On Wed, Mar 26, 2025 at 3:06 PM Tamir Duberstein <tam...@gmail.com> wrote: > > 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.
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.".
