From ea402a9d3aaf1d40b4ba384501ef38dd93368ded Mon Sep 17 00:00:00 2001
From: Lucas Franceschino <lucas@cryspen.dev>
Date: Thu, 20 Jun 2024 14:47:05 +0200
Subject: [PATCH] fix(frontend): make `path_to` breadth-first

---
 frontend/exporter/src/traits.rs | 122 ++++++++++++++++++--------------
 1 file changed, 70 insertions(+), 52 deletions(-)

diff --git a/frontend/exporter/src/traits.rs b/frontend/exporter/src/traits.rs
index 67d7cd47c..c108e7f8d 100644
--- a/frontend/exporter/src/traits.rs
+++ b/frontend/exporter/src/traits.rs
@@ -201,65 +201,83 @@ pub mod rustc {
                     .collect()
             }
 
-            #[tracing::instrument(level = "trace", skip(s))]
-            fn path_to(
-                self,
-                s: &S,
-                target: PolyTraitRef<'tcx>,
-                param_env: rustc_middle::ty::ParamEnv<'tcx>,
-            ) -> Option<Path<'tcx>> {
-                let tcx = s.base().tcx;
-                if predicate_equality(self.upcast(tcx), target.upcast(tcx), param_env, s) {
-                    return Some(vec![]);
+        #[tracing::instrument(level = "trace", skip(s))]
+        fn path_to(
+            self,
+            s: &S,
+            target: PolyTraitRef<'tcx>,
+            param_env: rustc_middle::ty::ParamEnv<'tcx>,
+        ) -> Option<Path<'tcx>> {
+            let tcx = s.base().tcx;
+
+            /// A candidate projects `self` along a path reaching some
+            /// predicate. A candidate is selected when its predicate
+            /// is the one expected, aka `target`.
+            #[derive(Debug)]
+            struct Candidate<'tcx> {
+                path: Path<'tcx>,
+                pred: PolyTraitPredicate<'tcx>,
+            }
+
+            use std::collections::VecDeque;
+            let mut candidates: VecDeque<Candidate<'tcx>> = vec![Candidate {
+                path: vec![],
+                pred: self,
+            }]
+            .into();
+
+            let target_pred = target.to_predicate(tcx);
+            let mut seen = std::collections::HashSet::new();
+
+            while let Some(candidate) = candidates.pop_front() {
+                {
+                    // If a predicate was already seen, we know it is
+                    // not the one we are looking for: we skip it.
+                    if seen.contains(&candidate.pred) {
+                        continue;
+                    }
+                    seen.insert(candidate.pred.clone());
                 }
+                tracing::trace!("candidate={:#?}", candidate);
 
-                let recurse = |p: Self| {
-                    if p == self {
-                        return None;
+                // if the candidate equals the target, let's return its path
+                if predicate_equality(candidate.pred.to_predicate(tcx), target_pred, param_env, s) {
+                    return Some(candidate.path);
+                }
+
+                // otherwise, we add to the queue all paths reachable from the candidate
+                for (index, parent_pred) in self.parents_trait_predicates(s) {
+                    let mut path = candidate.path.clone();
+                    path.push(PathChunk::Parent {
+                        predicate: parent_pred.clone(),
+                        predicate_id: parent_pred.predicate_id(s),
+                        index,
+                    });
+                    candidates.push_back(Candidate {
+                        pred: parent_pred.clone(),
+                        path,
+                    });
+                }
+                for (item, binder) in self.associated_items_trait_predicates(s) {
+                    for (index, parent_pred) in binder.skip_binder().into_iter() {
+                        let mut path = candidate.path.clone();
+                        path.push(PathChunk::AssocItem {
+                            item,
+                            predicate_id: parent_pred.predicate_id(s),
+                            predicate: parent_pred.clone(),
+                            index,
+                        });
+                        candidates.push_back(Candidate {
+                            pred: parent_pred.clone(),
+                            path,
+                        });
                     }
-                    p.path_to(s, target, param_env)
-                };
-                fn cons<T>(hd: T, tail: Vec<T>) -> Vec<T> {
-                    vec![hd].into_iter().chain(tail.into_iter()).collect()
                 }
-                self.parents_trait_predicates(s)
-                    .into_iter()
-                    .filter_map(|(index, p)| {
-                        recurse(p).map(|path| {
-                            cons(
-                                PathChunk::Parent {
-                                    predicate: p,
-                                    predicate_id: p.predicate_id(s),
-                                    index,
-                                },
-                                path,
-                            )
-                        })
-                    })
-                    .max_by_key(|path| path.len())
-                    .or_else(|| {
-                        self.associated_items_trait_predicates(s)
-                            .into_iter()
-                            .filter_map(|(item, binder)| {
-                                binder.skip_binder().into_iter().find_map(|(index, p)| {
-                                    recurse(p).map(|path| {
-                                        cons(
-                                            PathChunk::AssocItem {
-                                                item,
-                                                predicate_id: p.predicate_id(s),
-                                                predicate: p,
-                                                index,
-                                            },
-                                            path,
-                                        )
-                                    })
-                                })
-                            })
-                            .max_by_key(|path| path.len())
-                    })
             }
+            None
         }
     }
+}
 
     impl ImplExprAtom {
         fn with_args(self, args: Vec<ImplExpr>, r#trait: TraitRef) -> ImplExpr {