diff --git a/compiler/rustc_trait_selection/src/solve/project_goals.rs b/compiler/rustc_trait_selection/src/solve/project_goals.rs
index 435f2877fb3..0658836fb9c 100644
--- a/compiler/rustc_trait_selection/src/solve/project_goals.rs
+++ b/compiler/rustc_trait_selection/src/solve/project_goals.rs
@@ -82,6 +82,7 @@ fn term_is_fully_unconstrained(&self, goal: Goal<'tcx, ProjectionPredicate<'tcx>
             }
         };
 
+        // Guard against `<T as Trait<?0>>::Assoc = ?0>`.
         struct ContainsTerm<'tcx> {
             term: ty::Term<'tcx>,
         }