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

 // Seemingly inconsequential code changes to this file can lead to measurable
 // performance impact on compilation times, due at least in part to the fact
 // that the layout code gets called from many instantiations of the various
 // collections, resulting in having to optimize down excess IR multiple times.
 // Your performance intuition is useless. Run perf.

 use crate::cmp;
 use crate::error::Error;
 use crate::fmt;
 use crate::mem::{self, ValidAlign};
 use crate::ptr::NonNull;

 // While this function is used in one place and its implementation
 // could be inlined, the previous attempts to do so made rustc
 // slower:
 //
 // * https://github.com/rust-lang/rust/pull/72189
 // * https://github.com/rust-lang/rust/pull/79827
 const fn size_align<T>() -> (usize, usize) {
     (mem::size_of::<T>(), mem::align_of::<T>())
 }

 /// Layout of a block of memory.
 ///
 /// An instance of `Layout` describes a particular layout of memory.
 /// You build a `Layout` up as an input to give to an allocator.
 ///
 /// All layouts have an associated size and a power-of-two alignment.
 ///
 /// (Note that layouts are *not* required to have non-zero size,
 /// even though `GlobalAlloc` requires that all memory requests
 /// be non-zero in size. A caller must either ensure that conditions
 /// like this are met, use specific allocators with looser
 /// requirements, or use the more lenient `Allocator` interface.)
 #[stable(feature = "alloc_layout", since = "1.28.0")]
 #[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
 #[lang = "alloc_layout"]
 pub struct Layout {
     // size of the requested block of memory, measured in bytes.
     size: usize,

     // alignment of the requested block of memory, measured in bytes.
     // we ensure that this is always a power-of-two, because API's
     // like `posix_memalign` require it and it is a reasonable
     // constraint to impose on Layout constructors.
     //
     // (However, we do not analogously require `align >= sizeof(void*)`,
     //  even though that is *also* a requirement of `posix_memalign`.)
     align: ValidAlign,
 }

 impl Layout {
     /// Constructs a `Layout` from a given `size` and `align`,
     /// or returns `LayoutError` if any of the following conditions
     /// are not met:
     ///
     /// * `align` must not be zero,
     ///
     /// * `align` must be a power of two,
     ///
     /// * `size`, when rounded up to the nearest multiple of `align`,
     ///    must not overflow isize (i.e., the rounded value must be
     ///    less than or equal to `isize::MAX`).
     #[stable(feature = "alloc_layout", since = "1.28.0")]
     #[rustc_const_stable(feature = "const_alloc_layout_size_align", since = "1.50.0")]
     #[inline]
     #[rustc_allow_const_fn_unstable(ptr_alignment_type)]
     pub const fn from_size_align(size: usize, align: usize) -> Result<Self, LayoutError> {
         if !align.is_power_of_two() {
             return Err(LayoutError);
         }

         // SAFETY: just checked that align is a power of two.
         Layout::from_size_valid_align(size, unsafe { ValidAlign::new_unchecked(align) })
     }

     #[inline(always)]
     const fn max_size_for_align(align: ValidAlign) -> usize {
         // (power-of-two implies align != 0.)

         // Rounded up size is:
         //   size_rounded_up = (size + align - 1) & !(align - 1);
         //
         // We know from above that align != 0. If adding (align - 1)
         // does not overflow, then rounding up will be fine.
         //
         // Conversely, &-masking with !(align - 1) will subtract off
         // only low-order-bits. Thus if overflow occurs with the sum,
         // the &-mask cannot subtract enough to undo that overflow.
         //
         // Above implies that checking for summation overflow is both
         // necessary and sufficient.
         isize::MAX as usize - (align.as_usize() - 1)
     }

     /// Internal helper constructor to skip revalidating alignment validity.
     #[inline]
     const fn from_size_valid_align(size: usize, align: ValidAlign) -> Result<Self, LayoutError> {
         if size > Self::max_size_for_align(align) {
             return Err(LayoutError);
         }

         // SAFETY: Layout::size invariants checked above.
         Ok(Layout { size, align })
     }

     /// Creates a layout, bypassing all checks.
     ///
     /// # Safety
     ///
     /// This function is unsafe as it does not verify the preconditions from
     /// [`Layout::from_size_align`].
     #[stable(feature = "alloc_layout", since = "1.28.0")]
     #[rustc_const_stable(feature = "const_alloc_layout_unchecked", since = "1.36.0")]
     #[must_use]
     #[inline]
     #[rustc_allow_const_fn_unstable(ptr_alignment_type)]
     pub const unsafe fn from_size_align_unchecked(size: usize, align: usize) -> Self {
         // SAFETY: the caller is required to uphold the preconditions.
         unsafe { Layout { size, align: ValidAlign::new_unchecked(align) } }
     }

     /// The minimum size in bytes for a memory block of this layout.
     #[stable(feature = "alloc_layout", since = "1.28.0")]
     #[rustc_const_stable(feature = "const_alloc_layout_size_align", since = "1.50.0")]
     #[must_use]
     #[inline]
     pub const fn size(&self) -> usize {
         self.size
     }

     /// The minimum byte alignment for a memory block of this layout.
     #[stable(feature = "alloc_layout", since = "1.28.0")]
     #[rustc_const_stable(feature = "const_alloc_layout_size_align", since = "1.50.0")]
     #[must_use = "this returns the minimum alignment, \
                   without modifying the layout"]
     #[inline]
     #[rustc_allow_const_fn_unstable(ptr_alignment_type)]
     pub const fn align(&self) -> usize {
         self.align.as_usize()
     }

     /// Constructs a `Layout` suitable for holding a value of type `T`.
     #[stable(feature = "alloc_layout", since = "1.28.0")]
     #[rustc_const_stable(feature = "alloc_layout_const_new", since = "1.42.0")]
     #[must_use]
     #[inline]
     pub const fn new<T>() -> Self {
         let (size, align) = size_align::<T>();
         // SAFETY: if the type is instantiated, rustc already ensures that its
         // layout is valid. Use the unchecked constructor to avoid inserting a
         // panicking codepath that needs to be optimized out.
         unsafe { Layout::from_size_align_unchecked(size, align) }
     }

     /// Produces layout describing a record that could be used to
     /// allocate backing structure for `T` (which could be a trait
     /// or other unsized type like a slice).
     #[stable(feature = "alloc_layout", since = "1.28.0")]
     #[must_use]
     #[inline]
     pub fn for_value<T: ?Sized>(t: &T) -> Self {
         let (size, align) = (mem::size_of_val(t), mem::align_of_val(t));
         // SAFETY: see rationale in `new` for why this is using the unsafe variant
         unsafe { Layout::from_size_align_unchecked(size, align) }
     }

     /// Produces layout describing a record that could be used to
     /// allocate backing structure for `T` (which could be a trait
     /// or other unsized type like a slice).
     ///
     /// # Safety
     ///
     /// This function is only safe to call if the following conditions hold:
     ///
     /// - If `T` is `Sized`, this function is always safe to call.
     /// - If the unsized tail of `T` is:
     ///     - a [slice], then the length of the slice tail must be an initialized
     ///       integer, and the size of the *entire value*
     ///       (dynamic tail length + statically sized prefix) must fit in `isize`.
     ///     - a [trait object], then the vtable part of the pointer must point
     ///       to a valid vtable for the type `T` acquired by an unsizing coercion,
     ///       and the size of the *entire value*
     ///       (dynamic tail length + statically sized prefix) must fit in `isize`.
     ///     - an (unstable) [extern type], then this function is always safe to
     ///       call, but may panic or otherwise return the wrong value, as the
     ///       extern type's layout is not known. This is the same behavior as
     ///       [`Layout::for_value`] on a reference to an extern type tail.
     ///     - otherwise, it is conservatively not allowed to call this function.
     ///
     /// [trait object]: ../../book/ch17-02-trait-objects.html
     /// [extern type]: ../../unstable-book/language-features/extern-types.html
     #[unstable(feature = "layout_for_ptr", issue = "69835")]
     #[must_use]
     pub unsafe fn for_value_raw<T: ?Sized>(t: *const T) -> Self {
         // SAFETY: we pass along the prerequisites of these functions to the caller
         let (size, align) = unsafe { (mem::size_of_val_raw(t), mem::align_of_val_raw(t)) };
         // SAFETY: see rationale in `new` for why this is using the unsafe variant
         unsafe { Layout::from_size_align_unchecked(size, align) }
     }

     /// Creates a `NonNull` that is dangling, but well-aligned for this Layout.
     ///
     /// Note that the pointer value may potentially represent a valid pointer,
     /// which means this must not be used as a "not yet initialized"
     /// sentinel value. Types that lazily allocate must track initialization by
     /// some other means.
     #[unstable(feature = "alloc_layout_extra", issue = "55724")]
     #[rustc_const_unstable(feature = "alloc_layout_extra", issue = "55724")]
     #[must_use]
     #[inline]
     pub const fn dangling(&self) -> NonNull<u8> {
         // SAFETY: align is guaranteed to be non-zero
         unsafe { NonNull::new_unchecked(crate::ptr::invalid_mut::<u8>(self.align())) }
     }

     /// Creates a layout describing the record that can hold a value
     /// of the same layout as `self`, but that also is aligned to
     /// alignment `align` (measured in bytes).
     ///
     /// If `self` already meets the prescribed alignment, then returns
     /// `self`.
     ///
     /// Note that this method does not add any padding to the overall
     /// size, regardless of whether the returned layout has a different
     /// alignment. In other words, if `K` has size 16, `K.align_to(32)`
     /// will *still* have size 16.
     ///
     /// Returns an error if the combination of `self.size()` and the given
     /// `align` violates the conditions listed in [`Layout::from_size_align`].
     #[stable(feature = "alloc_layout_manipulation", since = "1.44.0")]
     #[inline]
     pub fn align_to(&self, align: usize) -> Result<Self, LayoutError> {
         Layout::from_size_align(self.size(), cmp::max(self.align(), align))
     }

     /// Returns the amount of padding we must insert after `self`
     /// to ensure that the following address will satisfy `align`
     /// (measured in bytes).
     ///
     /// e.g., if `self.size()` is 9, then `self.padding_needed_for(4)`
     /// returns 3, because that is the minimum number of bytes of
     /// padding required to get a 4-aligned address (assuming that the
     /// corresponding memory block starts at a 4-aligned address).
     ///
     /// The return value of this function has no meaning if `align` is
     /// not a power-of-two.
     ///
     /// Note that the utility of the returned value requires `align`
     /// to be less than or equal to the alignment of the starting
     /// address for the whole allocated block of memory. One way to
     /// satisfy this constraint is to ensure `align <= self.align()`.
     #[unstable(feature = "alloc_layout_extra", issue = "55724")]
     #[rustc_const_unstable(feature = "const_alloc_layout", issue = "67521")]
     #[must_use = "this returns the padding needed, \
                   without modifying the `Layout`"]
     #[inline]
     pub const fn padding_needed_for(&self, align: usize) -> usize {
         let len = self.size();

         // Rounded up value is:
         //   len_rounded_up = (len + align - 1) & !(align - 1);
         // and then we return the padding difference: `len_rounded_up - len`.
         //
         // We use modular arithmetic throughout:
         //
         // 1. align is guaranteed to be > 0, so align - 1 is always
         //    valid.
         //
         // 2. `len + align - 1` can overflow by at most `align - 1`,
         //    so the &-mask with `!(align - 1)` will ensure that in the
         //    case of overflow, `len_rounded_up` will itself be 0.
         //    Thus the returned padding, when added to `len`, yields 0,
         //    which trivially satisfies the alignment `align`.
         //
         // (Of course, attempts to allocate blocks of memory whose
         // size and padding overflow in the above manner should cause
         // the allocator to yield an error anyway.)

         let len_rounded_up = len.wrapping_add(align).wrapping_sub(1) & !align.wrapping_sub(1);
         len_rounded_up.wrapping_sub(len)
     }

     /// Creates a layout by rounding the size of this layout up to a multiple
     /// of the layout's alignment.
     ///
     /// This is equivalent to adding the result of `padding_needed_for`
     /// to the layout's current size.
     #[stable(feature = "alloc_layout_manipulation", since = "1.44.0")]
     #[must_use = "this returns a new `Layout`, \
                   without modifying the original"]
     #[inline]
     pub fn pad_to_align(&self) -> Layout {
         let pad = self.padding_needed_for(self.align());
         // This cannot overflow. Quoting from the invariant of Layout:
         // > `size`, when rounded up to the nearest multiple of `align`,
         // > must not overflow isize (i.e., the rounded value must be
         // > less than or equal to `isize::MAX`)
         let new_size = self.size() + pad;

         // SAFETY: padded size is guaranteed to not exceed `isize::MAX`.
         unsafe { Layout::from_size_align_unchecked(new_size, self.align()) }
     }

     /// Creates a layout describing the record for `n` instances of
     /// `self`, with a suitable amount of padding between each to
     /// ensure that each instance is given its requested size and
     /// alignment. On success, returns `(k, offs)` where `k` is the
     /// layout of the array and `offs` is the distance between the start
     /// of each element in the array.
     ///
     /// On arithmetic overflow, returns `LayoutError`.
     #[unstable(feature = "alloc_layout_extra", issue = "55724")]
     #[inline]
     pub fn repeat(&self, n: usize) -> Result<(Self, usize), LayoutError> {
         // This cannot overflow. Quoting from the invariant of Layout:
         // > `size`, when rounded up to the nearest multiple of `align`,
         // > must not overflow isize (i.e., the rounded value must be
         // > less than or equal to `isize::MAX`)
         let padded_size = self.size() + self.padding_needed_for(self.align());
         let alloc_size = padded_size.checked_mul(n).ok_or(LayoutError)?;

         // The safe constructor is called here to enforce the isize size limit.
         Layout::from_size_valid_align(alloc_size, self.align).map(|layout| (layout, padded_size))
     }

     /// Creates a layout describing the record for `self` followed by
     /// `next`, including any necessary padding to ensure that `next`
     /// will be properly aligned, but *no trailing padding*.
     ///
     /// In order to match C representation layout `repr(C)`, you should
     /// call `pad_to_align` after extending the layout with all fields.
     /// (There is no way to match the default Rust representation
     /// layout `repr(Rust)`, as it is unspecified.)
     ///
     /// Note that the alignment of the resulting layout will be the maximum of
     /// those of `self` and `next`, in order to ensure alignment of both parts.
     ///
     /// Returns `Ok((k, offset))`, where `k` is layout of the concatenated
     /// record and `offset` is the relative location, in bytes, of the
     /// start of the `next` embedded within the concatenated record
     /// (assuming that the record itself starts at offset 0).
     ///
     /// On arithmetic overflow, returns `LayoutError`.
     ///
     /// # Examples
     ///
     /// To calculate the layout of a `#[repr(C)]` structure and the offsets of
     /// the fields from its fields' layouts:
     ///
     /// ```rust
     /// # use std::alloc::{Layout, LayoutError};
     /// pub fn repr_c(fields: &[Layout]) -> Result<(Layout, Vec<usize>), LayoutError> {
     ///     let mut offsets = Vec::new();
     ///     let mut layout = Layout::from_size_align(0, 1)?;
     ///     for &field in fields {
     ///         let (new_layout, offset) = layout.extend(field)?;
     ///         layout = new_layout;
     ///         offsets.push(offset);
     ///     }
     ///     // Remember to finalize with `pad_to_align`!
     ///     Ok((layout.pad_to_align(), offsets))
     /// }
     /// # // test that it works
     /// # #[repr(C)] struct S { a: u64, b: u32, c: u16, d: u32 }
     /// # let s = Layout::new::<S>();
     /// # let u16 = Layout::new::<u16>();
     /// # let u32 = Layout::new::<u32>();
     /// # let u64 = Layout::new::<u64>();
     /// # assert_eq!(repr_c(&[u64, u32, u16, u32]), Ok((s, vec![0, 8, 12, 16])));
     /// ```
     #[stable(feature = "alloc_layout_manipulation", since = "1.44.0")]
     #[inline]
     pub fn extend(&self, next: Self) -> Result<(Self, usize), LayoutError> {
         let new_align = cmp::max(self.align, next.align);
         let pad = self.padding_needed_for(next.align());

         let offset = self.size().checked_add(pad).ok_or(LayoutError)?;
         let new_size = offset.checked_add(next.size()).ok_or(LayoutError)?;

         // The safe constructor is called here to enforce the isize size limit.
         let layout = Layout::from_size_valid_align(new_size, new_align)?;
         Ok((layout, offset))
     }

     /// Creates a layout describing the record for `n` instances of
     /// `self`, with no padding between each instance.
     ///
     /// Note that, unlike `repeat`, `repeat_packed` does not guarantee
     /// that the repeated instances of `self` will be properly
     /// aligned, even if a given instance of `self` is properly
     /// aligned. In other words, if the layout returned by
     /// `repeat_packed` is used to allocate an array, it is not
     /// guaranteed that all elements in the array will be properly
     /// aligned.
     ///
     /// On arithmetic overflow, returns `LayoutError`.
     #[unstable(feature = "alloc_layout_extra", issue = "55724")]
     #[inline]
     pub fn repeat_packed(&self, n: usize) -> Result<Self, LayoutError> {
         let size = self.size().checked_mul(n).ok_or(LayoutError)?;
         // The safe constructor is called here to enforce the isize size limit.
         Layout::from_size_valid_align(size, self.align)
     }

     /// Creates a layout describing the record for `self` followed by
     /// `next` with no additional padding between the two. Since no
     /// padding is inserted, the alignment of `next` is irrelevant,
     /// and is not incorporated *at all* into the resulting layout.
     ///
     /// On arithmetic overflow, returns `LayoutError`.
     #[unstable(feature = "alloc_layout_extra", issue = "55724")]
     #[inline]
     pub fn extend_packed(&self, next: Self) -> Result<Self, LayoutError> {
         let new_size = self.size().checked_add(next.size()).ok_or(LayoutError)?;
         // The safe constructor is called here to enforce the isize size limit.
         Layout::from_size_valid_align(new_size, self.align)
     }

     /// Creates a layout describing the record for a `[T; n]`.
     ///
     /// On arithmetic overflow or when the total size would exceed
     /// `isize::MAX`, returns `LayoutError`.
     #[stable(feature = "alloc_layout_manipulation", since = "1.44.0")]
     #[inline]
     pub fn array<T>(n: usize) -> Result<Self, LayoutError> {
         // Reduce the amount of code we need to monomorphize per `T`.
         return inner(mem::size_of::<T>(), ValidAlign::of::<T>(), n);

         #[inline]
         fn inner(element_size: usize, align: ValidAlign, n: usize) -> Result<Layout, LayoutError> {
             // We need to check two things about the size:
             //  - That the total size won't overflow a `usize`, and
             //  - That the total size still fits in an `isize`.
             // By using division we can check them both with a single threshold.
             // That'd usually be a bad idea, but thankfully here the element size
             // and alignment are constants, so the compiler will fold all of it.
             if element_size != 0 && n > Layout::max_size_for_align(align) / element_size {
                 return Err(LayoutError);
             }

             let array_size = element_size * n;

             // SAFETY: We just checked above that the `array_size` will not
             // exceed `isize::MAX` even when rounded up to the alignment.
             // And `ValidAlign` guarantees it's a power of two.
             unsafe { Ok(Layout::from_size_align_unchecked(array_size, align.as_usize())) }
         }
     }
 }

 #[stable(feature = "alloc_layout", since = "1.28.0")]
 #[deprecated(
     since = "1.52.0",
     note = "Name does not follow std convention, use LayoutError",
     suggestion = "LayoutError"
 )]
 pub type LayoutErr = LayoutError;

 /// The parameters given to `Layout::from_size_align`
 /// or some other `Layout` constructor
 /// do not satisfy its documented constraints.
 #[stable(feature = "alloc_layout_error", since = "1.50.0")]
 #[non_exhaustive]
 #[derive(Clone, PartialEq, Eq, Debug)]
 pub struct LayoutError;

 #[stable(feature = "alloc_layout", since = "1.28.0")]
 impl Error for LayoutError {}

 // (we need this for downstream impl of trait Error)
 #[stable(feature = "alloc_layout", since = "1.28.0")]
 impl fmt::Display for LayoutError {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         f.write_str("invalid parameters to Layout::from_size_align")
     }
 }
	// Seemingly inconsequential code changes to this file can lead to measurable
	// performance impact on compilation times, due at least in part to the fact
	// that the layout code gets called from many instantiations of the various
	// collections, resulting in having to optimize down excess IR multiple times.
	// Your performance intuition is useless. Run perf.

	use crate::cmp;
	use crate::error::Error;
	use crate::fmt;
	use crate::mem::{self, ValidAlign};
	use crate::ptr::NonNull;

	// While this function is used in one place and its implementation
	// could be inlined, the previous attempts to do so made rustc
	// slower:
	//
	// * https://github.com/rust-lang/rust/pull/72189
	// * https://github.com/rust-lang/rust/pull/79827
	const fn size_align<T>() -> (usize, usize) {
	(mem::size_of::<T>(), mem::align_of::<T>())
	}

	/// Layout of a block of memory.
	///
	/// An instance of `Layout` describes a particular layout of memory.
	/// You build a `Layout` up as an input to give to an allocator.
	///
	/// All layouts have an associated size and a power-of-two alignment.
	///
	/// (Note that layouts are not required to have non-zero size,
	/// even though `GlobalAlloc` requires that all memory requests
	/// be non-zero in size. A caller must either ensure that conditions
	/// like this are met, use specific allocators with looser
	/// requirements, or use the more lenient `Allocator` interface.)
	#[stable(feature = "alloc_layout", since = "1.28.0")]
	#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
	#[lang = "alloc_layout"]
	pub struct Layout {
	// size of the requested block of memory, measured in bytes.
	size: usize,

	// alignment of the requested block of memory, measured in bytes.
	// we ensure that this is always a power-of-two, because API's
	// like `posix_memalign` require it and it is a reasonable
	// constraint to impose on Layout constructors.
	//
	// (However, we do not analogously require `align >= sizeof(void*)`,
	// even though that is also a requirement of `posix_memalign`.)
	align: ValidAlign,
	}

	impl Layout {
	/// Constructs a `Layout` from a given `size` and `align`,
	/// or returns `LayoutError` if any of the following conditions
	/// are not met:
	///
	/// * `align` must not be zero,
	///
	/// * `align` must be a power of two,
	///
	/// * `size`, when rounded up to the nearest multiple of `align`,
	/// must not overflow isize (i.e., the rounded value must be
	/// less than or equal to `isize::MAX`).
	#[stable(feature = "alloc_layout", since = "1.28.0")]
	#[rustc_const_stable(feature = "const_alloc_layout_size_align", since = "1.50.0")]
	#[inline]
	#[rustc_allow_const_fn_unstable(ptr_alignment_type)]
	pub const fn from_size_align(size: usize, align: usize) -> Result<Self, LayoutError> {
	if !align.is_power_of_two() {
	return Err(LayoutError);
	}

	// SAFETY: just checked that align is a power of two.
	Layout::from_size_valid_align(size, unsafe { ValidAlign::new_unchecked(align) })
	}

	#[inline(always)]
	const fn max_size_for_align(align: ValidAlign) -> usize {
	// (power-of-two implies align != 0.)

	// Rounded up size is:
	// size_rounded_up = (size + align - 1) & !(align - 1);
	//
	// We know from above that align != 0. If adding (align - 1)
	// does not overflow, then rounding up will be fine.
	//
	// Conversely, &-masking with !(align - 1) will subtract off
	// only low-order-bits. Thus if overflow occurs with the sum,
	// the &-mask cannot subtract enough to undo that overflow.
	//
	// Above implies that checking for summation overflow is both
	// necessary and sufficient.
	isize::MAX as usize - (align.as_usize() - 1)
	}

	/// Internal helper constructor to skip revalidating alignment validity.
	#[inline]
	const fn from_size_valid_align(size: usize, align: ValidAlign) -> Result<Self, LayoutError> {
	if size > Self::max_size_for_align(align) {
	return Err(LayoutError);
	}

	// SAFETY: Layout::size invariants checked above.
	Ok(Layout { size, align })
	}

	/// Creates a layout, bypassing all checks.
	///
	/// # Safety
	///
	/// This function is unsafe as it does not verify the preconditions from
	/// [`Layout::from_size_align`].
	#[stable(feature = "alloc_layout", since = "1.28.0")]
	#[rustc_const_stable(feature = "const_alloc_layout_unchecked", since = "1.36.0")]
	#[must_use]
	#[inline]
	#[rustc_allow_const_fn_unstable(ptr_alignment_type)]
	pub const unsafe fn from_size_align_unchecked(size: usize, align: usize) -> Self {
	// SAFETY: the caller is required to uphold the preconditions.
	unsafe { Layout { size, align: ValidAlign::new_unchecked(align) } }
	}

	/// The minimum size in bytes for a memory block of this layout.
	#[stable(feature = "alloc_layout", since = "1.28.0")]
	#[rustc_const_stable(feature = "const_alloc_layout_size_align", since = "1.50.0")]
	#[must_use]
	#[inline]
	pub const fn size(&self) -> usize {
	self.size
	}

	/// The minimum byte alignment for a memory block of this layout.
	#[stable(feature = "alloc_layout", since = "1.28.0")]
	#[rustc_const_stable(feature = "const_alloc_layout_size_align", since = "1.50.0")]
	#[must_use = "this returns the minimum alignment, \
	without modifying the layout"]
	#[inline]
	#[rustc_allow_const_fn_unstable(ptr_alignment_type)]
	pub const fn align(&self) -> usize {
	self.align.as_usize()
	}

	/// Constructs a `Layout` suitable for holding a value of type `T`.
	#[stable(feature = "alloc_layout", since = "1.28.0")]
	#[rustc_const_stable(feature = "alloc_layout_const_new", since = "1.42.0")]
	#[must_use]
	#[inline]
	pub const fn new<T>() -> Self {
	let (size, align) = size_align::<T>();
	// SAFETY: if the type is instantiated, rustc already ensures that its
	// layout is valid. Use the unchecked constructor to avoid inserting a
	// panicking codepath that needs to be optimized out.
	unsafe { Layout::from_size_align_unchecked(size, align) }
	}

	/// Produces layout describing a record that could be used to
	/// allocate backing structure for `T` (which could be a trait
	/// or other unsized type like a slice).
	#[stable(feature = "alloc_layout", since = "1.28.0")]
	#[must_use]
	#[inline]
	pub fn for_value<T: ?Sized>(t: &T) -> Self {
	let (size, align) = (mem::size_of_val(t), mem::align_of_val(t));
	// SAFETY: see rationale in `new` for why this is using the unsafe variant
	unsafe { Layout::from_size_align_unchecked(size, align) }
	}

	/// Produces layout describing a record that could be used to
	/// allocate backing structure for `T` (which could be a trait
	/// or other unsized type like a slice).
	///
	/// # Safety
	///
	/// This function is only safe to call if the following conditions hold:
	///
	/// - If `T` is `Sized`, this function is always safe to call.
	/// - If the unsized tail of `T` is:
	/// - a [slice], then the length of the slice tail must be an initialized
	/// integer, and the size of the entire value
	/// (dynamic tail length + statically sized prefix) must fit in `isize`.
	/// - a [trait object], then the vtable part of the pointer must point
	/// to a valid vtable for the type `T` acquired by an unsizing coercion,
	/// and the size of the entire value
	/// (dynamic tail length + statically sized prefix) must fit in `isize`.
	/// - an (unstable) [extern type], then this function is always safe to
	/// call, but may panic or otherwise return the wrong value, as the
	/// extern type's layout is not known. This is the same behavior as
	/// [`Layout::for_value`] on a reference to an extern type tail.
	/// - otherwise, it is conservatively not allowed to call this function.
	///
	/// [trait object]: ../../book/ch17-02-trait-objects.html
	/// [extern type]: ../../unstable-book/language-features/extern-types.html
	#[unstable(feature = "layout_for_ptr", issue = "69835")]
	#[must_use]
	pub unsafe fn for_value_raw<T: ?Sized>(t: *const T) -> Self {
	// SAFETY: we pass along the prerequisites of these functions to the caller
	let (size, align) = unsafe { (mem::size_of_val_raw(t), mem::align_of_val_raw(t)) };
	// SAFETY: see rationale in `new` for why this is using the unsafe variant
	unsafe { Layout::from_size_align_unchecked(size, align) }
	}

	/// Creates a `NonNull` that is dangling, but well-aligned for this Layout.
	///
	/// Note that the pointer value may potentially represent a valid pointer,
	/// which means this must not be used as a "not yet initialized"
	/// sentinel value. Types that lazily allocate must track initialization by
	/// some other means.
	#[unstable(feature = "alloc_layout_extra", issue = "55724")]
	#[rustc_const_unstable(feature = "alloc_layout_extra", issue = "55724")]
	#[must_use]
	#[inline]
	pub const fn dangling(&self) -> NonNull<u8> {
	// SAFETY: align is guaranteed to be non-zero
	unsafe { NonNull::new_unchecked(crate::ptr::invalid_mut::<u8>(self.align())) }
	}

	/// Creates a layout describing the record that can hold a value
	/// of the same layout as `self`, but that also is aligned to
	/// alignment `align` (measured in bytes).
	///
	/// If `self` already meets the prescribed alignment, then returns
	/// `self`.
	///
	/// Note that this method does not add any padding to the overall
	/// size, regardless of whether the returned layout has a different
	/// alignment. In other words, if `K` has size 16, `K.align_to(32)`
	/// will still have size 16.
	///
	/// Returns an error if the combination of `self.size()` and the given
	/// `align` violates the conditions listed in [`Layout::from_size_align`].
	#[stable(feature = "alloc_layout_manipulation", since = "1.44.0")]
	#[inline]
	pub fn align_to(&self, align: usize) -> Result<Self, LayoutError> {
	Layout::from_size_align(self.size(), cmp::max(self.align(), align))
	}

	/// Returns the amount of padding we must insert after `self`
	/// to ensure that the following address will satisfy `align`
	/// (measured in bytes).
	///
	/// e.g., if `self.size()` is 9, then `self.padding_needed_for(4)`
	/// returns 3, because that is the minimum number of bytes of
	/// padding required to get a 4-aligned address (assuming that the
	/// corresponding memory block starts at a 4-aligned address).
	///
	/// The return value of this function has no meaning if `align` is
	/// not a power-of-two.
	///
	/// Note that the utility of the returned value requires `align`
	/// to be less than or equal to the alignment of the starting
	/// address for the whole allocated block of memory. One way to
	/// satisfy this constraint is to ensure `align <= self.align()`.
	#[unstable(feature = "alloc_layout_extra", issue = "55724")]
	#[rustc_const_unstable(feature = "const_alloc_layout", issue = "67521")]
	#[must_use = "this returns the padding needed, \
	without modifying the `Layout`"]
	#[inline]
	pub const fn padding_needed_for(&self, align: usize) -> usize {
	let len = self.size();

	// Rounded up value is:
	// len_rounded_up = (len + align - 1) & !(align - 1);
	// and then we return the padding difference: `len_rounded_up - len`.
	//
	// We use modular arithmetic throughout:
	//
	// 1. align is guaranteed to be > 0, so align - 1 is always
	// valid.
	//
	// 2. `len + align - 1` can overflow by at most `align - 1`,
	// so the &-mask with `!(align - 1)` will ensure that in the
	// case of overflow, `len_rounded_up` will itself be 0.
	// Thus the returned padding, when added to `len`, yields 0,
	// which trivially satisfies the alignment `align`.
	//
	// (Of course, attempts to allocate blocks of memory whose
	// size and padding overflow in the above manner should cause
	// the allocator to yield an error anyway.)

	let len_rounded_up = len.wrapping_add(align).wrapping_sub(1) & !align.wrapping_sub(1);
	len_rounded_up.wrapping_sub(len)
	}

	/// Creates a layout by rounding the size of this layout up to a multiple
	/// of the layout's alignment.
	///
	/// This is equivalent to adding the result of `padding_needed_for`
	/// to the layout's current size.
	#[stable(feature = "alloc_layout_manipulation", since = "1.44.0")]
	#[must_use = "this returns a new `Layout`, \
	without modifying the original"]
	#[inline]
	pub fn pad_to_align(&self) -> Layout {
	let pad = self.padding_needed_for(self.align());
	// This cannot overflow. Quoting from the invariant of Layout:
	// > `size`, when rounded up to the nearest multiple of `align`,
	// > must not overflow isize (i.e., the rounded value must be
	// > less than or equal to `isize::MAX`)
	let new_size = self.size() + pad;

	// SAFETY: padded size is guaranteed to not exceed `isize::MAX`.
	unsafe { Layout::from_size_align_unchecked(new_size, self.align()) }
	}

	/// Creates a layout describing the record for `n` instances of
	/// `self`, with a suitable amount of padding between each to
	/// ensure that each instance is given its requested size and
	/// alignment. On success, returns `(k, offs)` where `k` is the
	/// layout of the array and `offs` is the distance between the start
	/// of each element in the array.
	///
	/// On arithmetic overflow, returns `LayoutError`.
	#[unstable(feature = "alloc_layout_extra", issue = "55724")]
	#[inline]
	pub fn repeat(&self, n: usize) -> Result<(Self, usize), LayoutError> {
	// This cannot overflow. Quoting from the invariant of Layout:
	// > `size`, when rounded up to the nearest multiple of `align`,
	// > must not overflow isize (i.e., the rounded value must be
	// > less than or equal to `isize::MAX`)
	let padded_size = self.size() + self.padding_needed_for(self.align());
	let alloc_size = padded_size.checked_mul(n).ok_or(LayoutError)?;

	// The safe constructor is called here to enforce the isize size limit.
	Layout::from_size_valid_align(alloc_size, self.align).map(\|layout\| (layout, padded_size))
	}

	/// Creates a layout describing the record for `self` followed by
	/// `next`, including any necessary padding to ensure that `next`
	/// will be properly aligned, but no trailing padding.
	///
	/// In order to match C representation layout `repr(C)`, you should
	/// call `pad_to_align` after extending the layout with all fields.
	/// (There is no way to match the default Rust representation
	/// layout `repr(Rust)`, as it is unspecified.)
	///
	/// Note that the alignment of the resulting layout will be the maximum of
	/// those of `self` and `next`, in order to ensure alignment of both parts.
	///
	/// Returns `Ok((k, offset))`, where `k` is layout of the concatenated
	/// record and `offset` is the relative location, in bytes, of the
	/// start of the `next` embedded within the concatenated record
	/// (assuming that the record itself starts at offset 0).
	///
	/// On arithmetic overflow, returns `LayoutError`.
	///
	/// # Examples
	///
	/// To calculate the layout of a `#[repr(C)]` structure and the offsets of
	/// the fields from its fields' layouts:
	///
	/// ```rust
	/// # use std::alloc::{Layout, LayoutError};
	/// pub fn repr_c(fields: &[Layout]) -> Result<(Layout, Vec<usize>), LayoutError> {
	/// let mut offsets = Vec::new();
	/// let mut layout = Layout::from_size_align(0, 1)?;
	/// for &field in fields {
	/// let (new_layout, offset) = layout.extend(field)?;
	/// layout = new_layout;
	/// offsets.push(offset);
	/// }
	/// // Remember to finalize with `pad_to_align`!
	/// Ok((layout.pad_to_align(), offsets))
	/// }
	/// # // test that it works
	/// # #[repr(C)] struct S { a: u64, b: u32, c: u16, d: u32 }
	/// # let s = Layout::new::<S>();
	/// # let u16 = Layout::new::<u16>();
	/// # let u32 = Layout::new::<u32>();
	/// # let u64 = Layout::new::<u64>();
	/// # assert_eq!(repr_c(&[u64, u32, u16, u32]), Ok((s, vec![0, 8, 12, 16])));
	/// ```
	#[stable(feature = "alloc_layout_manipulation", since = "1.44.0")]
	#[inline]
	pub fn extend(&self, next: Self) -> Result<(Self, usize), LayoutError> {
	let new_align = cmp::max(self.align, next.align);
	let pad = self.padding_needed_for(next.align());

	let offset = self.size().checked_add(pad).ok_or(LayoutError)?;
	let new_size = offset.checked_add(next.size()).ok_or(LayoutError)?;

	// The safe constructor is called here to enforce the isize size limit.
	let layout = Layout::from_size_valid_align(new_size, new_align)?;
	Ok((layout, offset))
	}

	/// Creates a layout describing the record for `n` instances of
	/// `self`, with no padding between each instance.
	///
	/// Note that, unlike `repeat`, `repeat_packed` does not guarantee
	/// that the repeated instances of `self` will be properly
	/// aligned, even if a given instance of `self` is properly
	/// aligned. In other words, if the layout returned by
	/// `repeat_packed` is used to allocate an array, it is not
	/// guaranteed that all elements in the array will be properly
	/// aligned.
	///
	/// On arithmetic overflow, returns `LayoutError`.
	#[unstable(feature = "alloc_layout_extra", issue = "55724")]
	#[inline]
	pub fn repeat_packed(&self, n: usize) -> Result<Self, LayoutError> {
	let size = self.size().checked_mul(n).ok_or(LayoutError)?;
	// The safe constructor is called here to enforce the isize size limit.
	Layout::from_size_valid_align(size, self.align)
	}

	/// Creates a layout describing the record for `self` followed by
	/// `next` with no additional padding between the two. Since no
	/// padding is inserted, the alignment of `next` is irrelevant,
	/// and is not incorporated at all into the resulting layout.
	///
	/// On arithmetic overflow, returns `LayoutError`.
	#[unstable(feature = "alloc_layout_extra", issue = "55724")]
	#[inline]
	pub fn extend_packed(&self, next: Self) -> Result<Self, LayoutError> {
	let new_size = self.size().checked_add(next.size()).ok_or(LayoutError)?;
	// The safe constructor is called here to enforce the isize size limit.
	Layout::from_size_valid_align(new_size, self.align)
	}

	/// Creates a layout describing the record for a `[T; n]`.
	///
	/// On arithmetic overflow or when the total size would exceed
	/// `isize::MAX`, returns `LayoutError`.
	#[stable(feature = "alloc_layout_manipulation", since = "1.44.0")]
	#[inline]
	pub fn array<T>(n: usize) -> Result<Self, LayoutError> {
	// Reduce the amount of code we need to monomorphize per `T`.
	return inner(mem::size_of::<T>(), ValidAlign::of::<T>(), n);

	#[inline]
	fn inner(element_size: usize, align: ValidAlign, n: usize) -> Result<Layout, LayoutError> {
	// We need to check two things about the size:
	// - That the total size won't overflow a `usize`, and
	// - That the total size still fits in an `isize`.
	// By using division we can check them both with a single threshold.
	// That'd usually be a bad idea, but thankfully here the element size
	// and alignment are constants, so the compiler will fold all of it.
	if element_size != 0 && n > Layout::max_size_for_align(align) / element_size {
	return Err(LayoutError);
	}

	let array_size = element_size * n;

	// SAFETY: We just checked above that the `array_size` will not
	// exceed `isize::MAX` even when rounded up to the alignment.
	// And `ValidAlign` guarantees it's a power of two.
	unsafe { Ok(Layout::from_size_align_unchecked(array_size, align.as_usize())) }
	}
	}
	}

	#[stable(feature = "alloc_layout", since = "1.28.0")]
	#[deprecated(
	since = "1.52.0",
	note = "Name does not follow std convention, use LayoutError",
	suggestion = "LayoutError"
	)]
	pub type LayoutErr = LayoutError;

	/// The parameters given to `Layout::from_size_align`
	/// or some other `Layout` constructor
	/// do not satisfy its documented constraints.
	#[stable(feature = "alloc_layout_error", since = "1.50.0")]
	#[non_exhaustive]
	#[derive(Clone, PartialEq, Eq, Debug)]
	pub struct LayoutError;

	#[stable(feature = "alloc_layout", since = "1.28.0")]
	impl Error for LayoutError {}

	// (we need this for downstream impl of trait Error)
	#[stable(feature = "alloc_layout", since = "1.28.0")]
	impl fmt::Display for LayoutError {
	fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
	f.write_str("invalid parameters to Layout::from_size_align")
	}
	}