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?
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:
- transmute reference to reference.
- 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/2633b85ab2c89822d2c227fc9e81c6ec1c0...
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.".