Resolve multi-candidate methods on impls defined outside the type's module - #4778
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Resolve multi-candidate methods on impls defined outside the type's module#4778kasimte wants to merge 1 commit into
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…odule resolve_in_type_def refines multiple same-named inherent-impl candidates by comparing the user's turbofish generic arguments against the last two ::-separated segments of def_path_str(candidate). When a candidate's impl block lives in a different module than its self type, def_path_str renders it as `path::to::module::<impl path::to::Type<Args>>::method` instead of `Type::<Args>::method`, and no user-spellable turbofish can match that <impl ...> wrapper. Real instance: Box<dyn Any(+Send)(+Sync), A>::downcast_unchecked's three impls live in alloc/src/boxed/convert.rs while Box is defined in boxed.rs. Add a fallback in last_two_items_of_path_match: when the direct comparison fails and the candidate's second-to-last path segment is in <impl SELF_TYPE> form, extract SELF_TYPE's own generic arguments and retry the comparison against those. The retry strips the redundant parens def_path_str adds around a trait-object bound in a generic-argument list (e.g. `(dyn Any + 'static)` -> `dyn Any + 'static`) before comparing; tuple-type parens are semantic and preserved.
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Resolves #4777.
last_two_items_of_path_matchcompares the user's turbofish againstdef_path_str's rendering of each candidate. Impls living outside their type's home module render as<impl Type<Args>>, which no user-spellable path can equal — so multi-candidate methods in that position (e.g. the threeBox<dyn Any(+Send)(+Sync), A>::downcast_uncheckedimpls in alloc'sboxed/convert.rs) fail to resolve under every spelling, while single-candidate methods skip refinement and work.This is the out-of-module half of #3773 (whose fix and
multiple_inherent_impls.rstest cover the same-module rendering). It adds a fallback that unwraps the<impl SELF_TYPE>form, extracts SELF_TYPE's generic arguments, strips the disambiguation parenthesesdef_path_stradds around trait-object bounds (tuple-type parentheses are semantic and preserved), and retries the comparison.Scope: this keeps the existing string-comparison approach and only unblocks the out-of-module rendering; the milder same-module symptom (concrete turbofish arguments not matching a structured self-type such as
MaybeUninit<u32>vs the impl'sMaybeUninit<T>— the generic-parameter spelling works there) is unchanged, and a semantic-resolution rewrite would subsume both. Happy to take direction if the deeper rewrite is preferred.Tests: four unit tests in the existing
simple_last_two_items_of_path_matchmodule (dyn-args match + mismatch, tuple parens preserved, non-generic no-fallback) and a new end-to-end regression testtests/kani/FunctionContracts/cross_module_multiple_impls.rs(fails to resolve before the fix, verifies after). Also validated on the motivating case: all threeBox<dyn Any…>::downcast_uncheckedcontracts in verify-rust-std resolve and verify asproof_for_contracttargets under the patched resolver.Applies cleanly on current
main(last_two_items_of_path_matchis unchanged there). One call-out: candidates whose rendered path contains->(fn-pointer /Fn-sugar types) are not unwrapped by the fallback's bracket counting — they keep the existing failed-to-resolve behavior rather than matching a different candidate.Motivating case: model-checking/verify-rust-std#669 — the three
Box<dyn Any…>::downcast_uncheckedcontract targets there.By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.