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

Tamir Duberstein
2025-03-26T10:35:57Z

On Wed, Mar 26, 2025 at 6:31 AM Benno Lossin <benno.lossin@proton.me> wrote:
>
> On Wed Mar 26, 2025 at 12:54 AM CET, Tamir Duberstein wrote:
> > 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?
>
> It's right in the docs: "`transmute` should be the absolute last
> resort." [1]. IIRC, Gary was a bit more lenient in its use, but I think
> we should avoid it as much as possible such that people copying code or
> taking inspiration also don't use it.
>
> So for both cases I'd prefer an `as` cast.
>
> [1]: https://doc.rust-lang.org/std/mem/fn.transmute.html

I don't follow the logic. The trouble with `as` casts is that they are
very lenient in what they allow, and to do these conversions with `as`
casts requires ref -> pointer -> pointer -> pointer deref versus a
single transmute. The safety comment perfectly describes why it's OK
to do: the types are transparent. So why is `as` casting pointers
better? It's just as unchecked as transmuting, and worse, it requires
a raw pointer dereference.