two-phase borrows creates extra error messages

[nikomatsakis]

As uncovered in #47689, two-phase borrows as implemented can lead to "extra" and confusing error messages. The problem is that each time we have a "gen " of the activation, we do the activation check -- but if there are two activations in a row, each of them can emit an error, when we only want an error from the first.

The HEAD~1 commit in branch issue-48418 on my repository fixes the problem, but probably not in a sound way, as discussed in this commit (and its successor).

The proper fix either involves adding a dominator check (suppress activation of bit B at A2 if it is dominated by another activation of bit B at A1), or else a "must activate" sort of analysis.

cc @pnkfelix

[nikomatsakis]

cc @rust-lang/wg-compiler-nll -- this is a bit of an opaque description, but maybe someone is interested in taking a crack at this? If so, @pnkfelix or I can try to hammer out more precise instructions if needed, though perhaps we ought to settle on the fix we actually want. =)

[nikomatsakis]

I believe @bobtwinkles fixed this.

This test now has one error:

https://github.com/rust-lang/rust/blob/master/src/test/ui/issue-45157.stderr

Whereas before it had two:

https://github.com/davidtwco/rust/blob/e6376a15b4f22bc296a832d232937add7e04511d/src/test/ui/issue-45157.stderr