fix bug in DepthFirstSearch where start node was not visited

This could cause us to visit the start node twice.

Author: nikomatsakis

compiler/rustc_data_structures/src/graph/iterate/mod.rs

@@ -84,7 +84,10 @@ where
     G: ?Sized + DirectedGraph + WithNumNodes + WithSuccessors,
 {
     pub fn new(graph: &'graph G, start_node: G::Node) -> Self {
-        Self { graph, stack: vec![start_node], visited: BitSet::new_empty(graph.num_nodes()) }
+        let mut visited = BitSet::new_empty(graph.num_nodes());
+        visited.insert(start_node);
+        Self { graph, stack: vec![start_node], visited }
+    }
     }
 }
 

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

@@ -20,3 +20,11 @@ fn is_cyclic() {
     assert!(!is_cyclic(&diamond_acyclic));
     assert!(is_cyclic(&diamond_cyclic));
 }
+
+#[test]
+fn dfs() {
+    let graph = TestGraph::new(0, &[(0, 1), (0, 2), (1, 3), (2, 3), (3, 0)]);
+
+    let result: Vec<usize> = DepthFirstSearch::new(&graph, 0).collect();
+    assert_eq!(result, vec![0, 2, 3, 1]);
+}