Avoid chain() in find_constraint_paths_between_regions().

This iterator can be hot, and chained iterators are slow. The second
half of the chain is almost always empty, so this commit specializes the
code to avoid the chained iteration.

This change reduces instruction counts for the `wg-grammar` benchmark by
up to 1.5%.

Author: nnethercote

src/librustc_mir/borrow_check/nll/region_infer/error_reporting/mod.rs

@@ -253,28 +253,40 @@ impl<'tcx> RegionInferenceContext<'tcx> {
             let outgoing_edges_from_graph = self.constraint_graph
                 .outgoing_edges(r, &self.constraints, fr_static);
 
-
-            // But member constraints can also give rise to `'r: 'x`
-            // edges that were not part of the graph initially, so
-            // watch out for those.
-            let outgoing_edges_from_picks = self.applied_member_constraints(r)
-                .iter()
-                .map(|&AppliedMemberConstraint { min_choice, member_constraint_index, .. }| {
-                    let p_c = &self.member_constraints[member_constraint_index];
-                    OutlivesConstraint {
-                        sup: r,
-                        sub: min_choice,
-                        locations: Locations::All(p_c.definition_span),
-                        category: ConstraintCategory::OpaqueType,
+            macro_rules! handle_constraint {
+                ($constraint:ident) => {
+                    debug_assert_eq!($constraint.sup, r);
+                    let sub_region = $constraint.sub;
+                    if let Trace::NotVisited = context[sub_region] {
+                        context[sub_region] = Trace::FromOutlivesConstraint($constraint);
+                        deque.push_back(sub_region);
                     }
-                });
-
-            for constraint in outgoing_edges_from_graph.chain(outgoing_edges_from_picks) {
-                debug_assert_eq!(constraint.sup, r);
-                let sub_region = constraint.sub;
-                if let Trace::NotVisited = context[sub_region] {
-                    context[sub_region] = Trace::FromOutlivesConstraint(constraint);
-                    deque.push_back(sub_region);
+                }
+            }
+
+            for constraint in outgoing_edges_from_graph {
+                handle_constraint!(constraint);
+            }
+
+            // But member constraints can also give rise to `'r: 'x` edges that
+            // were not part of the graph initially, so watch out for those.
+            // (But they are extremely rare.)
+            let applied_member_constraints = self.applied_member_constraints(r);
+            if !applied_member_constraints.is_empty() {
+                let outgoing_edges_from_picks = self.applied_member_constraints(r)
+                    .iter()
+                    .map(|&AppliedMemberConstraint { min_choice, member_constraint_index, .. }| {
+                        let p_c = &self.member_constraints[member_constraint_index];
+                        OutlivesConstraint {
+                            sup: r,
+                            sub: min_choice,
+                            locations: Locations::All(p_c.definition_span),
+                            category: ConstraintCategory::OpaqueType,
+                        }
+                    });
+
+                for constraint in outgoing_edges_from_picks {
+                    handle_constraint!(constraint);
                 }
             }
         }