
+///
+/// [example data format]: https://serde.rs/data-format.html
+pub trait SerializeStruct {
+    /// Must match the `Ok` type of our `Serializer`.
+    type Ok;
+
+    /// Must match the `Error` type of our `Serializer`.
+    type Error: Error;
+
+    /// Serialize a struct field.
+    fn serialize_field<T: ?Sized>(
+        &mut self,
+        key: &'static str,
+        value: &T,
+    ) -> Result<(), Self::Error>
+    where
+        T: Serialize;
+
+    /// Indicate that a struct field has been skipped.
+    #[inline]
+    fn skip_field(&mut self, key: &'static str) -> Result<(), Self::Error> {
+        let _ = key;
+        Ok(())
+    }
+
+    /// Finish serializing a struct.
+    fn end(self) -> Result<Self::Ok, Self::Error>;
+}
+
+/// Returned from `Serializer::serialize_struct_variant`.
+///
+/// # Example use
+///
+/// ```edition2018
+/// use serde::ser::{Serialize, SerializeStructVariant, Serializer};
+///
+/// enum E {
+///     S { r: u8, g: u8, b: u8 },
+/// }
+///
+/// impl Serialize for E {
+///     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+///     where
+///         S: Serializer,
+///     {
+///         match *self {
+///             E::S {
+///                 ref r,
+///                 ref g,
+///                 ref b,
+///             } => {
+///                 let mut sv = serializer.serialize_struct_variant("E", 0, "S", 3)?;
+///                 sv.serialize_field("r", r)?;
+///                 sv.serialize_field("g", g)?;
+///                 sv.serialize_field("b", b)?;
+///                 sv.end()
+///             }
+///         }
+///     }
+/// }
+/// ```
+///
+/// # Example implementation
+///
+/// The [example data format] presented on the website demonstrates an
+/// implementation of `SerializeStructVariant` for a basic JSON data format.
+///
+/// [example data format]: https://serde.rs/data-format.html
+pub trait SerializeStructVariant {
+    /// Must match the `Ok` type of our `Serializer`.
+    type Ok;
+
+    /// Must match the `Error` type of our `Serializer`.
+    type Error: Error;
+
+    /// Serialize a struct variant field.
+    fn serialize_field<T: ?Sized>(
+        &mut self,
+        key: &'static str,
+        value: &T,
+    ) -> Result<(), Self::Error>
+    where
+        T: Serialize;
+
+    /// Indicate that a struct variant field has been skipped.
+    #[inline]
+    fn skip_field(&mut self, key: &'static str) -> Result<(), Self::Error> {
+        let _ = key;
+        Ok(())
+    }
+
+    /// Finish serializing a struct variant.
+    fn end(self) -> Result<Self::Ok, Self::Error>;
+}
+
+fn iterator_len_hint<I>(iter: &I) -> Option<usize>
+where
+    I: Iterator,
+{
+    match iter.size_hint() {
+        (lo, Some(hi)) if lo == hi => Some(lo),
+        _ => None,
+    }
+}
diff --git a/src/std_error.rs b/src/std_error.rs
new file mode 100644
index 0000000..1055e0f
--- /dev/null
+++ b/src/std_error.rs
@@ -0,0 +1,48 @@
+use lib::{Debug, Display};
+
+/// Either a re-export of std::error::Error or a new identical trait, depending
+/// on whether Serde's "std" feature is enabled.
+///
+/// Serde's error traits [`serde::ser::Error`] and [`serde::de::Error`] require
+/// [`std::error::Error`] as a supertrait, but only when Serde is built with
+/// "std" enabled. Data formats that don't care about no\_std support should
+/// generally provide their error types with a `std::error::Error` impl
+/// directly:
+///
+/// ```edition2018
+/// #[derive(Debug)]
+/// struct MySerError {...}
+///
+/// impl serde::ser::Error for MySerError {...}
+///
+/// impl std::fmt::Display for MySerError {...}
+///
+/// // We don't support no_std!
+/// impl std::error::Error for MySerError {}
+/// ```
+///
+/// Data formats that *do* support no\_std may either have a "std" feature of
+/// their own:
+///
+/// ```toml
+/// [features]
+/// std = ["serde/std"]
+/// ```
+///
+/// ```edition2018
+/// #[cfg(feature = "std")]
+/// impl std::error::Error for MySerError {}
+/// ```
+///
+/// ... or else provide the std Error impl unconditionally via Serde's
+/// re-export:
+///
+/// ```edition2018
+/// impl serde::ser::StdError for MySerError {}
+/// ```
+pub trait Error: Debug + Display {
+    /// The underlying cause of this error, if any.
+    fn source(&self) -> Option<&(Error + 'static)> {
+        None
+    }
+}