trait_solver: Preserve concrete candidates in ambiguous ORs - #162442
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A reflexive `'a: 'a` leaf is always satisfied, so filter it out in `And::new` instead of pattern matching for it in the places which happen to build such a constraint. An AND which ends up empty is trivially true, which makes the OR containing it true. This is how a reflexive candidate discharges a root type outlives constraint, so `destructure_type_outlives_constraints_in_root` no longer has to look at region outlives leaves at all.
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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. |
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Stacked on #161988. That PR contains the first six commits in this branch, covering reflexive region constraints and the canonicalization needed around them. The only new work here is the final mixed-OR fix. Once #161988 lands, that is all that remains.
propagate_ambiguityused to collapse an entire OR when one alternative was ambiguous. That could throw away a concrete candidate before the root assumptions got a chance to check it. Now ambiguity propagates only when every alternative is ambiguous. A mixed OR keeps its concrete candidates, but still stays ambiguous if none of those candidates can be proved.I kept the dependency visible because these fixes are related, but they are easier to review separately. The regression test covers both OR orders and the AND case where ambiguity is required. I like this split because it keeps the evaluator from turning a candidate that might work into
Ambigjust because another branch is unclear.This is the follow-up Boxy asked to split out in this comment. cc @BoxyUwU