Coroutine desugaring creates mutable reference to uninitialized fields

This code triggers a UB error in Miri with -Zmiri-recursive-validation:

fn uninit_fields() {
    // Test that uninhabited saved local doesn't make the entire variant uninhabited.
    // (https://github.com/rust-lang/rust/issues/115145, https://github.com/rust-lang/rust/pull/118871)
    fn conjure<T>() -> T {
        loop {}
    }

    fn run<T>(x: bool, y: bool) {
        let mut c = #[coroutine]
        || {
            if x {
                let _a: T;
                if y {
                    _a = conjure::<T>();
                }
                yield ();
            } else {
                let _a: T;
                if y {
                    _a = conjure::<T>();
                }
                yield ();
            }
        };
        assert!(matches!(Pin::new(&mut c).resume(()), CoroutineState::Yielded(())));
        assert!(matches!(Pin::new(&mut c).resume(()), CoroutineState::Complete(())));
    }

    run::<!>(false, false);
}

error: Undefined Behavior: constructing invalid value of type &mut {coroutine@src/tools/miri/tests/pass/coroutine.rs:296:9: 296:11}: at .<deref>.<coroutine-state(4)>.<captured-var(x)>, encountered a value of the never type `!`
   --> src/tools/miri/tests/pass/coroutine.rs:312:35
    |
312 |         assert!(matches!(Pin::new(&mut c).resume(()), CoroutineState::Complete(())));
    |                                   ^^^^^^ Undefined Behavior occurred here
    |

This is in the grey area between sound and unsound since we have not yet decided whether the data behind a reference has to be valid or not.

I am not fully sure what happens here. I thought coroutines wrap their fields in MaybeUninit exactly to avoid this. Also it is very strange that apparently the problem is with the field x, which should be a bool...

Is there some way to dump a bit more information about the generated coroutine to better understand what is going on?

Tracking issue: https://github.com/rust-lang/rust/issues/43122

Start with the reproducer in src/tools/miri/tests/pass/coroutine.rs and run it under Miri with -Zmiri-recursive-validation. Inspect the generated coroutine and desugaring to determine why the captured x field is treated as containing an uninitialized never value; done means explaining the validity issue and adding or updating a regression test if a compiler fix is identified.