Add fixme note

Author: pierwill

chalk-solve/src/coherence.rs

@@ -107,8 +107,9 @@ where
 
     // Build the forest of specialization relationships.
     fn build_specialization_forest(&self) -> Result<Graph<ImplId<I>, ()>, CoherenceError<I>> {
-        // The forest is returned as a graph but built as a GraphMap; this is
+        // FIXME(pierwill): Previously, the forest was built as a GraphMap
         // so that we never add multiple nodes with the same ItemId.
+        // Will need to add a check for this.
         let mut forest = DiGraph::new();
 
         // Find all specializations (implemented in coherence/solve)