NLL Diagnostic Review 3: Unions not reinitialized after assignment into field

[davidtwco]

@pnkfelix's review comments:

ui/borrowck/borrowck-union-move-assign.nll.stderr rejecting sound uses of unions; it reports 3 errors when only one expected. We should make a run-pass test from the two cases we expect to accept.

For the record, here's the relevant part of the test in question (play):

#![feature(untagged_unions)]
struct A;
struct B;
union U {
    a: A,
    b: B,
}
fn main() {
    unsafe {
        {
            let mut u = U { a: A };
            let _a = u.a;
            u.a = A;
            let _a = u.a; // OK
        }
        {
            let mut u = U { a: A };
            let _a = u.a;
            u.b = B;
            let _a = u.a; // OK
        }
    }
}

[pnkfelix]

This should be NLL-complete, not NLL-sound, right?

[davidtwco]

@pnkfelix Yeah, it should. My bad.

[pnkfelix]

triage: P-high, not not Release milestone. This issue is bad enough for fix quickly, but there's no reason to beta-backport (as I explained on the PR itself here #55657 (comment) )