Type annotation on `let _` binding materializes the value

This program is fine:

fn main() {
    let mut x = &1;
    unsafe { (&raw mut x).cast::<usize>().write(0) };
    let _ = x; // fine
}

x contains invalid data, but we're just mentioning the place without loading from it.

However, this program has UB according to Miri:

fn main() {
    let mut x = &1;
    unsafe { (&raw mut x).cast::<usize>().write(0) };
    let _: &_ = x; // errors
}

Somehow, with a type annotation, the mentioned place becomes an actually loaded-from place.

For this case that seems like an odd inconsistency, but things become more tricky if we consider that there can be coercions involved, including coercions from uninhabited types to any types. Those must actually materialize the input value, obviously. (I seems to remember we have an issue for such coercions somewhere?)

Cc @Nadrieril

Start by running both Rust examples under Miri and compare the behavior of the unannotated and annotated let _ bindings. Investigate how type annotations and coercions determine whether the place is materialized, including coercions from uninhabited types. Done means the language behavior is clarified and the inconsistency is resolved or documented with coverage for these cases.