do not add nodes that already exist

chalk-solve/src/coherence.rs

@@ -107,28 +107,19 @@ where
 
     // Build the forest of specialization relationships.
     fn build_specialization_forest(&self) -> Result<Graph<ImplId<I>, ()>, CoherenceError<I>> {
-        // 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)
         // Record them in the forest by adding an edge from the less special
         // to the more special.
         self.visit_specializations_of_trait(|less_special, more_special| {
-            let nodes = forest.raw_nodes();
-            let mut goodnodes: Vec<ImplId<_>> = vec![];
+            let node_impls: Vec<ImplId<_>> = forest.raw_nodes().iter().map(|x| x.weight).collect();
 
-            for node in nodes {
-                goodnodes.push(node.weight);
-            }
-
-            if !goodnodes.contains(&less_special) && !goodnodes.contains(&more_special) {
+            // Check so that we never add multiple nodes with the same ImplId.
+            if !node_impls.contains(&less_special) && !node_impls.contains(&more_special) {
                 let l = forest.add_node(less_special);
                 let m = forest.add_node(more_special);
 
-                #[rustfmt::skip]
-                println!( "adding an edge from {:?} to {:?}", less_special, more_special );
                 forest.add_edge(l, m, ());
             }
         })?;