rustc VecGraph: require the index type to implement Ord

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

@@ -1,3 +1,5 @@
+use std::cmp::Ord;
+
 use crate::graph::{DirectedGraph, GraphSuccessors, WithNumEdges, WithNumNodes, WithSuccessors};
 use rustc_index::vec::{Idx, IndexVec};
 
@@ -17,10 +19,10 @@ pub struct VecGraph<N: Idx> {
     edge_targets: Vec<N>,
 }
 
-impl<N: Idx> VecGraph<N> {
+impl<N: Idx + Ord> VecGraph<N> {
     pub fn new(num_nodes: usize, mut edge_pairs: Vec<(N, N)>) -> Self {
         // Sort the edges by the source -- this is important.
-        edge_pairs.sort_by_key(|&edge_pairs| (edge_pairs.0.index(), edge_pairs.1.index()));
+        edge_pairs.sort();
 
         let num_edges = edge_pairs.len();
 
@@ -100,7 +102,7 @@ impl<'graph, N: Idx> GraphSuccessors<'graph> for VecGraph<N> {
     type Iter = std::iter::Cloned<std::slice::Iter<'graph, N>>;
 }
 
-impl<N: Idx> WithSuccessors for VecGraph<N> {
+impl<N: Idx + Ord> WithSuccessors for VecGraph<N> {
     fn successors(&self, node: N) -> <Self as GraphSuccessors<'_>>::Iter {
         self.successors(node).iter().cloned()
     }