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

[Benno Lossin]

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@xxxxxxxxx> 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@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.
>>
>> 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

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Cheers,
Benno