insert_graph fails with self-edges

[acl-cqc]

I applied the following diff to the insert_graph test in src/portgraph.rs (commit 71453e8 on branch main):

@@ -1616,12 +1616,13 @@ pub mod test {
         g.link_nodes(node0g, 0, node1g, 0)?;
 
         let mut h = PortGraph::new();
-        let node0h = h.add_node(1, 2);
-        let node1h = h.add_node(2, 1);
-        h.link_nodes(node0h, 0, node1h, 1)?;
-        h.link_nodes(node0h, 1, node1h, 0)?;
-        h.link_nodes(node1h, 0, node0h, 0)?;
+        let node0h = h.add_node(2, 2);
+        let node1h = h.add_node(1, 1);
+        h.link_nodes(node0h, 0, node1h, 0)?;
+        h.link_nodes(node0h, 1, node0h, 0)?;
+        h.link_nodes(node1h, 0, node0h, 1)?;
 
+        // The following fails with an AlreadyLinked:
         let map = g.insert_graph(&h)?;
         assert_eq!(map.len(), 2);

The edges are now a cycle from node0h back to itself as well as a cycle from node0h via node1h back to node0h. The error happens because in insert_graph all_links for node0h returns two views of the same edge (one from the outgoing port and one from the incoming port), both of which lead to trying to add the edge.

I also tried with MultiPortGraph using this diff on the above, and insert_graph succeeds but is adding two copies of the edge:

@@ -1607,7 +1607,7 @@ pub mod test {
 
     #[test]
     fn insert_graph() -> Result<(), Box<dyn std::error::Error>> {
-        let mut g = PortGraph::new();
+        let mut g = crate::MultiPortGraph::new();
         // Add dummy nodes to produce different node ids than in the other graph.
         g.add_node(0, 0);
         g.add_node(0, 0);

@@ -1622,18 +1622,26 @@ pub mod test {
         h.link_nodes(node0h, 1, node0h, 0)?;
         h.link_nodes(node1h, 0, node0h, 1)?;
 
-        // The following fails with an AlreadyLinked:
+        // This now succeeds as one would expect:
         let map = g.insert_graph(&h)?;
         assert_eq!(map.len(), 2);
 
         assert_eq!(g.node_count(), 6);
-        assert_eq!(g.link_count(), 4);
+        //This fails (link_count is 5):
+        //assert_eq!(g.link_count(), 4);
         assert!(g.contains_node(map[&node0h]));
         assert!(g.contains_node(map[&node1h]));
-        assert!(g.input_neighbours(map[&node0h]).eq([map[&node1h]]));
-        assert!(g
-            .output_neighbours(map[&node0h])
-            .eq([map[&node1h], map[&node1h]]));
+        // Modified the assert to give better debug output.
+        // The LHS now has three elements: map[&node0h] *twice* and map[&node1h]
+        assert_eq!(
+            g.input_neighbours(map[&node0h]).collect::<Vec<_>>(),
+            vec![map[&node0h], map[&node1h]]
+        );
+        // Similarly, here the LHS now has map[&node1h], map[&node0h], map[&node0h]
+        assert_eq!(
+            g.output_neighbours(map[&node0h]).collect::<Vec<_>>(),
+            vec![map[&node1h], map[&node0h]]
+        );
 
         Ok(())
     }