[PATCH v4 02/22] rust: transmute: add `cast_slice[_mut]` functions

[alistair23]

From: Benno Lossin <benno.lossin@xxxxxxxxx>

Add functions to make casting slices only one `unsafe` block.

Signed-off-by: Benno Lossin <benno.lossin@xxxxxxxxx>
Message-ID: <20250814124424.516191-2-lossin@xxxxxxxxxx>
Signed-off-by: Alistair Francis <alistair.francis@xxxxxxx>
---
 rust/kernel/transmute.rs | 71 +++++++++++++++++++++++++++++++++++++++-
 1 file changed, 70 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/transmute.rs b/rust/kernel/transmute.rs
index 654b5ede2fe2..68e96c635ed7 100644
--- a/rust/kernel/transmute.rs
+++ b/rust/kernel/transmute.rs
@@ -2,7 +2,10 @@
 
 //! Traits for transmuting types.
 
-use core::mem::size_of;
+use core::{
+    mem::size_of,
+    slice, //
+};
 
 /// Types for which any bit pattern is valid.
 ///
@@ -227,3 +230,69 @@ macro_rules! impl_asbytes {
     {<T: AsBytes>} [T],
     {<T: AsBytes, const N: usize>} [T; N],
 }
+
+/// Casts the type of a slice to another.
+///
+/// Also see [`cast_slice_mut`].
+///
+/// # Examples
+///
+/// ```rust
+/// # use kernel::transmute::cast_slice;
+/// #[repr(transparent)]
+/// #[derive(Debug)]
+/// struct Container<T>(T);
+///
+/// let array = [0u32; 42];
+/// let slice = &array;
+/// // SAFETY: `Container<u32>` transparently wraps a `u32`.
+/// let container_slice = unsafe { cast_slice::<u32, Container<u32>>(slice) };
+/// pr_info!("{container_slice:?}");
+/// ```
+///
+/// # Safety
+///
+/// - `T` and `U` must have the same layout.
+/// - Every bit pattern that is valid for `T` must also be valid for `U`.
+/// - Neither `T` nor `U` may have interior mutability.
+#[inline]
+pub unsafe fn cast_slice<T, U>(slice: &[T]) -> &[U] {
+    // CAST: by the safety requirements, `T` and `U` have the same layout.
+    let ptr = slice.as_ptr().cast::<U>();
+    // SAFETY: `ptr` and `len` come from the same slice reference.
+    unsafe { slice::from_raw_parts(ptr, slice.len()) }
+}
+
+/// Casts the type of a slice to another.
+///
+/// Also see [`cast_slice`].
+///
+/// # Examples
+///
+/// ```rust
+/// # use kernel::transmute::cast_slice_mut;
+/// #[repr(transparent)]
+/// #[derive(Debug)]
+/// struct Container<T>(T);
+///
+/// let mut array = [0u32; 42];
+/// let slice = &mut array;
+/// // SAFETY: `Container<u32>` transparently wraps a `u32`.
+/// let container_slice = unsafe { cast_slice_mut::<u32, Container<u32>>(slice) };
+/// pr_info!("{container_slice:?}");
+/// ```
+///
+/// # Safety
+///
+/// - `T` and `U` must have the same layout.
+/// - Every bit pattern that is valid for `T` must also be valid for `U`, and
+///   vice versa, so that values written through the returned slice are again
+///   valid for `T`.
+/// - Neither `T` nor `U` may have interior mutability.
+#[inline]
+pub unsafe fn cast_slice_mut<T, U>(slice: &mut [T]) -> &mut [U] {
+    // CAST: by the safety requirements, `T` and `U` have the same layout.
+    let ptr = slice.as_mut_ptr().cast::<U>();
+    // SAFETY: `ptr` and `len` come from the same slice reference.
+    unsafe { slice::from_raw_parts_mut(ptr, slice.len()) }
+}
-- 
2.55.0