On Wed, Mar 26, 2025 at 1:36 PM Benno Lossin <benno.los...@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.
