linux-kernel - Re: [PATCH v7 7/7] rust: enable `clippy::ref_as

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Message-ID: <CAJ-ks9k6220j6CQSOF4TDrgY9qq4PfV9uaMXz1Qk4m=eeSr5Ag@mail.gmail.com>
Date: Tue, 25 Mar 2025 19:54:14 -0400
From: Tamir Duberstein <tamird@...il.com>
To: Benno Lossin <benno.lossin@...ton.me>
Cc: Masahiro Yamada <masahiroy@...nel.org>, Nathan Chancellor <nathan@...nel.org>, 
	Nicolas Schier <nicolas@...sle.eu>, Miguel Ojeda <ojeda@...nel.org>, 
	Alex Gaynor <alex.gaynor@...il.com>, Boqun Feng <boqun.feng@...il.com>, 
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Subject: Re: [PATCH v7 7/7] rust: enable `clippy::ref_as_ptr` lint

On Tue, Mar 25, 2025 at 6:40 PM Benno Lossin <benno.lossin@...ton.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@...ton.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?