9fde33a09f
### Explanation In the case that `assignSyntheticOpaque := true` and the given metavariable is `syntheticOpaque` and the depth of the metavariable is not the current depth, `isReadOnlyOrSyntheticOpaque` returns false, even though the metavariable is read-only because of being declared at a smaller depth. This causes the metavariable to (wrongly) be able to be instantiated by `isDefEq`. This bug was found at the proof of [RingHom.PropertyIsLocal.sourceAffineLocally_of_source_openCover](https://leanprover-community.github.io/mathlib4_docs/Mathlib/AlgebraicGeometry/Morphisms/RingHomProperties.html#RingHom.PropertyIsLocal.sourceAffineLocally_of_source_openCover), which involves a type class synthesis for `CommRing ?m.2404`, and the synthesis manages to instantiate this metavariable into different values, even though `synthInstance?` increases the metavariable depth. This synthesis fails after 1 second. I found the bug while modifying the instance synthesis code: the modified code spent several minutes on this failed synthesis. ### Test The problem can be verified with the test: ``` run_meta do let m ← mkFreshExprMVar (Expr.sort levelOne) MetavarKind.syntheticOpaque withAssignableSyntheticOpaque do withNewMCtxDepth do let eq ← isDefEq m (.const ``Nat []) Lean.logInfo m! "{eq}" ``` this unification used to succeed, giving `true`, and this fix makes it return `false`. ### Impact on Mathlib This fix causes a change in the behaviour of `congr`, `convert` and friends, which breaks a couple of proofs in mathlib. Most of these are fixed by supplying more arguments. I fixed these proofs, and [benched](http://speed.lean-fro.org/mathlib4/compare/b821bfd9-3769-4930-b77f-0adc6f9d218f/to/e7b27246-a3e6-496a-b552-ff4b45c7236e?hash2=4f3c460cc1668820c9af8418a87a23db44c7acab) mathlib. The result is that most files are unaffected, but some files are significantly improved. This is most prominent in Mathlib.RingTheory.Jacobson, where the number of instructions has decreased by 28%. The overall improvement is a 0.3% reduction in instructions. [Zulip message](https://leanprover.zulipchat.com/#narrow/stream/287929-mathlib4/topic/Ways.20to.20speed.20up.20Mathlib/near/439218960)
12 lines
319 B
Text
12 lines
319 B
Text
import Lean
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open Lean Meta
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-- In a new MCtx depth, metavariables should not be assignable by `isDefEq`.
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run_meta do
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let m ← mkFreshExprMVar (Expr.sort levelOne) MetavarKind.syntheticOpaque
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withAssignableSyntheticOpaque do
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withNewMCtxDepth do
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let eq ← isDefEq m (.const ``Nat [])
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Lean.logInfo m! "{eq}"
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