# Struct Layout

1.28.0 · Source

    pub struct Layout { /* private fields */ }

Expand description

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. The size, when rounded up to the nearest multiple of `align`, does not overflow `isize` (i.e., the rounded value will always be less than or equal to `isize::MAX`).

(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.)

## Implementations§

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### impl Layout

1.28.0 (const: 1.50.0) · Source

#### pub 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`).

1.28.0 (const: 1.36.0) · Source

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

Creates a layout, bypassing all checks.

##### §Safety

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

1.28.0 (const: 1.50.0) · Source

#### pub const fn size(&self) -> usize

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

1.28.0 (const: 1.50.0) · Source

#### pub 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.

1.28.0 (const: 1.42.0) · Source

#### pub const fn new<T>() -> Self

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

1.28.0 (const: 1.85.0) · Source

#### pub 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).

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#### pub const unsafe fn for_value_raw<T: ?Sized>(t: *const T) -> Self

🔬This is a nightly-only experimental API. (`layout_for_ptr` #69835)

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`. For the special case where the dynamic tail length is 0, this function is safe to call.
  - 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.

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#### pub const fn dangling(&self) -> NonNull<u8>

🔬This is a nightly-only experimental API. (`alloc_layout_extra` #55724)

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.

1.44.0 (const: 1.85.0) · Source

#### pub 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`.

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#### pub const fn padding_needed_for(&self, align: usize) -> usize

🔬This is a nightly-only experimental API. (`alloc_layout_extra` #55724)

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()`.

1.44.0 (const: 1.85.0) · Source

#### pub 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.

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

🔬This is a nightly-only experimental API. (`alloc_layout_extra` #55724)

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.

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

On arithmetic overflow, returns `LayoutError`.

##### §Examples

    #![feature(alloc_layout_extra)]
    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(24, 4).unwrap(), 8));

1.44.0 (const: 1.85.0) · Source

#### pub 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:

    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))
    }

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

🔬This is a nightly-only experimental API. (`alloc_layout_extra` #55724)

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`.

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

🔬This is a nightly-only experimental API. (`alloc_layout_extra` #55724)

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`.

1.44.0 (const: 1.85.0) · Source

#### pub 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`.

## Trait Implementations§

1.28.0 · Source§

### impl Clone for Layout

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#### fn clone(&self) -> Layout

Returns a duplicate of the value. Read more
1.0.0 · Source§

#### fn clone_from(&mut self, source: &Self)
where Self:,

Performs copy-assignment from `source`. Read more
1.28.0 · Source§

### impl Debug for Layout

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#### fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter. Read more
1.28.0 · Source§

### impl Hash for Layout

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#### fn hash<__H: Hasher>(&self, state: &mut __H)

Feeds this value into the given `Hasher`. Read more
1.3.0 · Source§

#### fn hash_slice<H: Hasher>(data: &[Self], state: &mut H)
where Self: Sized,

Feeds a slice of this type into the given `Hasher`. Read more
1.28.0 · Source§

### impl PartialEq for Layout

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#### fn eq(&self, other: &Layout) -> bool

Tests for `self` and `other` values to be equal, and is used by `==`.
1.0.0 · Source§

#### fn ne(&self, other: &Rhs) -> bool

Tests for `!=`. The default implementation is almost always sufficient, and should not be overridden without very good reason.
1.28.0 · Source§

### impl Copy for Layout

1.28.0 · Source§

### impl Eq for Layout

1.28.0 · Source§

### impl StructuralPartialEq for Layout

## Auto Trait Implementations§

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### impl Freeze for Layout

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### impl RefUnwindSafe for Layout

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### impl Send for Layout

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### impl Sync for Layout

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### impl Unpin for Layout

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### impl UnwindSafe for Layout

## Blanket Implementations§

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### impl<T> Any for T
where T: 'static + ?Sized,

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#### fn type_id(&self) -> TypeId

Gets the `TypeId` of `self`. Read more
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### impl<T> Borrow<T> for T
where T: ?Sized,

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#### fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more
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### impl<T> BorrowMut<T> for T
where T: ?Sized,

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#### fn borrow_mut(&mut self) -> &mut T

Mutably borrows from an owned value. Read more
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### impl<T> CloneToUninit for T
where T: Clone,

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#### unsafe fn clone_to_uninit(&self, dest: *mut u8)

🔬This is a nightly-only experimental API. (`clone_to_uninit` #126799)
Performs copy-assignment from `self` to `dest`. Read more
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### impl<T> From<T> for T

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#### fn from(t: T) -> T

Returns the argument unchanged.

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### impl<T, U> Into<U> for T
where U: From<T>,

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#### fn into(self) -> U

Calls `U::from(self)`.

That is, this conversion is whatever the implementation of `From<T> for U` chooses to do.

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### impl<T, U> TryFrom<U> for T
where U: Into<T>,

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#### type Error = Infallible

The type returned in the event of a conversion error.
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#### fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

Performs the conversion.
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### impl<T, U> TryInto<U> for T
where U: TryFrom<T>,

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#### type Error = <U as TryFrom<T>>::Error

The type returned in the event of a conversion error.
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#### fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>

Performs the conversion.