impl Layout

const fn from_size_align(size: usize, align: usize) -> Result<Self, LayoutError>

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).

const fn is_size_align_valid(size: usize, align: usize) -> bool

const fn is_size_alignment_valid(size: usize, alignment: Alignment) -> bool

const fn max_size_for_alignment(alignment: Alignment) -> usize

const fn from_size_alignment(size: usize, alignment: Alignment) -> Result<Self, LayoutError>

Constructs a Layout from a given size and alignment, or returns LayoutError if any of the following conditions are not met:

• size, when rounded up to the nearest multiple of alignment, must not overflow isize (i.e., the rounded value must be less than or equal to isize::MAX).

const unsafe fn from_size_align_unchecked(size: usize, align: usize) -> Self

Creates a layout, bypassing all checks.

Safety

The arguments passed must meet the preconditions from Layout::from_size_align.

const unsafe fn from_size_alignment_unchecked(size: usize, alignment: Alignment) -> Self

Creates a layout, bypassing all checks.

Safety

This function is unsafe as it does not verify the preconditions from Layout::from_size_alignment.

const fn size(&self) -> usize

The minimum size in bytes for a memory block of this layout.

const fn align(&self) -> usize

The minimum byte alignment for a memory block of this layout.

The returned alignment is guaranteed to be a power of two.

const fn alignment(&self) -> Alignment

The minimum byte alignment for a memory block of this layout.

The returned alignment is guaranteed to be a power of two.

const fn new<T>() -> Self

Constructs a Layout suitable for holding a value of type T.

const fn for_value<T: ?Sized>(t: &T) -> Self

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).

const unsafe fn for_value_raw<T: ?Sized>(val: *const T) -> Self

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 safe to call if the pointer is safe to reborrow as &T (in which case you could also call [for_value][Self::for_value]). Otherwise, the following conditions must hold:

• If T is Sized, this function is always safe to call.
• If the unsized tail of T is:
  • a [slice] [U], str, or a trait object dyn Trait, then the size of the entire value (dynamic tail length + statically sized prefix) must fit in isize. For the special case where the dynamic tail length is 0, this function is safe to call.
  • No other kind of unsized tail currently exists that satisfies the trait bounds for this function. If more kinds of unsized tails get introduced in the future, the documentation of this function will have to be extended before it can be used for such types.

Here, unsized tail refers to the type obtained by recursively descending through the last field of a tuple or struct until we arrived at a built-in unsized type.

As a consequence of these rules, it is the case that whenever it is allowed to convert val into a shared reference, then it is also allowed to invoke this function.

const fn dangling_ptr(&self) -> NonNull<u8>

Creates a NonNull that is dangling, but well-aligned for this Layout.

Note that the address of the returned pointer may potentially be that of 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.

const fn align_to(&self, align: usize) -> Result<Self, LayoutError>

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.

const fn adjust_alignment_to(&self, alignment: Alignment) -> Result<Self, LayoutError>

Creates a layout describing the record that can hold a value of the same layout as self, but that also is aligned to alignment alignment.

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 alignment violates the conditions listed in Layout::from_size_alignment.

const fn padding_needed_for(&self, alignment: Alignment) -> usize

Returns the amount of padding we must insert after self to ensure that the following address will satisfy alignment.

e.g., if self.size() is 9, then self.padding_needed_for(alignment4) (where alignment4.as_usize() == 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).

Note that the utility of the returned value requires alignment 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 alignment.as_usize() <= self.align().

const fn size_rounded_up_to_custom_alignment(&self, alignment: Alignment) -> usize

Returns the smallest multiple of align greater than or equal to self.size().

This can return at most Alignment::MAX (aka isize::MAX + 1) because the original size is at most isize::MAX.

const fn pad_to_align(&self) -> Layout

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.

const fn repeat(&self, n: usize) -> Result<(Self, usize), LayoutError>

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.

Does not include padding after the trailing element.

(That distance between elements is sometimes known as "stride".)

On arithmetic overflow, returns LayoutError.

Examples

use std::alloc::Layout;

// All rust types have a size that's a multiple of their alignment.
let normal = Layout::from_size_align(12, 4).unwrap();
let repeated = normal.repeat(3).unwrap();
assert_eq!(repeated, (Layout::from_size_align(36, 4).unwrap(), 12));

// But you can manually make layouts which don't meet that rule.
let padding_needed = Layout::from_size_align(6, 4).unwrap();
let repeated = padding_needed.repeat(3).unwrap();
assert_eq!(repeated, (Layout::from_size_align(22, 4).unwrap(), 8));

// Repeating an element zero times has zero size, but keeps the alignment (like `[T; 0]`)
let repeated = normal.repeat(0).unwrap();
assert_eq!(repeated, (Layout::from_size_align(0, 4).unwrap(), 12));
let repeated = padding_needed.repeat(0).unwrap();
assert_eq!(repeated, (Layout::from_size_align(0, 4).unwrap(), 8));

const fn extend(&self, next: Self) -> Result<(Self, usize), LayoutError>

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:

# 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])));

const fn repeat_packed(&self, n: usize) -> Result<Self, LayoutError>

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.

const fn extend_packed(&self, next: Self) -> Result<Self, LayoutError>

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.

const fn array<T>(n: usize) -> Result<Self, LayoutError>

Creates a layout describing the record for a [T; n].

On arithmetic overflow or when the total size would exceed isize::MAX, returns LayoutError.