Add tests for predecessor-aware `VecGraph` mode

compiler/rustc_data_structures/src/graph/vec_graph/tests.rs

@@ -18,10 +18,18 @@ fn create_graph() -> VecGraph<usize> {
     VecGraph::new(7, vec![(0, 1), (1, 2), (1, 3), (3, 4), (5, 1)])
 }
 
+fn create_graph_with_back_refs() -> VecGraph<usize, true> {
+    // Same as above
+    VecGraph::new(7, vec![(0, 1), (1, 2), (1, 3), (3, 4), (5, 1)])
+}
+
 #[test]
 fn num_nodes() {
     let graph = create_graph();
     assert_eq!(graph.num_nodes(), 7);
+
+    let graph = create_graph_with_back_refs();
+    assert_eq!(graph.num_nodes(), 7);
 }
 
 #[test]
@@ -34,6 +42,27 @@ fn successors() {
     assert_eq!(graph.successors(4), &[] as &[usize]);
     assert_eq!(graph.successors(5), &[1]);
     assert_eq!(graph.successors(6), &[] as &[usize]);
+
+    let graph = create_graph_with_back_refs();
+    assert_eq!(graph.successors(0), &[1]);
+    assert_eq!(graph.successors(1), &[2, 3]);
+    assert_eq!(graph.successors(2), &[] as &[usize]);
+    assert_eq!(graph.successors(3), &[4]);
+    assert_eq!(graph.successors(4), &[] as &[usize]);
+    assert_eq!(graph.successors(5), &[1]);
+    assert_eq!(graph.successors(6), &[] as &[usize]);
+}
+
+#[test]
+fn predecessors() {
+    let graph = create_graph_with_back_refs();
+    assert_eq!(graph.predecessors(0), &[]);
+    assert_eq!(graph.predecessors(1), &[0, 5]);
+    assert_eq!(graph.predecessors(2), &[1]);
+    assert_eq!(graph.predecessors(3), &[1]);
+    assert_eq!(graph.predecessors(4), &[3]);
+    assert_eq!(graph.predecessors(5), &[]);
+    assert_eq!(graph.predecessors(6), &[]);
 }
 
 #[test]
@@ -41,4 +70,8 @@ fn dfs() {
     let graph = create_graph();
     let dfs: Vec<_> = graph::depth_first_search(&graph, 0).collect();
     assert_eq!(dfs, vec![0, 1, 3, 4, 2]);
+
+    let graph = create_graph_with_back_refs();
+    let dfs: Vec<_> = graph::depth_first_search(&graph, 0).collect();
+    assert_eq!(dfs, vec![0, 1, 3, 4, 2]);
 }