review

compiler/rustc_next_trait_solver/src/canonicalizer.rs

@@ -67,6 +67,12 @@ pub struct Canonicalizer<'a, D: SolverDelegate<Interner = I>, I: Interner> {
     variables: &'a mut Vec<I::GenericArg>,
     var_kinds: Vec<CanonicalVarKind<I>>,
     variable_lookup_table: HashMap<I::GenericArg, usize>,
+    /// Maps each `sub_unification_table_root_var` to the index of the first
+    /// variable which used it.
+    ///
+    /// This means in case two type variables have the same sub relations root,
+    /// we set the `sub_root` of the second variable to the position of the first.
+    /// Otherwise the `sub_root` of each type variable is just its own position.
     sub_root_lookup_table: HashMap<ty::TyVid, usize>,
     binder_index: ty::DebruijnIndex,
 
@@ -273,7 +279,7 @@ impl<'a, D: SolverDelegate<Interner = I>, I: Interner> Canonicalizer<'a, D, I> {
     }
 
     fn get_or_insert_sub_root(&mut self, vid: ty::TyVid) -> ty::BoundVar {
-        let root_vid = self.delegate.sub_root_ty_var(vid);
+        let root_vid = self.delegate.sub_unification_table_root_var(vid);
         let idx =
             *self.sub_root_lookup_table.entry(root_vid).or_insert_with(|| self.variables.len());
         ty::BoundVar::from(idx)

compiler/rustc_next_trait_solver/src/solve/eval_ctxt/mod.rs

@@ -900,8 +900,8 @@ where
             && goal.param_env.visit_with(&mut visitor).is_continue()
     }
 
-    pub(super) fn sub_ty_vids_raw(&self, a: ty::TyVid, b: ty::TyVid) {
-        self.delegate.sub_ty_vids_raw(a, b)
+    pub(super) fn sub_unify_ty_vids_raw(&self, a: ty::TyVid, b: ty::TyVid) {
+        self.delegate.sub_unify_ty_vids_raw(a, b)
     }
 
     #[instrument(level = "trace", skip(self, param_env), ret)]

compiler/rustc_next_trait_solver/src/solve/mod.rs

@@ -121,7 +121,7 @@ where
     fn compute_subtype_goal(&mut self, goal: Goal<I, ty::SubtypePredicate<I>>) -> QueryResult<I> {
         match (goal.predicate.a.kind(), goal.predicate.b.kind()) {
             (ty::Infer(ty::TyVar(a_vid)), ty::Infer(ty::TyVar(b_vid))) => {
-                self.sub_ty_vids_raw(a_vid, b_vid);
+                self.sub_unify_ty_vids_raw(a_vid, b_vid);
                 self.evaluate_added_goals_and_make_canonical_response(Certainty::AMBIGUOUS)
             }
             _ => {