Re: [PATCH v7 7/7] rust: enable `clippy::ref_as_ptr` lint

[Benno Lossin <benno.lossin@proton.me>]

On Tue Mar 25, 2025 at 9:07 PM CET, Tamir Duberstein wrote:
> diff --git a/rust/kernel/str.rs b/rust/kernel/str.rs
> index 40034f77fc2f..6233af50bab7 100644
> --- a/rust/kernel/str.rs
> +++ b/rust/kernel/str.rs
> @@ -29,7 +29,7 @@ pub const fn is_empty(&self) -> bool {
>      #[inline]
>      pub const fn from_bytes(bytes: &[u8]) -> &Self {
>          // SAFETY: `BStr` is transparent to `[u8]`.
> -        unsafe { &*(bytes as *const [u8] as *const BStr) }
> +        unsafe { &*(core::mem::transmute::<*const [u8], *const Self>(bytes)) }

Hmm I'm not sure about using `transmute` here. Yes the types are
transparent, but I don't think that we should use it here.

>      }
>  
>      /// Strip a prefix from `self`. Delegates to [`slice::strip_prefix`].
> @@ -290,7 +290,7 @@ pub const fn from_bytes_with_nul(bytes: &[u8]) -> Result<&Self, CStrConvertError
>      #[inline]
>      pub unsafe fn from_bytes_with_nul_unchecked_mut(bytes: &mut [u8]) -> &mut CStr {
>          // SAFETY: Properties of `bytes` guaranteed by the safety precondition.
> -        unsafe { &mut *(bytes as *mut [u8] as *mut CStr) }
> +        unsafe { &mut *(core::mem::transmute::<*mut [u8], *mut Self>(bytes)) }
>      }
>  
>      /// Returns a C pointer to the string.
> diff --git a/rust/kernel/uaccess.rs b/rust/kernel/uaccess.rs
> index 80a9782b1c6e..c042b1fe499e 100644
> --- a/rust/kernel/uaccess.rs
> +++ b/rust/kernel/uaccess.rs
> @@ -242,7 +242,7 @@ pub fn read_raw(&mut self, out: &mut [MaybeUninit<u8>]) -> Result {
>      pub fn read_slice(&mut self, out: &mut [u8]) -> Result {
>          // SAFETY: The types are compatible and `read_raw` doesn't write uninitialized bytes to
>          // `out`.
> -        let out = unsafe { &mut *(out as *mut [u8] as *mut [MaybeUninit<u8>]) };
> +        let out = unsafe { &mut *(core::mem::transmute::<*mut [u8], *mut [MaybeUninit<u8>]>(out)) };

I have a patch that adds a `cast_slice_mut` method that could be used
here, so I can fix it in that series. But let's not use `transmute` here
either.

---
Cheers,
Benno

>          self.read_raw(out)
>      }
>