Miri misses UB in place expressions when there are "_" patterns

[RalfJung]

The following example should both be UB, but Miri finds neither of it: (this got fixed by rust-lang/rust#104844)

fn main() {
    let ptr = 32 as *const u8;
    unsafe { let _ = *ptr; }
}

This is blocked on rust-lang/rust#99180.

Here's another variant: (that is not fixed yet, will be fixed by rust-lang/rust#103208)

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

[RalfJung]

rust-lang/rust#102256 preserves these place expressions in MIR, which is a step towards fixing this.

@JakobDegen

We currently spec FakeRead as being a semantic nop; we could make an exception for the ForWildcard case, but it might be cleaner to have a new StatementKind::PlaceComputation or something like that

Would anything go wrong if we spec'ed them all to be evaluating the place expression and then discarding it (i.e., not actually doing a read, never producing a value)?

[JakobDegen]

Can't be sure without trying, but I experimented for a bit and wasn't able to come up with any problems.

[RalfJung]

Now that rust-lang/rust#104844 landed, this is actually fixed, we just need tests.

[RalfJung]

Actually only the let _ version is fixed. match with a _ pattern does not seem to be preserved in MIR.

@cjgillot is that expected?

EDIT: Oh I see, rust-lang/rust#103208 seems to be taking care of this.

[RalfJung]

rust-lang/rust#103208 landed. Due to rust-lang/reference#1387, the original example is not UB any more, but similar things are, so this will be fixed after adding a test.