better test

Author: pierwill

chalk-solve/src/coherence.rs

@@ -116,10 +116,21 @@ where
         // 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 l = forest.add_node(less_special);
-            let m = forest.add_node(more_special);
+            let nodes = forest.raw_nodes();
+            let mut goodnodes: Vec<ImplId<_>> = vec![];
 
-            forest.add_edge(l, m, ());
+            for node in nodes {
+                goodnodes.push(node.weight);
+            }
+
+            if !goodnodes.contains(&less_special) && !goodnodes.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, ());
+            }
         })?;
 
         Ok(forest)