From 94bf8f04f402a2410ab85a6e6b9e542e3942b2a2 Mon Sep 17 00:00:00 2001
From: lcnr <rust@lcnr.de>
Date: Fri, 17 Jan 2025 10:01:45 +0100
Subject: [PATCH] add cache to `AmbiguityCausesVisitor`

---
 .../rustc_trait_selection/src/traits/coherence.rs  | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/compiler/rustc_trait_selection/src/traits/coherence.rs b/compiler/rustc_trait_selection/src/traits/coherence.rs
index 190e34618d2af..e27143f139641 100644
--- a/compiler/rustc_trait_selection/src/traits/coherence.rs
+++ b/compiler/rustc_trait_selection/src/traits/coherence.rs
@@ -6,7 +6,7 @@
 
 use std::fmt::Debug;
 
-use rustc_data_structures::fx::FxIndexSet;
+use rustc_data_structures::fx::{FxHashSet, FxIndexSet};
 use rustc_errors::{Diag, EmissionGuarantee};
 use rustc_hir::def::DefKind;
 use rustc_hir::def_id::DefId;
@@ -626,6 +626,7 @@ fn compute_intercrate_ambiguity_causes<'tcx>(
 }
 
 struct AmbiguityCausesVisitor<'a, 'tcx> {
+    cache: FxHashSet<Goal<'tcx, ty::Predicate<'tcx>>>,
     causes: &'a mut FxIndexSet<IntercrateAmbiguityCause<'tcx>>,
 }
 
@@ -635,6 +636,10 @@ impl<'a, 'tcx> ProofTreeVisitor<'tcx> for AmbiguityCausesVisitor<'a, 'tcx> {
     }
 
     fn visit_goal(&mut self, goal: &InspectGoal<'_, 'tcx>) {
+        if !self.cache.insert(goal.goal()) {
+            return;
+        }
+
         let infcx = goal.infcx();
         for cand in goal.candidates() {
             cand.visit_nested_in_probe(self);
@@ -759,5 +764,10 @@ fn search_ambiguity_causes<'tcx>(
     goal: Goal<'tcx, ty::Predicate<'tcx>>,
     causes: &mut FxIndexSet<IntercrateAmbiguityCause<'tcx>>,
 ) {
-    infcx.probe(|_| infcx.visit_proof_tree(goal, &mut AmbiguityCausesVisitor { causes }));
+    infcx.probe(|_| {
+        infcx.visit_proof_tree(goal, &mut AmbiguityCausesVisitor {
+            cache: Default::default(),
+            causes,
+        })
+    });
 }