Skip next-solver fulfillment when no type var was instantiated - #161480
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Skip next-solver fulfillment when no type var was instantiated#161480anonrig wants to merge 1 commit into
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Some changes occurred to the core trait solver cc @rust-lang/initiative-trait-system-refactor |
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Thanks for the pull request, and welcome! The Rust Project has assigned @lcnr (or someone else) to review your changes, you should hear from them (or someone else) within the next two weeks. Please see the contribution instructions for more information. |
Unknown-unknown equate and sub-unify do not make single-var stalled goals progress. Track instantiations in the infer undo log and skip the pending-queue walk when every pending goal is type-var-only with at most one stalled vid. This removes the remaining quadratic rescan on rustc#159933: each `push(Default::default())` equates a fresh vid with the vec element vid, so goals stay stalled until a later instantiate. Two-var goals still take the existing path.
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Refs #159933
r? lcnr
Next-solver fulfillment walks the pending queue on every
try_evaluate_obligations. On #159933 (v.push(Default::default())thenv.push(0u8)), each push equates a fresh infer vid with the vec element vid. Those equates are unknown-unknown:ty_or_const_infer_var_changedstays false until a vid is instantiated, butsub_rootsgo stale and force a full rescan.This records instantiations in the infer undo log and skips the
retain_mutwalk when:Two-var goals (for example
Mirrorwith distinct self and parameter infer vars that later unify) still take the existing path.-Zdisable-fast-pathsdisables the skip.