Fix graph logic

Author: pierwill

chalk-solve/src/coherence.rs

@@ -110,16 +110,47 @@ where
 
         let node_impls: Vec<ImplId<_>> = forest.raw_nodes().iter().map(|x| x.weight).collect();
 
-        // Find all specializations (implemented in coherence/solve)
-        // Record them in the forest by adding an edge from the less special
-        // to the more special.
+        // Find all specializations. 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| {
-            // 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);
-
-                forest.add_edge(l, m, ());
+            match (
+                node_impls.contains(&less_special),
+                node_impls.contains(&more_special),
+            ) {
+                (true, true) => {
+                    // but how do we get indices for l and m?
+                    let l = forest
+                        .node_indices()
+                        .find(|i| forest[*i] == less_special)
+                        .unwrap();
+                    let m = forest
+                        .node_indices()
+                        .find(|i| forest[*i] == more_special)
+                        .unwrap();
+                    forest.add_edge(l, m, ());
+                }
+                (true, false) => {
+                    let m = forest.add_node(more_special);
+                    let l = forest
+                        .node_indices()
+                        .find(|i| forest[*i] == less_special)
+                        .unwrap();
+                    forest.add_edge(l, m, ());
+                }
+                (false, true) => {
+                    let l = forest.add_node(less_special);
+                    let m = forest
+                        .node_indices()
+                        .find(|i| forest[*i] == more_special)
+                        .unwrap();
+                    forest.add_edge(l, m, ());
+                }
+                (false, false) => {
+                    // add L and M
+                    let l = forest.add_node(less_special);
+                    let m = forest.add_node(more_special);
+                    forest.add_edge(l, m, ());
+                }
             }
         })?;