src/libcore/alloc/layout.rs - third_party/rust - Git at Google

 use crate::cmp;
 use crate::fmt;
 use crate::mem;
 use crate::num::NonZeroUsize;
 use crate::ptr::NonNull;

 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 `AllocRef` interface.)
 #[stable(feature = "alloc_layout", since = "1.28.0")]
 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
 #[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_: NonZeroUsize,
 }

 impl Layout {
     /// Constructs a `Layout` from a given `size` and `align`,
     /// or returns `LayoutErr` 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 (i.e., the rounded value must be less than
     ///    or equal to `usize::MAX`).
     #[stable(feature = "alloc_layout", since = "1.28.0")]
     #[rustc_const_unstable(feature = "const_alloc_layout", issue = "67521")]
     #[inline]
     pub const fn from_size_align(size: usize, align: usize) -> Result<Self, LayoutErr> {
         if !align.is_power_of_two() {
             return Err(LayoutErr { private: () });
         }

         // (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.
         if size > usize::MAX - (align - 1) {
             return Err(LayoutErr { private: () });
         }

         // SAFETY: the conditions for `from_size_align_unchecked` have been
         // checked above.
         unsafe { Ok(Layout::from_size_align_unchecked(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`](#method.from_size_align).
     #[stable(feature = "alloc_layout", since = "1.28.0")]
     #[rustc_const_stable(feature = "alloc_layout", since = "1.28.0")]
     #[inline]
     pub const unsafe fn from_size_align_unchecked(size: usize, align: usize) -> Self {
         // SAFETY: the caller must ensure that `align` is greater than zero.
         Layout { size_: size, align_: unsafe { NonZeroUsize::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_unstable(feature = "const_alloc_layout", issue = "67521")]
     #[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_unstable(feature = "const_alloc_layout", issue = "67521")]
     #[inline]
     pub const fn align(&self) -> usize {
         self.align_.get()
     }

     /// 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")]
     #[inline]
     pub const fn new<T>() -> Self {
         let (size, align) = size_align::<T>();
         // SAFETY: the align is guaranteed by Rust to be a power of two and
         // the size+align combo is guaranteed to fit in our address space. As a
         // result use the unchecked constructor here to avoid inserting code
         // that panics if it isn't optimized well enough.
         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")]
     #[inline]
     pub fn for_value<T: ?Sized>(t: &T) -> Self {
         let (size, align) = (mem::size_of_val(t), mem::align_of_val(t));
         debug_assert!(Layout::from_size_align(size, align).is_ok());
         // SAFETY: see rationale in `new` for why this is using an unsafe variant below
         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")]
     #[inline]
     pub const fn dangling(&self) -> NonNull<u8> {
         // SAFETY: align is guaranteed to be non-zero
         unsafe { NonNull::new_unchecked(self.align() as *mut u8) }
     }

     /// 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`](#method.from_size_align).
     #[stable(feature = "alloc_layout_manipulation", since = "1.44.0")]
     #[inline]
     pub fn align_to(&self, align: usize) -> Result<Self, LayoutErr> {
         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")]
     #[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")]
     #[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 (i.e., the rounded value must be less than
         // > `usize::MAX`)
         let new_size = self.size() + pad;

         Layout::from_size_align(new_size, self.align()).unwrap()
     }

     /// 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 `LayoutErr`.
     #[unstable(feature = "alloc_layout_extra", issue = "55724")]
     #[inline]
     pub fn repeat(&self, n: usize) -> Result<(Self, usize), LayoutErr> {
         // This cannot overflow. Quoting from the invariant of Layout:
         // > `size`, when rounded up to the nearest multiple of `align`,
         // > must not overflow (i.e., the rounded value must be less than
         // > `usize::MAX`)
         let padded_size = self.size() + self.padding_needed_for(self.align());
         let alloc_size = padded_size.checked_mul(n).ok_or(LayoutErr { private: () })?;

         // SAFETY: self.align is already known to be valid and alloc_size has been
         // padded already.
         unsafe { Ok((Layout::from_size_align_unchecked(alloc_size, self.align()), 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 `LayoutErr`.
     ///
     /// # 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, LayoutErr};
     /// pub fn repr_c(fields: &[Layout]) -> Result<(Layout, Vec<usize>), LayoutErr> {
     ///     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), LayoutErr> {
         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(LayoutErr { private: () })?;
         let new_size = offset.checked_add(next.size()).ok_or(LayoutErr { private: () })?;

         let layout = Layout::from_size_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 `LayoutErr`.
     #[unstable(feature = "alloc_layout_extra", issue = "55724")]
     #[inline]
     pub fn repeat_packed(&self, n: usize) -> Result<Self, LayoutErr> {
         let size = self.size().checked_mul(n).ok_or(LayoutErr { private: () })?;
         Layout::from_size_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 `LayoutErr`.
     #[unstable(feature = "alloc_layout_extra", issue = "55724")]
     #[inline]
     pub fn extend_packed(&self, next: Self) -> Result<Self, LayoutErr> {
         let new_size = self.size().checked_add(next.size()).ok_or(LayoutErr { private: () })?;
         Layout::from_size_align(new_size, self.align())
     }

     /// Creates a layout describing the record for a `[T; n]`.
     ///
     /// On arithmetic overflow, returns `LayoutErr`.
     #[stable(feature = "alloc_layout_manipulation", since = "1.44.0")]
     #[inline]
     pub fn array<T>(n: usize) -> Result<Self, LayoutErr> {
         let (layout, offset) = Layout::new::<T>().repeat(n)?;
         debug_assert_eq!(offset, mem::size_of::<T>());
         Ok(layout.pad_to_align())
     }
 }

 /// The parameters given to `Layout::from_size_align`
 /// or some other `Layout` constructor
 /// do not satisfy its documented constraints.
 #[stable(feature = "alloc_layout", since = "1.28.0")]
 #[derive(Clone, PartialEq, Eq, Debug)]
 pub struct LayoutErr {
     private: (),
 }

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

	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 `AllocRef` interface.)
	#[stable(feature = "alloc_layout", since = "1.28.0")]
	#[derive(Copy, Clone, Debug, PartialEq, Eq)]
	#[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_: NonZeroUsize,
	}

	impl Layout {
	/// Constructs a `Layout` from a given `size` and `align`,
	/// or returns `LayoutErr` 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 (i.e., the rounded value must be less than
	/// or equal to `usize::MAX`).
	#[stable(feature = "alloc_layout", since = "1.28.0")]
	#[rustc_const_unstable(feature = "const_alloc_layout", issue = "67521")]
	#[inline]
	pub const fn from_size_align(size: usize, align: usize) -> Result<Self, LayoutErr> {
	if !align.is_power_of_two() {
	return Err(LayoutErr { private: () });
	}

	// (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.
	if size > usize::MAX - (align - 1) {
	return Err(LayoutErr { private: () });
	}

	// SAFETY: the conditions for `from_size_align_unchecked` have been
	// checked above.
	unsafe { Ok(Layout::from_size_align_unchecked(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`](#method.from_size_align).
	#[stable(feature = "alloc_layout", since = "1.28.0")]
	#[rustc_const_stable(feature = "alloc_layout", since = "1.28.0")]
	#[inline]
	pub const unsafe fn from_size_align_unchecked(size: usize, align: usize) -> Self {
	// SAFETY: the caller must ensure that `align` is greater than zero.
	Layout { size_: size, align_: unsafe { NonZeroUsize::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_unstable(feature = "const_alloc_layout", issue = "67521")]
	#[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_unstable(feature = "const_alloc_layout", issue = "67521")]
	#[inline]
	pub const fn align(&self) -> usize {
	self.align_.get()
	}

	/// 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")]
	#[inline]
	pub const fn new<T>() -> Self {
	let (size, align) = size_align::<T>();
	// SAFETY: the align is guaranteed by Rust to be a power of two and
	// the size+align combo is guaranteed to fit in our address space. As a
	// result use the unchecked constructor here to avoid inserting code
	// that panics if it isn't optimized well enough.
	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")]
	#[inline]
	pub fn for_value<T: ?Sized>(t: &T) -> Self {
	let (size, align) = (mem::size_of_val(t), mem::align_of_val(t));
	debug_assert!(Layout::from_size_align(size, align).is_ok());
	// SAFETY: see rationale in `new` for why this is using an unsafe variant below
	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")]
	#[inline]
	pub const fn dangling(&self) -> NonNull<u8> {
	// SAFETY: align is guaranteed to be non-zero
	unsafe { NonNull::new_unchecked(self.align() as *mut u8) }
	}

	/// 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`](#method.from_size_align).
	#[stable(feature = "alloc_layout_manipulation", since = "1.44.0")]
	#[inline]
	pub fn align_to(&self, align: usize) -> Result<Self, LayoutErr> {
	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")]
	#[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")]
	#[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 (i.e., the rounded value must be less than
	// > `usize::MAX`)
	let new_size = self.size() + pad;

	Layout::from_size_align(new_size, self.align()).unwrap()
	}

	/// 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 `LayoutErr`.
	#[unstable(feature = "alloc_layout_extra", issue = "55724")]
	#[inline]
	pub fn repeat(&self, n: usize) -> Result<(Self, usize), LayoutErr> {
	// This cannot overflow. Quoting from the invariant of Layout:
	// > `size`, when rounded up to the nearest multiple of `align`,
	// > must not overflow (i.e., the rounded value must be less than
	// > `usize::MAX`)
	let padded_size = self.size() + self.padding_needed_for(self.align());
	let alloc_size = padded_size.checked_mul(n).ok_or(LayoutErr { private: () })?;

	// SAFETY: self.align is already known to be valid and alloc_size has been
	// padded already.
	unsafe { Ok((Layout::from_size_align_unchecked(alloc_size, self.align()), 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 `LayoutErr`.
	///
	/// # 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, LayoutErr};
	/// pub fn repr_c(fields: &[Layout]) -> Result<(Layout, Vec<usize>), LayoutErr> {
	/// 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), LayoutErr> {
	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(LayoutErr { private: () })?;
	let new_size = offset.checked_add(next.size()).ok_or(LayoutErr { private: () })?;

	let layout = Layout::from_size_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 `LayoutErr`.
	#[unstable(feature = "alloc_layout_extra", issue = "55724")]
	#[inline]
	pub fn repeat_packed(&self, n: usize) -> Result<Self, LayoutErr> {
	let size = self.size().checked_mul(n).ok_or(LayoutErr { private: () })?;
	Layout::from_size_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 `LayoutErr`.
	#[unstable(feature = "alloc_layout_extra", issue = "55724")]
	#[inline]
	pub fn extend_packed(&self, next: Self) -> Result<Self, LayoutErr> {
	let new_size = self.size().checked_add(next.size()).ok_or(LayoutErr { private: () })?;
	Layout::from_size_align(new_size, self.align())
	}

	/// Creates a layout describing the record for a `[T; n]`.
	///
	/// On arithmetic overflow, returns `LayoutErr`.
	#[stable(feature = "alloc_layout_manipulation", since = "1.44.0")]
	#[inline]
	pub fn array<T>(n: usize) -> Result<Self, LayoutErr> {
	let (layout, offset) = Layout::new::<T>().repeat(n)?;
	debug_assert_eq!(offset, mem::size_of::<T>());
	Ok(layout.pad_to_align())
	}
	}

	/// The parameters given to `Layout::from_size_align`
	/// or some other `Layout` constructor
	/// do not satisfy its documented constraints.
	#[stable(feature = "alloc_layout", since = "1.28.0")]
	#[derive(Clone, PartialEq, Eq, Debug)]
	pub struct LayoutErr {
	private: (),
	}

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