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

Tamir Duberstein
2025-03-25T23:54:52Z

On Tue, Mar 25, 2025 at 6:40 PM Benno Lossin <benno.lossin@proton.me> wrote:
>
> On Tue Mar 25, 2025 at 11:33 PM CET, Tamir Duberstein wrote:
> > On Tue, Mar 25, 2025 at 6:11 PM Benno Lossin <benno.lossin@proton.me> 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.
>
> This is AFAIK one of the only places where we cannot get rid of the `as`
> cast. So:
>
> let bytes: *const [u8] = bytes;
> // CAST: `BStr` transparently wraps `[u8]`.
> let bytes = bytes as *const BStr;
> // SAFETY: `bytes` is derived from a reference.
> unsafe { &*bytes }
>
> IMO a `transmute` is worse than an `as` cast :)

Hmm, looking at this again we can just transmute ref-to-ref and avoid
pointers entirely. We're already doing that in
`CStr::from_bytes_with_nul_unchecked`

Why is transmute worse than an `as` cast?