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

Tamir Duberstein
2025-03-26T20:47:57Z

On Wed, Mar 26, 2025 at 3:06 PM Tamir Duberstein <tamird@gmail.com> wrote:
>
> On Wed, Mar 26, 2025 at 1:36 PM Benno Lossin <benno.lossin@proton.me> wrote:
> >
> > On Wed Mar 26, 2025 at 5:57 PM CET, Tamir Duberstein wrote:
> > >
> > >
> > > In the current code you're looking at, yes. But in the code I have
> > > locally I'm transmuting `[u8]` to `BStr`. See my earlier reply where I
> > > said "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`".
> >
> > `CStr::from_bytes_with_nul_unchecked` does the transmute with
> > references. That is a usage that the docs of `transmute` explicitly
> > recommend to change to an `as` cast [1].
>
> RIght. That guidance was written in 2016
> (https://github.com/rust-lang/rust/pull/34609) and doesn't present any
> rationale for `as` casts being preferred to transmute. I posted a
> comment in the most relevant issue I could find:
> https://github.com/rust-lang/rust/issues/34249#issuecomment-2755316610.
>
> > No idea about provenance still.
>
> Well that's not surprising, nobody was thinking about provenance in
> 2016. But I really don't think we should blindly follow the advice in
> this case. It doesn't make an iota of sense to me - does it make sense
> to you?
>
> >
> > [1]: https://doc.rust-lang.org/std/mem/fn.transmute.html#alternatives
> >
> > >> I tried to find some existing issues about the topic and found that
> > >> there exists a clippy lint `transmute_ptr_to_ptr`. There is an issue
> > >> asking for a better justification [1] and it seems like nobody provided
> > >> one there. Maybe we should ask the opsem team what happens to provenance
> > >> when transmuting?
> > >
> > > Yeah, we should do this - but again: not relevant in this discussion.
> >
> > I think it's pretty relevant.
>
> It's not relevant because we're no longer talking about transmuting
> pointer to pointer. The two options are:
> 1. transmute reference to reference.
> 2. coerce reference to pointer, `as` cast pointer to pointer (triggers
> `ptr_as_ptr`), reborrow pointer to reference.
>
> If anyone can help me understand why (2) is better than (1), I'd
> certainly appreciate it.

Turns out there's a tortured past even in the standard library. In
2017 someone replaces trasmutes with pointer casts:

https://github.com/rust-lang/rust/commit/2633b85ab2c89822d2c227fc9e81c6ec1c0ed9b6

In 2020 someone changes it back to transmute:

https://github.com/rust-lang/rust/pull/75157/files

See also https://github.com/rust-lang/rust/pull/34609#issuecomment-230559871
which makes my point better than I have, particularly this snippet:
"In addition, casting through raw pointers removes the check that both
types have the same size that transmute does provide.".