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| -rw-r--r-- | compiler/rustc_trait_selection/src/solve/eval_ctxt/mod.rs | 14 |
1 files changed, 14 insertions, 0 deletions
diff --git a/compiler/rustc_trait_selection/src/solve/eval_ctxt/mod.rs b/compiler/rustc_trait_selection/src/solve/eval_ctxt/mod.rs index ee0fc8139a2..76c50a11102 100644 --- a/compiler/rustc_trait_selection/src/solve/eval_ctxt/mod.rs +++ b/compiler/rustc_trait_selection/src/solve/eval_ctxt/mod.rs @@ -395,6 +395,20 @@ impl<'a, 'tcx> EvalCtxt<'a, 'tcx> { original_values: Vec<ty::GenericArg<'tcx>>, response: CanonicalResponse<'tcx>, ) -> Result<(Certainty, bool, Vec<Goal<'tcx, ty::Predicate<'tcx>>>), NoSolution> { + // The old solver did not evaluate nested goals when normalizing. + // It returned the selection constraints allowing a `Projection` + // obligation to not hold in coherence while avoiding the fatal error + // from overflow. + // + // We match this behavior here by considering all constraints + // from nested goals which are not from where-bounds. We will already + // need to track which nested goals are required by impl where-bounds + // for coinductive cycles, so we simply reuse that here. + // + // While we could consider overflow constraints in more cases, this should + // not be necessary for backcompat and results in better perf. It also + // avoids a potential inconsistency which would otherwise require some + // tracking for root goals as well. See #119071 for an example. let keep_overflow_constraints = || { self.search_graph.current_goal_is_normalizes_to() && source != GoalSource::ImplWhereBound |