Avoid allocation in loop.

crates/accelerate/src/sabre_swap/mod.rs

@@ -171,11 +171,12 @@ fn populate_extended_set(
     let mut decremented: IndexMap<usize, u32, ahash::RandomState> =
         IndexMap::with_hasher(ahash::RandomState::default());
     let mut i = 0;
+    let mut visit_now: Vec<NodeIndex> = Vec::new();
     while i < to_visit.len() && extended_set.len() < EXTENDED_SET_SIZE {
         // Visit runs of non-2Q gates fully before moving on to children
         // of 2Q gates. This way, traversal order is a BFS of 2Q gates rather
         // than of all gates.
-        let mut visit_now = vec![to_visit[i]];
+        visit_now.push(to_visit[i]);
         let mut j = 0;
         while let Some(node) = visit_now.get(j) {
             for edge in dag.dag.edges_directed(*node, Direction::Outgoing) {
@@ -185,7 +186,8 @@ fn populate_extended_set(
                 required_predecessors[successor_index] -= 1;
                 if required_predecessors[successor_index] == 0 {
                     if !dag.dag[successor_node].directive
-		        && !dag.node_blocks.contains_key(&successor_index) {
+                        && !dag.node_blocks.contains_key(&successor_index)
+                    {
                         if let [a, b] = dag.dag[successor_node].qubits[..] {
                             extended_set.push([a.to_phys(layout), b.to_phys(layout)]);
                             to_visit.push(successor_node);
@@ -197,6 +199,7 @@ fn populate_extended_set(
             }
             j += 1;
         }
+        visit_now.clear();
         i += 1;
     }
     for (node, amount) in decremented.iter() {