Fix move checking for nested union fields

[petrochenkov]

Fixes #41126
r? @arielb1

[arielb1]

I'll rather r? @pnkfelix - he best understands the borrow checker.

BTW, do we have an rpass test that shows what patterns are acceptable?

[petrochenkov]

@arielb1

BTW, do we have an rpass test that shows what patterns are acceptable?

There are few tests, but they are mostly "compile-pass" style tests:
https://github.com/rust-lang/rust/blob/master/src/test/compile-fail/borrowck/borrowck-union-borrow-nested.rs
https://github.com/rust-lang/rust/blob/master/src/test/compile-fail/borrowck/borrowck-union-borrow.rs
https://github.com/rust-lang/rust/blob/master/src/test/compile-fail/borrowck/borrowck-union-move-assign.rs
https://github.com/rust-lang/rust/blob/master/src/test/compile-fail/borrowck/borrowck-union-move.rs
https://github.com/rust-lang/rust/blob/master/src/test/compile-fail/borrowck/borrowck-union-uninitialized.rs (this one is a FIXME test)
https://github.com/rust-lang/rust/blob/master/src/test/run-pass/union/union-drop-assign.rs
https://github.com/rust-lang/rust/blob/master/src/test/run-pass/union/union-overwrite.rs

I didn't add many runtime tests executing drop code for partial moves in the original PR due to issues with drop flags that I couldn't fix quickly.

[pnkfelix]

This looks fine, in the sense that it is a natural extension of the logic that was previously implemented.

So I'm going to r+, but I also agree with @arielb1's comment that we may want to revisit the rules regarding how borrows and unions interact.

[pnkfelix]

@bors r+

[bors]

📌 Commit ad58d37 has been approved by pnkfelix

[bors]

⌛ Testing commit ad58d37 with merge ba37798...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: pnkfelix
Pushing ba37798 to master...