// SPDX-License-Identifier: GPL-2.0
//! Memory layout.
//!
//! Custom layout types extending or improving [`Layout`].
use core::{alloc::Layout, marker::PhantomData};
/// Error when constructing an [`ArrayLayout`].
pub struct LayoutError;
/// A layout for an array `[T; n]`.
///
/// # Invariants
///
/// - `len * size_of::<T>() <= isize::MAX`.
pub struct ArrayLayout<T> {
len: usize,
_phantom: PhantomData<fn() -> T>,
}
impl<T> Clone for ArrayLayout<T> {
fn clone(&self) -> Self {
*self
}
}
impl<T> Copy for ArrayLayout<T> {}
const ISIZE_MAX: usize = isize::MAX as usize;
impl<T> ArrayLayout<T> {
/// Creates a new layout for `[T; 0]`.
pub const fn empty() -> Self {
// INVARIANT: `0 * size_of::<T>() <= isize::MAX`.
Self {
len: 0,
_phantom: PhantomData,
}
}
/// Creates a new layout for `[T; len]`.
///
/// # Errors
///
/// When `len * size_of::<T>()` overflows or when `len * size_of::<T>() > isize::MAX`.
///
/// # Examples
///
/// ```
/// # use kernel::alloc::layout::{ArrayLayout, LayoutError};
/// let layout = ArrayLayout::<i32>::new(15)?;
/// assert_eq!(layout.len(), 15);
///
/// // Errors because `len * size_of::<T>()` overflows.
/// let layout = ArrayLayout::<i32>::new(isize::MAX as usize);
/// assert!(layout.is_err());
///
/// // Errors because `len * size_of::<i32>() > isize::MAX`,
/// // even though `len < isize::MAX`.
/// let layout = ArrayLayout::<i32>::new(isize::MAX as usize / 2);
/// assert!(layout.is_err());
///
/// # Ok::<(), Error>(())
/// ```
pub const fn new(len: usize) -> Result<Self, LayoutError> {
match len.checked_mul(core::mem::size_of::<T>()) {
Some(size) if size <= ISIZE_MAX => {
// INVARIANT: We checked above that `len * size_of::<T>() <= isize::MAX`.
Ok(Self {
len,
_phantom: PhantomData,
})
}
_ => Err(LayoutError),
}
}
/// Creates a new layout for `[T; len]`.
///
/// # Safety
///
/// `len` must be a value, for which `len * size_of::<T>() <= isize::MAX` is true.
pub unsafe fn new_unchecked(len: usize) -> Self {
// INVARIANT: By the safety requirements of this function
// `len * size_of::<T>() <= isize::MAX`.
Self {
len,
_phantom: PhantomData,
}
}
/// Returns the number of array elements represented by this layout.
pub const fn len(&self) -> usize {
self.len
}
/// Returns `true` when no array elements are represented by this layout.
pub const fn is_empty(&self) -> bool {
self.len == 0
}
}
impl<T> From<ArrayLayout<T>> for Layout {
fn from(value: ArrayLayout<T>) -> Self {
let res = Layout::array::<T>(value.len);
// SAFETY: By the type invariant of `ArrayLayout` we have
// `len * size_of::<T>() <= isize::MAX` and thus the result must be `Ok`.
unsafe { res.unwrap_unchecked() }
}
}
