Meeting proposal: behavior of _ in match patterns

[RalfJung]

Summary

Does this code have UB?

fn main() {
    let ptr = 32 as *const (u8, u8);
    unsafe {
        match (*ptr).1 { _ => {} }
    }
}

Currently the answer Miri gives is no. That is inconsistent with let _ = (*ptr).1, which creates a PlaceMention that is picked up by Miri and hence raises UB (the UB error will change with rust-lang/rust#114330, since it will be UB at the projection rather than the deref, but it will remain UB).

We should determine whether we want to make this behavior consistent.

Reading

Meeting document

Issues/PRs:

• Allow partially moved values in match rust#103208
• Miri misses UB in place expressions when there are "_" patterns miri#2360

Comment policy

These issues are meant to be used as an "announcements channel" regarding the proposal, and not as a
place to discuss the technical details. Feel free to subscribe to updates. We'll post comments when
reviewing the proposal in meetings or making a scheduling decision. In the meantime, if you have
questions or ideas, ping the proposers on Zulip (or elsewhere).

[RalfJung]

Meeting summary: everyone is fine with the opsem implications of rust-lang/rust#103208.