Incorporate iter_nodes into graph::DirectedGraph

This assumes that the set of valid node IDs is exactly `0..num_nodes`.

In practice, we have a lot of graph-algorithm code that already assumes that
nodes are densely numbered, by using `num_nodes` to allocate per-node indexed
data structures.

compiler/rustc_data_structures/src/graph/mod.rs

@@ -14,7 +14,23 @@ mod tests;
 pub trait DirectedGraph {
     type Node: Idx;
 
+    /// Returns the total number of nodes in this graph.
+    ///
+    /// Several graph algorithm implementations assume that every node ID is
+    /// strictly less than the number of nodes, i.e. nodes are densely numbered.
+    /// That assumption allows them to use `num_nodes` to allocate per-node
+    /// data structures, indexed by node.
     fn num_nodes(&self) -> usize;
+
+    /// Iterates over all nodes of a graph in ascending numeric order.
+    ///
+    /// Assumes that nodes are densely numbered, i.e. every index in
+    /// `0..num_nodes` is a valid node.
+    fn iter_nodes(
+        &self,
+    ) -> impl Iterator<Item = Self::Node> + DoubleEndedIterator + ExactSizeIterator {
+        (0..self.num_nodes()).map(<Self::Node as Idx>::new)
+    }
 }
 
 pub trait NumEdges: DirectedGraph {

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

@@ -333,8 +333,8 @@ where
             to_annotation,
         };
 
-        let scc_indices = (0..num_nodes)
-            .map(G::Node::new)
+        let scc_indices = graph
+            .iter_nodes()
             .map(|node| match this.start_walk_from(node) {
                 WalkReturn::Complete { scc_index, .. } => scc_index,
                 WalkReturn::Cycle { min_depth, .. } => {