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

[Benno Lossin <benno.lossin@proton.me>]

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 :)

---
Cheers,
Benno