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stepping into impls for norm is unproductive

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lcnr 2025-04-16 10:19:14 +02:00
parent efb1e3d676
commit 48e119ef5a
6 changed files with 201 additions and 8 deletions
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21
compiler/rustc_next_trait_solver/src/solve/eval_ctxt/mod.rs
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@ -286,18 +286,23 @@ where
// fixing it may cause inference breakage or introduce ambiguity.
GoalSource::Misc => PathKind::Unknown,
GoalSource::NormalizeGoal(path_kind) => path_kind,
GoalSource::ImplWhereBound => {
GoalSource::ImplWhereBound => match self.current_goal_kind {
// We currently only consider a cycle coinductive if it steps
// into a where-clause of a coinductive trait.
CurrentGoalKind::CoinductiveTrait => PathKind::Coinductive,
// While normalizing via an impl does step into a where-clause of
// an impl, accessing the associated item immediately steps out of
// it again. This means cycles/recursive calls are not guarded
// by impls used for normalization.
//
// See tests/ui/traits/next-solver/cycles/normalizes-to-is-not-productive.rs
// for how this can go wrong.
CurrentGoalKind::NormalizesTo => PathKind::Inductive,
// We probably want to make all traits coinductive in the future,
// so we treat cycles involving their where-clauses as ambiguous.
if let CurrentGoalKind::CoinductiveTrait = self.current_goal_kind {
PathKind::Coinductive
} else {
PathKind::Unknown
}
}
// so we treat cycles involving where-clauses of not-yet coinductive
// traits as ambiguous for now.
CurrentGoalKind::Misc => PathKind::Unknown,
},
// Relating types is always unproductive. If we were to map proof trees to
// corecursive functions as explained in #136824, relating types never
// introduces a constructor which could cause the recursion to be guarded.