Re: [PATCH] rust: retain pointer mut-ness in `container_of!`

[Alice Ryhl <aliceryhl@google.com>]

On Wed, Apr 09, 2025 at 10:43:16AM -0400, Tamir Duberstein wrote:
> Avoid casting the input pointer to `*const _`, allowing the output
> pointer to be `*mut` if the input is `*mut`. This allows a number of
> `*const` to `*mut` conversions to be removed at the cost of slightly
> worse ergonomics when the macro is used with a reference rather than a
> pointer; the only example of this was in the macro's own doctest.
> 
> Reviewed-by: Benno Lossin <benno.lossin@proton.me>
> Reviewed-by: Alice Ryhl <aliceryhl@google.com>
> Signed-off-by: Tamir Duberstein <tamird@gmail.com>
> ---
> This patch is extracted from 3 other series to reduce duplication.
> ---
>  rust/kernel/lib.rs    |  5 ++---
>  rust/kernel/rbtree.rs | 23 ++++++++++-------------
>  2 files changed, 12 insertions(+), 16 deletions(-)
> 
> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> index de07aadd1ff5..1df11156302a 100644
> --- a/rust/kernel/lib.rs
> +++ b/rust/kernel/lib.rs
> @@ -190,7 +190,7 @@ fn panic(info: &core::panic::PanicInfo<'_>) -> ! {
>  /// }
>  ///
>  /// let test = Test { a: 10, b: 20 };
> -/// let b_ptr = &test.b;
> +/// let b_ptr: *const _ = &test.b;
>  /// // SAFETY: The pointer points at the `b` field of a `Test`, so the resulting pointer will be
>  /// // in-bounds of the same allocation as `b_ptr`.
>  /// let test_alias = unsafe { container_of!(b_ptr, Test, b) };
> @@ -199,9 +199,8 @@ fn panic(info: &core::panic::PanicInfo<'_>) -> ! {
>  #[macro_export]
>  macro_rules! container_of {
>      ($ptr:expr, $type:ty, $($f:tt)*) => {{
> -        let ptr = $ptr as *const _ as *const u8;
>          let offset: usize = ::core::mem::offset_of!($type, $($f)*);
> -        ptr.sub(offset) as *const $type
> +        $ptr.byte_sub(offset).cast::<$type>()
>      }}
>  }

This implementation does not check the type of `ptr`. Would we not want
it to have the type of the field?

Alice