Fix marker trait winnowing depending on impl order#153847
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traviscross wants to merge 1 commit intorust-lang:mainfrom
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Fix marker trait winnowing depending on impl order#153847traviscross wants to merge 1 commit intorust-lang:mainfrom
traviscross wants to merge 1 commit intorust-lang:mainfrom
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Since this mentions the old solver, is there an equivalent next solver test that this one was derived from?
Either way, should probably run on both solvers
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None that I could find. I added the directives to test with both solvers.
The `TypeOutlives` handler in `evaluate_predicate_recursively` checks whether a type in a `T: 'a` predicate has free regions, bound regions, or inference variables -- and if so, returns `EvaluatedToOkModuloRegions` rather than `EvaluatedToOk`. The comment says "no free lifetimes or generic parameters", but the code checked `has_non_region_infer` twice instead of checking `has_non_region_param()`. This meant that `TypeOutlives(T, 'static)` where `T` is a type parameter returned `EvaluatedToOk` -- claiming the result holds unconditionally -- when the correct answer is `EvaluatedToOkModuloRegions`. The distinction matters during marker trait winnowing in `prefer_lhs_over_victim`, which uses `must_apply_considering_regions()` (true only for `EvaluatedToOk`) to decide whether one overlapping impl beats another. With the bug, a `T: 'static`-bounded impl appeared equally as strong as an unrestricted impl, making the winner depend on source order. This caused spurious E0310 errors when the more-constrained impl happened to appear after the less-constrained one. Fixes Rust issue 109481. This same symptom was originally filed as issue 84917 and fixed in PR 88139. Then PR 102472 rewrote the `TypeOutlives` handler, introducing the duplicate `has_non_region_infer()` and losing the param check, regressing this. Shortly after, issue 109481 was filed. It noted the connection to PR 102472 -- that it was only relevant after it -- but the duplicate condition was not noticed.
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Story: I was looking through this code for an unrelated reason and happened to notice the duplicate condition. That seemed obviously wrong, especially in light of the comment, so I worked backward to what it affected, wrote a test that failed, and then found that the test matched an existing
known-bugtest, which pointed me to #109481.The
TypeOutliveshandler inevaluate_predicate_recursivelychecks whether a type in aT: 'apredicate has free regions, bound regions, or inference variables -- and if so, returnsEvaluatedToOkModuloRegionsrather thanEvaluatedToOk. The comment says "no free lifetimes or generic parameters", but the code checkedhas_non_region_infer()twice instead of checkinghas_non_region_param().This meant that
TypeOutlives(T, 'static)whereTis a type parameter returnedEvaluatedToOk-- claiming the result holds unconditionally -- when the correct answer isEvaluatedToOkModuloRegions.The distinction matters during marker trait winnowing in
prefer_lhs_over_victim, which usesmust_apply_considering_regions()(true only forEvaluatedToOk) to decide whether one overlapping impl beats another. With the bug, aT: 'static-bounded impl appeared equally as strong as an unrestricted impl, making the winner depend on source order. This caused spurious E0310 errors when the more-constrained impl happened to appear after the less-constrained one.Fixes #109481.
This same symptom was originally filed as #84917 and fixed in PR #88139. Then PR #102472 rewrote the
TypeOutliveshandler, introducing the duplicatehas_non_region_infer()and losing the param check, regressing this. Shortly after, #109481 was filed. It noted the connection to #102472 -- that it was only relevant after it -- but the duplicate condition was not noticed.r? @lcnr
cc @oli-obk @nikomatsakis