match on an enum with a single inhabited variant does not read discriminant

[RalfJung]

Examples by @meithecatte; Cc @Nadrieril
The following code should likely trigger UB, but currently Miri raises no error:

#![feature(never_type)]

#[repr(C)]
#[allow(dead_code)]
enum E {
  V1, // discriminant: 0
  V2(!), // 1
}

fn main() {
    assert_eq!(std::mem::size_of::<E>(), 4);

    let val = 1u32;
    let ptr = (&raw const val).cast::<E>();
    let r = unsafe { &*ptr };
    match r { //~ we'd probably want ERROR: read discriminant of an uninhabited enum variant
        E::V1 => {}
        E::V2(_) => {}
    }
}

The same goes for this variant that adds a closure (to check interactions with rust-lang/rust#138961):

#![feature(never_type)]

#[repr(C)]
#[allow(dead_code)]
enum E {
  V0, // discriminant: 0
  V2(!), // 2
}

fn main() {
    assert_eq!(std::mem::size_of::<E>(), 4);

    let val = 2u32;
    let ptr = (&raw const val).cast::<E>();
    let r = unsafe { &*ptr };
    let f = || {
        // After rust-lang/rust#138961, constructing the closure performs a reborrow of r.
        // Nevertheless, the discriminant is only actually inspected when the closure
        // is called.
        match r { //~ ERROR: read discriminant of an uninhabited enum variant
            E::V0 => {}
            E::V2(_) => {}
        }
    };

    f();
}

[theemathas]

This is similar to rust-lang/rust#142394

[RalfJung]

Ah, that is the issue I could not find. :) Let's track this on the Miri side as well.