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perf: skip re-attempting pending TC mvars with unchanged instantiated goals (quadratic resumption, #14448) #14449

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perf: skip re-attempting pending TC mvars with unchanged instantiated goals (quadratic resumption, #14448) #14449

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11 changes: 11 additions & 0 deletions src/Lean/Elab/SyntheticMVars.lean
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Original file line number Diff line number Diff line change
Expand Up @@ -76,8 +76,19 @@ private def resumePostponed (savedContext : SavedContext) (stx : Syntax) (mvarId
/--
Similar to `synthesizeInstMVarCore`, but makes sure that `instMVar` local context and instances
are used. It also logs any error message produced. -/
register_builtin_option Elab.tcSkipUnchanged : Bool := {
defValue := false
descr := "skip re-attempting a pending typeclass metavariable whose instantiated goal type is unchanged since its last failed attempt (the outcome is deterministic in the goal, so re-attempting is wasted work; the resumption loop is otherwise quadratic in the number of chained pending instances)"
}

private def synthesizePendingInstMVar (instMVar : MVarId) (extraErrorMsg? : Option MessageData := none): TermElabM Bool :=
instMVar.withContext do
if Elab.tcSkipUnchanged.get (← getOptions) then
let type ← instantiateMVars (← instMVar.getType)
if let some prev := (← get).tcSynthAttempt.get? instMVar then
if prev == type then
return false
modify fun s => { s with tcSynthAttempt := s.tcSynthAttempt.insert instMVar type }
try
synthesizeInstMVarCore instMVar (extraErrorMsg? := extraErrorMsg?)
catch
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5 changes: 5 additions & 0 deletions src/Lean/Elab/Term/TermElabM.lean
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Expand Up @@ -181,6 +181,11 @@ structure State where
levelNames : List Name := []
syntheticMVars : MVarIdMap SyntheticMVarDecl := {}
pendingMVars : List MVarId := {}
/-- Last instantiated goal type per still-pending `.typeClass` mvar whose synthesis
attempt failed; used (option `Elab.tcSkipUnchanged`) to skip re-attempts when the
goal is unchanged — the resumption loop otherwise re-tries every pending TC mvar
after each single success, which is quadratic in chained-literal statements. -/
tcSynthAttempt : MVarIdMap Expr := {}
/-- List of errors associated to a metavariable that are shown to the user if the metavariable could not be fully instantiated -/
mvarErrorInfos : List MVarErrorInfo := []
/-- List of data to be able to localize universe level metavariable errors to particular expressions. -/
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