library/core/src/error.rs - toolchain/rustc - Git at Google

 #![doc = include_str!("error.md")]
 #![unstable(feature = "error_in_core", issue = "none")]

 #[cfg(test)]
 mod tests;

 use crate::any::{Demand, Provider, TypeId};
 use crate::fmt::{Debug, Display};

 /// `Error` is a trait representing the basic expectations for error values,
 /// i.e., values of type `E` in [`Result<T, E>`].
 ///
 /// Errors must describe themselves through the [`Display`] and [`Debug`]
 /// traits. Error messages are typically concise lowercase sentences without
 /// trailing punctuation:
 ///
 /// ```
 /// let err = "NaN".parse::<u32>().unwrap_err();
 /// assert_eq!(err.to_string(), "invalid digit found in string");
 /// ```
 ///
 /// Errors may provide cause information. [`Error::source()`] is generally
 /// used when errors cross "abstraction boundaries". If one module must report
 /// an error that is caused by an error from a lower-level module, it can allow
 /// accessing that error via [`Error::source()`]. This makes it possible for the
 /// high-level module to provide its own errors while also revealing some of the
 /// implementation for debugging.
 #[stable(feature = "rust1", since = "1.0.0")]
 #[cfg_attr(not(test), rustc_diagnostic_item = "Error")]
 #[rustc_has_incoherent_inherent_impls]
 pub trait Error: Debug + Display {
     /// The lower-level source of this error, if any.
     ///
     /// # Examples
     ///
     /// ```
     /// use std::error::Error;
     /// use std::fmt;
     ///
     /// #[derive(Debug)]
     /// struct SuperError {
     ///     source: SuperErrorSideKick,
     /// }
     ///
     /// impl fmt::Display for SuperError {
     ///     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
     ///         write!(f, "SuperError is here!")
     ///     }
     /// }
     ///
     /// impl Error for SuperError {
     ///     fn source(&self) -> Option<&(dyn Error + 'static)> {
     ///         Some(&self.source)
     ///     }
     /// }
     ///
     /// #[derive(Debug)]
     /// struct SuperErrorSideKick;
     ///
     /// impl fmt::Display for SuperErrorSideKick {
     ///     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
     ///         write!(f, "SuperErrorSideKick is here!")
     ///     }
     /// }
     ///
     /// impl Error for SuperErrorSideKick {}
     ///
     /// fn get_super_error() -> Result<(), SuperError> {
     ///     Err(SuperError { source: SuperErrorSideKick })
     /// }
     ///
     /// fn main() {
     ///     match get_super_error() {
     ///         Err(e) => {
     ///             println!("Error: {e}");
     ///             println!("Caused by: {}", e.source().unwrap());
     ///         }
     ///         _ => println!("No error"),
     ///     }
     /// }
     /// ```
     #[stable(feature = "error_source", since = "1.30.0")]
     fn source(&self) -> Option<&(dyn Error + 'static)> {
         None
     }

     /// Gets the `TypeId` of `self`.
     #[doc(hidden)]
     #[unstable(
         feature = "error_type_id",
         reason = "this is memory-unsafe to override in user code",
         issue = "60784"
     )]
     fn type_id(&self, _: private::Internal) -> TypeId
     where
         Self: 'static,
     {
         TypeId::of::<Self>()
     }

     /// ```
     /// if let Err(e) = "xc".parse::<u32>() {
     ///     // Print `e` itself, no need for description().
     ///     eprintln!("Error: {e}");
     /// }
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     #[deprecated(since = "1.42.0", note = "use the Display impl or to_string()")]
     fn description(&self) -> &str {
         "description() is deprecated; use Display"
     }

     #[stable(feature = "rust1", since = "1.0.0")]
     #[deprecated(
         since = "1.33.0",
         note = "replaced by Error::source, which can support downcasting"
     )]
     #[allow(missing_docs)]
     fn cause(&self) -> Option<&dyn Error> {
         self.source()
     }

     /// Provides type based access to context intended for error reports.
     ///
     /// Used in conjunction with [`Demand::provide_value`] and [`Demand::provide_ref`] to extract
     /// references to member variables from `dyn Error` trait objects.
     ///
     /// # Example
     ///
     /// ```rust
     /// #![feature(provide_any)]
     /// #![feature(error_generic_member_access)]
     /// use core::fmt;
     /// use core::any::Demand;
     ///
     /// #[derive(Debug)]
     /// struct MyBacktrace {
     ///     // ...
     /// }
     ///
     /// impl MyBacktrace {
     ///     fn new() -> MyBacktrace {
     ///         // ...
     ///         # MyBacktrace {}
     ///     }
     /// }
     ///
     /// #[derive(Debug)]
     /// struct SourceError {
     ///     // ...
     /// }
     ///
     /// impl fmt::Display for SourceError {
     ///     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
     ///         write!(f, "Example Source Error")
     ///     }
     /// }
     ///
     /// impl std::error::Error for SourceError {}
     ///
     /// #[derive(Debug)]
     /// struct Error {
     ///     source: SourceError,
     ///     backtrace: MyBacktrace,
     /// }
     ///
     /// impl fmt::Display for Error {
     ///     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
     ///         write!(f, "Example Error")
     ///     }
     /// }
     ///
     /// impl std::error::Error for Error {
     ///     fn provide<'a>(&'a self, demand: &mut Demand<'a>) {
     ///         demand
     ///             .provide_ref::<MyBacktrace>(&self.backtrace)
     ///             .provide_ref::<dyn std::error::Error + 'static>(&self.source);
     ///     }
     /// }
     ///
     /// fn main() {
     ///     let backtrace = MyBacktrace::new();
     ///     let source = SourceError {};
     ///     let error = Error { source, backtrace };
     ///     let dyn_error = &error as &dyn std::error::Error;
     ///     let backtrace_ref = dyn_error.request_ref::<MyBacktrace>().unwrap();
     ///
     ///     assert!(core::ptr::eq(&error.backtrace, backtrace_ref));
     /// }
     /// ```
     #[unstable(feature = "error_generic_member_access", issue = "99301")]
     #[allow(unused_variables)]
     fn provide<'a>(&'a self, demand: &mut Demand<'a>) {}
 }

 #[unstable(feature = "error_generic_member_access", issue = "99301")]
 impl<E> Provider for E
 where
     E: Error + ?Sized,
 {
     fn provide<'a>(&'a self, demand: &mut Demand<'a>) {
         self.provide(demand)
     }
 }

 mod private {
     // This is a hack to prevent `type_id` from being overridden by `Error`
     // implementations, since that can enable unsound downcasting.
     #[unstable(feature = "error_type_id", issue = "60784")]
     #[derive(Debug)]
     pub struct Internal;
 }

 #[unstable(feature = "never_type", issue = "35121")]
 impl Error for ! {}

 impl<'a> dyn Error + 'a {
     /// Request a reference of type `T` as context about this error.
     #[unstable(feature = "error_generic_member_access", issue = "99301")]
     pub fn request_ref<T: ?Sized + 'static>(&'a self) -> Option<&'a T> {
         core::any::request_ref(self)
     }

     /// Request a value of type `T` as context about this error.
     #[unstable(feature = "error_generic_member_access", issue = "99301")]
     pub fn request_value<T: 'static>(&'a self) -> Option<T> {
         core::any::request_value(self)
     }
 }

 // Copied from `any.rs`.
 impl dyn Error + 'static {
     /// Returns `true` if the inner type is the same as `T`.
     #[stable(feature = "error_downcast", since = "1.3.0")]
     #[inline]
     pub fn is<T: Error + 'static>(&self) -> bool {
         // Get `TypeId` of the type this function is instantiated with.
         let t = TypeId::of::<T>();

         // Get `TypeId` of the type in the trait object (`self`).
         let concrete = self.type_id(private::Internal);

         // Compare both `TypeId`s on equality.
         t == concrete
     }

     /// Returns some reference to the inner value if it is of type `T`, or
     /// `None` if it isn't.
     #[stable(feature = "error_downcast", since = "1.3.0")]
     #[inline]
     pub fn downcast_ref<T: Error + 'static>(&self) -> Option<&T> {
         if self.is::<T>() {
             // SAFETY: `is` ensures this type cast is correct
             unsafe { Some(&*(self as *const dyn Error as *const T)) }
         } else {
             None
         }
     }

     /// Returns some mutable reference to the inner value if it is of type `T`, or
     /// `None` if it isn't.
     #[stable(feature = "error_downcast", since = "1.3.0")]
     #[inline]
     pub fn downcast_mut<T: Error + 'static>(&mut self) -> Option<&mut T> {
         if self.is::<T>() {
             // SAFETY: `is` ensures this type cast is correct
             unsafe { Some(&mut *(self as *mut dyn Error as *mut T)) }
         } else {
             None
         }
     }
 }

 impl dyn Error + 'static + Send {
     /// Forwards to the method defined on the type `dyn Error`.
     #[stable(feature = "error_downcast", since = "1.3.0")]
     #[inline]
     pub fn is<T: Error + 'static>(&self) -> bool {
         <dyn Error + 'static>::is::<T>(self)
     }

     /// Forwards to the method defined on the type `dyn Error`.
     #[stable(feature = "error_downcast", since = "1.3.0")]
     #[inline]
     pub fn downcast_ref<T: Error + 'static>(&self) -> Option<&T> {
         <dyn Error + 'static>::downcast_ref::<T>(self)
     }

     /// Forwards to the method defined on the type `dyn Error`.
     #[stable(feature = "error_downcast", since = "1.3.0")]
     #[inline]
     pub fn downcast_mut<T: Error + 'static>(&mut self) -> Option<&mut T> {
         <dyn Error + 'static>::downcast_mut::<T>(self)
     }

     /// Request a reference of type `T` as context about this error.
     #[unstable(feature = "error_generic_member_access", issue = "99301")]
     pub fn request_ref<T: ?Sized + 'static>(&self) -> Option<&T> {
         <dyn Error>::request_ref(self)
     }

     /// Request a value of type `T` as context about this error.
     #[unstable(feature = "error_generic_member_access", issue = "99301")]
     pub fn request_value<T: 'static>(&self) -> Option<T> {
         <dyn Error>::request_value(self)
     }
 }

 impl dyn Error + 'static + Send + Sync {
     /// Forwards to the method defined on the type `dyn Error`.
     #[stable(feature = "error_downcast", since = "1.3.0")]
     #[inline]
     pub fn is<T: Error + 'static>(&self) -> bool {
         <dyn Error + 'static>::is::<T>(self)
     }

     /// Forwards to the method defined on the type `dyn Error`.
     #[stable(feature = "error_downcast", since = "1.3.0")]
     #[inline]
     pub fn downcast_ref<T: Error + 'static>(&self) -> Option<&T> {
         <dyn Error + 'static>::downcast_ref::<T>(self)
     }

     /// Forwards to the method defined on the type `dyn Error`.
     #[stable(feature = "error_downcast", since = "1.3.0")]
     #[inline]
     pub fn downcast_mut<T: Error + 'static>(&mut self) -> Option<&mut T> {
         <dyn Error + 'static>::downcast_mut::<T>(self)
     }

     /// Request a reference of type `T` as context about this error.
     #[unstable(feature = "error_generic_member_access", issue = "99301")]
     pub fn request_ref<T: ?Sized + 'static>(&self) -> Option<&T> {
         <dyn Error>::request_ref(self)
     }

     /// Request a value of type `T` as context about this error.
     #[unstable(feature = "error_generic_member_access", issue = "99301")]
     pub fn request_value<T: 'static>(&self) -> Option<T> {
         <dyn Error>::request_value(self)
     }
 }

 impl dyn Error {
     /// Returns an iterator starting with the current error and continuing with
     /// recursively calling [`Error::source`].
     ///
     /// If you want to omit the current error and only use its sources,
     /// use `skip(1)`.
     ///
     /// # Examples
     ///
     /// ```
     /// #![feature(error_iter)]
     /// use std::error::Error;
     /// use std::fmt;
     ///
     /// #[derive(Debug)]
     /// struct A;
     ///
     /// #[derive(Debug)]
     /// struct B(Option<Box<dyn Error + 'static>>);
     ///
     /// impl fmt::Display for A {
     ///     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
     ///         write!(f, "A")
     ///     }
     /// }
     ///
     /// impl fmt::Display for B {
     ///     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
     ///         write!(f, "B")
     ///     }
     /// }
     ///
     /// impl Error for A {}
     ///
     /// impl Error for B {
     ///     fn source(&self) -> Option<&(dyn Error + 'static)> {
     ///         self.0.as_ref().map(|e| e.as_ref())
     ///     }
     /// }
     ///
     /// let b = B(Some(Box::new(A)));
     ///
     /// // let err : Box<Error> = b.into(); // or
     /// let err = &b as &(dyn Error);
     ///
     /// let mut iter = err.sources();
     ///
     /// assert_eq!("B".to_string(), iter.next().unwrap().to_string());
     /// assert_eq!("A".to_string(), iter.next().unwrap().to_string());
     /// assert!(iter.next().is_none());
     /// assert!(iter.next().is_none());
     /// ```
     #[unstable(feature = "error_iter", issue = "58520")]
     #[inline]
     pub fn sources(&self) -> Source<'_> {
         // You may think this method would be better in the Error trait, and you'd be right.
         // Unfortunately that doesn't work, not because of the object safety rules but because we
         // save a reference to self in Sources below as a trait object. If this method was
         // declared in Error, then self would have the type &T where T is some concrete type which
         // implements Error. We would need to coerce self to have type &dyn Error, but that requires
         // that Self has a known size (i.e., Self: Sized). We can't put that bound on Error
         // since that would forbid Error trait objects, and we can't put that bound on the method
         // because that means the method can't be called on trait objects (we'd also need the
         // 'static bound, but that isn't allowed because methods with bounds on Self other than
         // Sized are not object-safe). Requiring an Unsize bound is not backwards compatible.

         Source { current: Some(self) }
     }
 }

 /// An iterator over an [`Error`] and its sources.
 ///
 /// If you want to omit the initial error and only process
 /// its sources, use `skip(1)`.
 #[unstable(feature = "error_iter", issue = "58520")]
 #[derive(Clone, Debug)]
 pub struct Source<'a> {
     current: Option<&'a (dyn Error + 'static)>,
 }

 #[unstable(feature = "error_iter", issue = "58520")]
 impl<'a> Iterator for Source<'a> {
     type Item = &'a (dyn Error + 'static);

     fn next(&mut self) -> Option<Self::Item> {
         let current = self.current;
         self.current = self.current.and_then(Error::source);
         current
     }
 }

 #[stable(feature = "error_by_ref", since = "1.51.0")]
 impl<'a, T: Error + ?Sized> Error for &'a T {
     #[allow(deprecated, deprecated_in_future)]
     fn description(&self) -> &str {
         Error::description(&**self)
     }

     #[allow(deprecated)]
     fn cause(&self) -> Option<&dyn Error> {
         Error::cause(&**self)
     }

     fn source(&self) -> Option<&(dyn Error + 'static)> {
         Error::source(&**self)
     }

     fn provide<'b>(&'b self, demand: &mut Demand<'b>) {
         Error::provide(&**self, demand);
     }
 }

 #[stable(feature = "fmt_error", since = "1.11.0")]
 impl Error for crate::fmt::Error {
     #[allow(deprecated)]
     fn description(&self) -> &str {
         "an error occurred when formatting an argument"
     }
 }

 #[stable(feature = "try_borrow", since = "1.13.0")]
 impl Error for crate::cell::BorrowError {
     #[allow(deprecated)]
     fn description(&self) -> &str {
         "already mutably borrowed"
     }
 }

 #[stable(feature = "try_borrow", since = "1.13.0")]
 impl Error for crate::cell::BorrowMutError {
     #[allow(deprecated)]
     fn description(&self) -> &str {
         "already borrowed"
     }
 }

 #[stable(feature = "try_from", since = "1.34.0")]
 impl Error for crate::char::CharTryFromError {
     #[allow(deprecated)]
     fn description(&self) -> &str {
         "converted integer out of range for `char`"
     }
 }

 #[stable(feature = "char_from_str", since = "1.20.0")]
 impl Error for crate::char::ParseCharError {
     #[allow(deprecated)]
     fn description(&self) -> &str {
         self.__description()
     }
 }

 #[stable(feature = "duration_checked_float", since = "1.66.0")]
 impl Error for crate::time::TryFromFloatSecsError {}

 #[stable(feature = "frombyteswithnulerror_impls", since = "1.17.0")]
 impl Error for crate::ffi::FromBytesWithNulError {
     #[allow(deprecated)]
     fn description(&self) -> &str {
         self.__description()
     }
 }

 #[unstable(feature = "cstr_from_bytes_until_nul", issue = "95027")]
 impl Error for crate::ffi::FromBytesUntilNulError {}
	#![doc = include_str!("error.md")]
	#![unstable(feature = "error_in_core", issue = "none")]

	#[cfg(test)]
	mod tests;

	use crate::any::{Demand, Provider, TypeId};
	use crate::fmt::{Debug, Display};

	/// `Error` is a trait representing the basic expectations for error values,
	/// i.e., values of type `E` in [`Result<T, E>`].
	///
	/// Errors must describe themselves through the [`Display`] and [`Debug`]
	/// traits. Error messages are typically concise lowercase sentences without
	/// trailing punctuation:
	///
	/// ```
	/// let err = "NaN".parse::<u32>().unwrap_err();
	/// assert_eq!(err.to_string(), "invalid digit found in string");
	/// ```
	///
	/// Errors may provide cause information. [`Error::source()`] is generally
	/// used when errors cross "abstraction boundaries". If one module must report
	/// an error that is caused by an error from a lower-level module, it can allow
	/// accessing that error via [`Error::source()`]. This makes it possible for the
	/// high-level module to provide its own errors while also revealing some of the
	/// implementation for debugging.
	#[stable(feature = "rust1", since = "1.0.0")]
	#[cfg_attr(not(test), rustc_diagnostic_item = "Error")]
	#[rustc_has_incoherent_inherent_impls]
	pub trait Error: Debug + Display {
	/// The lower-level source of this error, if any.
	///
	/// # Examples
	///
	/// ```
	/// use std::error::Error;
	/// use std::fmt;
	///
	/// #[derive(Debug)]
	/// struct SuperError {
	/// source: SuperErrorSideKick,
	/// }
	///
	/// impl fmt::Display for SuperError {
	/// fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
	/// write!(f, "SuperError is here!")
	/// }
	/// }
	///
	/// impl Error for SuperError {
	/// fn source(&self) -> Option<&(dyn Error + 'static)> {
	/// Some(&self.source)
	/// }
	/// }
	///
	/// #[derive(Debug)]
	/// struct SuperErrorSideKick;
	///
	/// impl fmt::Display for SuperErrorSideKick {
	/// fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
	/// write!(f, "SuperErrorSideKick is here!")
	/// }
	/// }
	///
	/// impl Error for SuperErrorSideKick {}
	///
	/// fn get_super_error() -> Result<(), SuperError> {
	/// Err(SuperError { source: SuperErrorSideKick })
	/// }
	///
	/// fn main() {
	/// match get_super_error() {
	/// Err(e) => {
	/// println!("Error: {e}");
	/// println!("Caused by: {}", e.source().unwrap());
	/// }
	/// _ => println!("No error"),
	/// }
	/// }
	/// ```
	#[stable(feature = "error_source", since = "1.30.0")]
	fn source(&self) -> Option<&(dyn Error + 'static)> {
	None
	}

	/// Gets the `TypeId` of `self`.
	#[doc(hidden)]
	#[unstable(
	feature = "error_type_id",
	reason = "this is memory-unsafe to override in user code",
	issue = "60784"
	)]
	fn type_id(&self, _: private::Internal) -> TypeId
	where
	Self: 'static,
	{
	TypeId::of::<Self>()
	}

	/// ```
	/// if let Err(e) = "xc".parse::<u32>() {
	/// // Print `e` itself, no need for description().
	/// eprintln!("Error: {e}");
	/// }
	/// ```
	#[stable(feature = "rust1", since = "1.0.0")]
	#[deprecated(since = "1.42.0", note = "use the Display impl or to_string()")]
	fn description(&self) -> &str {
	"description() is deprecated; use Display"
	}

	#[stable(feature = "rust1", since = "1.0.0")]
	#[deprecated(
	since = "1.33.0",
	note = "replaced by Error::source, which can support downcasting"
	)]
	#[allow(missing_docs)]
	fn cause(&self) -> Option<&dyn Error> {
	self.source()
	}

	/// Provides type based access to context intended for error reports.
	///
	/// Used in conjunction with [`Demand::provide_value`] and [`Demand::provide_ref`] to extract
	/// references to member variables from `dyn Error` trait objects.
	///
	/// # Example
	///
	/// ```rust
	/// #![feature(provide_any)]
	/// #![feature(error_generic_member_access)]
	/// use core::fmt;
	/// use core::any::Demand;
	///
	/// #[derive(Debug)]
	/// struct MyBacktrace {
	/// // ...
	/// }
	///
	/// impl MyBacktrace {
	/// fn new() -> MyBacktrace {
	/// // ...
	/// # MyBacktrace {}
	/// }
	/// }
	///
	/// #[derive(Debug)]
	/// struct SourceError {
	/// // ...
	/// }
	///
	/// impl fmt::Display for SourceError {
	/// fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
	/// write!(f, "Example Source Error")
	/// }
	/// }
	///
	/// impl std::error::Error for SourceError {}
	///
	/// #[derive(Debug)]
	/// struct Error {
	/// source: SourceError,
	/// backtrace: MyBacktrace,
	/// }
	///
	/// impl fmt::Display for Error {
	/// fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
	/// write!(f, "Example Error")
	/// }
	/// }
	///
	/// impl std::error::Error for Error {
	/// fn provide<'a>(&'a self, demand: &mut Demand<'a>) {
	/// demand
	/// .provide_ref::<MyBacktrace>(&self.backtrace)
	/// .provide_ref::<dyn std::error::Error + 'static>(&self.source);
	/// }
	/// }
	///
	/// fn main() {
	/// let backtrace = MyBacktrace::new();
	/// let source = SourceError {};
	/// let error = Error { source, backtrace };
	/// let dyn_error = &error as &dyn std::error::Error;
	/// let backtrace_ref = dyn_error.request_ref::<MyBacktrace>().unwrap();
	///
	/// assert!(core::ptr::eq(&error.backtrace, backtrace_ref));
	/// }
	/// ```
	#[unstable(feature = "error_generic_member_access", issue = "99301")]
	#[allow(unused_variables)]
	fn provide<'a>(&'a self, demand: &mut Demand<'a>) {}
	}

	#[unstable(feature = "error_generic_member_access", issue = "99301")]
	impl<E> Provider for E
	where
	E: Error + ?Sized,
	{
	fn provide<'a>(&'a self, demand: &mut Demand<'a>) {
	self.provide(demand)
	}
	}

	mod private {
	// This is a hack to prevent `type_id` from being overridden by `Error`
	// implementations, since that can enable unsound downcasting.
	#[unstable(feature = "error_type_id", issue = "60784")]
	#[derive(Debug)]
	pub struct Internal;
	}

	#[unstable(feature = "never_type", issue = "35121")]
	impl Error for ! {}

	impl<'a> dyn Error + 'a {
	/// Request a reference of type `T` as context about this error.
	#[unstable(feature = "error_generic_member_access", issue = "99301")]
	pub fn request_ref<T: ?Sized + 'static>(&'a self) -> Option<&'a T> {
	core::any::request_ref(self)
	}

	/// Request a value of type `T` as context about this error.
	#[unstable(feature = "error_generic_member_access", issue = "99301")]
	pub fn request_value<T: 'static>(&'a self) -> Option<T> {
	core::any::request_value(self)
	}
	}

	// Copied from `any.rs`.
	impl dyn Error + 'static {
	/// Returns `true` if the inner type is the same as `T`.
	#[stable(feature = "error_downcast", since = "1.3.0")]
	#[inline]
	pub fn is<T: Error + 'static>(&self) -> bool {
	// Get `TypeId` of the type this function is instantiated with.
	let t = TypeId::of::<T>();

	// Get `TypeId` of the type in the trait object (`self`).
	let concrete = self.type_id(private::Internal);

	// Compare both `TypeId`s on equality.
	t == concrete
	}

	/// Returns some reference to the inner value if it is of type `T`, or
	/// `None` if it isn't.
	#[stable(feature = "error_downcast", since = "1.3.0")]
	#[inline]
	pub fn downcast_ref<T: Error + 'static>(&self) -> Option<&T> {
	if self.is::<T>() {
	// SAFETY: `is` ensures this type cast is correct
	unsafe { Some(&(self as const dyn Error as *const T)) }
	} else {
	None
	}
	}

	/// Returns some mutable reference to the inner value if it is of type `T`, or
	/// `None` if it isn't.
	#[stable(feature = "error_downcast", since = "1.3.0")]
	#[inline]
	pub fn downcast_mut<T: Error + 'static>(&mut self) -> Option<&mut T> {
	if self.is::<T>() {
	// SAFETY: `is` ensures this type cast is correct
	unsafe { Some(&mut (self as mut dyn Error as *mut T)) }
	} else {
	None
	}
	}
	}

	impl dyn Error + 'static + Send {
	/// Forwards to the method defined on the type `dyn Error`.
	#[stable(feature = "error_downcast", since = "1.3.0")]
	#[inline]
	pub fn is<T: Error + 'static>(&self) -> bool {
	<dyn Error + 'static>::is::<T>(self)
	}

	/// Forwards to the method defined on the type `dyn Error`.
	#[stable(feature = "error_downcast", since = "1.3.0")]
	#[inline]
	pub fn downcast_ref<T: Error + 'static>(&self) -> Option<&T> {
	<dyn Error + 'static>::downcast_ref::<T>(self)
	}

	/// Forwards to the method defined on the type `dyn Error`.
	#[stable(feature = "error_downcast", since = "1.3.0")]
	#[inline]
	pub fn downcast_mut<T: Error + 'static>(&mut self) -> Option<&mut T> {
	<dyn Error + 'static>::downcast_mut::<T>(self)
	}

	/// Request a reference of type `T` as context about this error.
	#[unstable(feature = "error_generic_member_access", issue = "99301")]
	pub fn request_ref<T: ?Sized + 'static>(&self) -> Option<&T> {
	<dyn Error>::request_ref(self)
	}

	/// Request a value of type `T` as context about this error.
	#[unstable(feature = "error_generic_member_access", issue = "99301")]
	pub fn request_value<T: 'static>(&self) -> Option<T> {
	<dyn Error>::request_value(self)
	}
	}

	impl dyn Error + 'static + Send + Sync {
	/// Forwards to the method defined on the type `dyn Error`.
	#[stable(feature = "error_downcast", since = "1.3.0")]
	#[inline]
	pub fn is<T: Error + 'static>(&self) -> bool {
	<dyn Error + 'static>::is::<T>(self)
	}

	/// Forwards to the method defined on the type `dyn Error`.
	#[stable(feature = "error_downcast", since = "1.3.0")]
	#[inline]
	pub fn downcast_ref<T: Error + 'static>(&self) -> Option<&T> {
	<dyn Error + 'static>::downcast_ref::<T>(self)
	}

	/// Forwards to the method defined on the type `dyn Error`.
	#[stable(feature = "error_downcast", since = "1.3.0")]
	#[inline]
	pub fn downcast_mut<T: Error + 'static>(&mut self) -> Option<&mut T> {
	<dyn Error + 'static>::downcast_mut::<T>(self)
	}

	/// Request a reference of type `T` as context about this error.
	#[unstable(feature = "error_generic_member_access", issue = "99301")]
	pub fn request_ref<T: ?Sized + 'static>(&self) -> Option<&T> {
	<dyn Error>::request_ref(self)
	}

	/// Request a value of type `T` as context about this error.
	#[unstable(feature = "error_generic_member_access", issue = "99301")]
	pub fn request_value<T: 'static>(&self) -> Option<T> {
	<dyn Error>::request_value(self)
	}
	}

	impl dyn Error {
	/// Returns an iterator starting with the current error and continuing with
	/// recursively calling [`Error::source`].
	///
	/// If you want to omit the current error and only use its sources,
	/// use `skip(1)`.
	///
	/// # Examples
	///
	/// ```
	/// #![feature(error_iter)]
	/// use std::error::Error;
	/// use std::fmt;
	///
	/// #[derive(Debug)]
	/// struct A;
	///
	/// #[derive(Debug)]
	/// struct B(Option<Box<dyn Error + 'static>>);
	///
	/// impl fmt::Display for A {
	/// fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
	/// write!(f, "A")
	/// }
	/// }
	///
	/// impl fmt::Display for B {
	/// fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
	/// write!(f, "B")
	/// }
	/// }
	///
	/// impl Error for A {}
	///
	/// impl Error for B {
	/// fn source(&self) -> Option<&(dyn Error + 'static)> {
	/// self.0.as_ref().map(\|e\| e.as_ref())
	/// }
	/// }
	///
	/// let b = B(Some(Box::new(A)));
	///
	/// // let err : Box<Error> = b.into(); // or
	/// let err = &b as &(dyn Error);
	///
	/// let mut iter = err.sources();
	///
	/// assert_eq!("B".to_string(), iter.next().unwrap().to_string());
	/// assert_eq!("A".to_string(), iter.next().unwrap().to_string());
	/// assert!(iter.next().is_none());
	/// assert!(iter.next().is_none());
	/// ```
	#[unstable(feature = "error_iter", issue = "58520")]
	#[inline]
	pub fn sources(&self) -> Source<'_> {
	// You may think this method would be better in the Error trait, and you'd be right.
	// Unfortunately that doesn't work, not because of the object safety rules but because we
	// save a reference to self in Sources below as a trait object. If this method was
	// declared in Error, then self would have the type &T where T is some concrete type which
	// implements Error. We would need to coerce self to have type &dyn Error, but that requires
	// that Self has a known size (i.e., Self: Sized). We can't put that bound on Error
	// since that would forbid Error trait objects, and we can't put that bound on the method
	// because that means the method can't be called on trait objects (we'd also need the
	// 'static bound, but that isn't allowed because methods with bounds on Self other than
	// Sized are not object-safe). Requiring an Unsize bound is not backwards compatible.

	Source { current: Some(self) }
	}
	}

	/// An iterator over an [`Error`] and its sources.
	///
	/// If you want to omit the initial error and only process
	/// its sources, use `skip(1)`.
	#[unstable(feature = "error_iter", issue = "58520")]
	#[derive(Clone, Debug)]
	pub struct Source<'a> {
	current: Option<&'a (dyn Error + 'static)>,
	}

	#[unstable(feature = "error_iter", issue = "58520")]
	impl<'a> Iterator for Source<'a> {
	type Item = &'a (dyn Error + 'static);

	fn next(&mut self) -> Option<Self::Item> {
	let current = self.current;
	self.current = self.current.and_then(Error::source);
	current
	}
	}

	#[stable(feature = "error_by_ref", since = "1.51.0")]
	impl<'a, T: Error + ?Sized> Error for &'a T {
	#[allow(deprecated, deprecated_in_future)]
	fn description(&self) -> &str {
	Error::description(&**self)
	}

	#[allow(deprecated)]
	fn cause(&self) -> Option<&dyn Error> {
	Error::cause(&**self)
	}

	fn source(&self) -> Option<&(dyn Error + 'static)> {
	Error::source(&**self)
	}

	fn provide<'b>(&'b self, demand: &mut Demand<'b>) {
	Error::provide(&**self, demand);
	}
	}

	#[stable(feature = "fmt_error", since = "1.11.0")]
	impl Error for crate::fmt::Error {
	#[allow(deprecated)]
	fn description(&self) -> &str {
	"an error occurred when formatting an argument"
	}
	}

	#[stable(feature = "try_borrow", since = "1.13.0")]
	impl Error for crate::cell::BorrowError {
	#[allow(deprecated)]
	fn description(&self) -> &str {
	"already mutably borrowed"
	}
	}

	#[stable(feature = "try_borrow", since = "1.13.0")]
	impl Error for crate::cell::BorrowMutError {
	#[allow(deprecated)]
	fn description(&self) -> &str {
	"already borrowed"
	}
	}

	#[stable(feature = "try_from", since = "1.34.0")]
	impl Error for crate::char::CharTryFromError {
	#[allow(deprecated)]
	fn description(&self) -> &str {
	"converted integer out of range for `char`"
	}
	}

	#[stable(feature = "char_from_str", since = "1.20.0")]
	impl Error for crate::char::ParseCharError {
	#[allow(deprecated)]
	fn description(&self) -> &str {
	self.__description()
	}
	}

	#[stable(feature = "duration_checked_float", since = "1.66.0")]
	impl Error for crate::time::TryFromFloatSecsError {}

	#[stable(feature = "frombyteswithnulerror_impls", since = "1.17.0")]
	impl Error for crate::ffi::FromBytesWithNulError {
	#[allow(deprecated)]
	fn description(&self) -> &str {
	self.__description()
	}
	}

	#[unstable(feature = "cstr_from_bytes_until_nul", issue = "95027")]
	impl Error for crate::ffi::FromBytesUntilNulError {}