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When allocating memory for arrays using allocators, the `Layout::array` function is typically used. It returns a result, since the given size might be too big. However, `Vec` and its iterators store their allocated capacity and thus they already did check that the size is not too big. The `ArrayLayout` type provides this exact behavior, as it can be infallibly converted into a `Layout`. Instead of a `usize` capacity, `Vec` and other similar array-storing types can use `ArrayLayout` instead. Reviewed-by: Gary Guo <gary@garyguo.net> Signed-off-by: Benno Lossin <benno.lossin@proton.me> Signed-off-by: Danilo Krummrich <dakr@kernel.org> Link: https://lore.kernel.org/r/20241004154149.93856-16-dakr@kernel.org [ Formatted a few comments. - Miguel ] Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
92 lines
2.4 KiB
Rust
92 lines
2.4 KiB
Rust
// SPDX-License-Identifier: GPL-2.0
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//! Memory layout.
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//!
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//! Custom layout types extending or improving [`Layout`].
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use core::{alloc::Layout, marker::PhantomData};
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/// Error when constructing an [`ArrayLayout`].
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pub struct LayoutError;
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/// A layout for an array `[T; n]`.
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///
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/// # Invariants
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///
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/// - `len * size_of::<T>() <= isize::MAX`.
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pub struct ArrayLayout<T> {
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len: usize,
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_phantom: PhantomData<fn() -> T>,
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}
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impl<T> Clone for ArrayLayout<T> {
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fn clone(&self) -> Self {
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*self
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}
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}
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impl<T> Copy for ArrayLayout<T> {}
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const ISIZE_MAX: usize = isize::MAX as usize;
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impl<T> ArrayLayout<T> {
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/// Creates a new layout for `[T; 0]`.
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pub const fn empty() -> Self {
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// INVARIANT: `0 * size_of::<T>() <= isize::MAX`.
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Self {
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len: 0,
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_phantom: PhantomData,
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}
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}
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/// Creates a new layout for `[T; len]`.
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///
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/// # Errors
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///
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/// When `len * size_of::<T>()` overflows or when `len * size_of::<T>() > isize::MAX`.
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pub const fn new(len: usize) -> Result<Self, LayoutError> {
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match len.checked_mul(core::mem::size_of::<T>()) {
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Some(len) if len <= ISIZE_MAX => {
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// INVARIANT: We checked above that `len * size_of::<T>() <= isize::MAX`.
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Ok(Self {
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len,
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_phantom: PhantomData,
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})
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}
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_ => Err(LayoutError),
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}
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}
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/// Creates a new layout for `[T; len]`.
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///
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/// # Safety
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///
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/// `len` must be a value, for which `len * size_of::<T>() <= isize::MAX` is true.
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pub unsafe fn new_unchecked(len: usize) -> Self {
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// INVARIANT: By the safety requirements of this function
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// `len * size_of::<T>() <= isize::MAX`.
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Self {
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len,
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_phantom: PhantomData,
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}
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}
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/// Returns the number of array elements represented by this layout.
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pub const fn len(&self) -> usize {
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self.len
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}
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/// Returns `true` when no array elements are represented by this layout.
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pub const fn is_empty(&self) -> bool {
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self.len == 0
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}
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}
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impl<T> From<ArrayLayout<T>> for Layout {
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fn from(value: ArrayLayout<T>) -> Self {
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let res = Layout::array::<T>(value.len);
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// SAFETY: By the type invariant of `ArrayLayout` we have
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// `len * size_of::<T>() <= isize::MAX` and thus the result must be `Ok`.
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unsafe { res.unwrap_unchecked() }
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}
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}
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