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

[Tamir Duberstein]

On Tue, Mar 25, 2025 at 6:11 PM Benno Lossin <benno.lossin@xxxxxxxxx> wrote:
>
> 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.

What's your suggestion? I initially tried

let bytes: *const [u8] = bytes;
unsafe { &*bytes.cast() }

but that doesn't compile because of the implicit Sized bound on pointer::cast.

>
> >      }
> >
> >      /// 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.

See above - I don't know what else I could write here.