Merge opaque hidden type candidates before member constraints

[amirHdev]

fixes rust-lang/trait-system-refactor-initiative#227

from what i understand the issue is that with the next solver we can get multiple hidden type candidates for what is effectively the same opaque instantiation but with fresh non-captured parent lifetimes. member constraints are currently applied to those candidates separately, so an unconstrained region can pick a valid but wrong lifetime before information from the other candidate is available. this changes the order a bit: compatible candidates are related first then RegionCtxt is rebuilt and member constraints are applied
non-member lifetime positions are replaced with fresh local NLL vars during that equality so we don't accidentally constrain regions that are supposed to be ignored. i'm not fully sure this is the best place to restore that behavior especially around how the candidates are grouped and feedback there would be appreciated

[rustbot]

r? @mu001999

rustbot has assigned @mu001999.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: compiler
• compiler expanded to 75 candidates
• Random selection from 18 candidates

[mu001999]

@rustbot reroll

[JohnTitor]

@rustbot reroll

[folkertdev]

r? types

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[jackh726]

r? lcnr

[lcnr]

this fix feels odd to me. I would assume us to instead split member constraints into a "complete" variant and a "non-complete" variant. But this is also interesting because different opaque type args can have different region assumptions in scope.

I don't have the capacity to deeply engage with this, blocked on next-solver stabilization.

[rust-bors]

☔ The latest upstream changes (presumably #163945) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.