rust: alloc: introduce ArrayLayout

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>

rust/kernel/alloc.rs

@@ -5,6 +5,7 @@
 #[cfg(not(any(test, testlib)))]
 pub mod allocator;
 pub mod kbox;
+pub mod layout;
 pub mod vec_ext;
 
 #[cfg(any(test, testlib))]

rust/kernel/alloc/layout.rs

@@ -0,0 +1,91 @@
+// 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`.
+    pub const fn new(len: usize) -> Result<Self, LayoutError> {
+        match len.checked_mul(core::mem::size_of::<T>()) {
+            Some(len) if len <= 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() }
+    }
+}