From 9edaa7dba3181ae68c60204e1e47a2d20e86236e Mon Sep 17 00:00:00 2001
From: Patrick Walton <pcwalton@mimiga.net>
Date: Fri, 22 Apr 2011 10:37:51 -0700
Subject: [PATCH] rustc: Add a fast path if there are no type substitutions to
 be made

---
 src/comp/middle/ty.rs | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/src/comp/middle/ty.rs b/src/comp/middle/ty.rs
index 59de77686124..96479c4096d6 100644
--- a/src/comp/middle/ty.rs
+++ b/src/comp/middle/ty.rs
@@ -2362,9 +2362,15 @@ mod Unify {
 
         auto ures = unify_step(cx, expected, actual);
         alt (ures) {
-        case (ures_ok(?t)) {
+        case (ures_ok(?typ)) {
+            // Fast path: if there are no local variables, don't perform
+            // substitutions.
+            if (_vec.len[mutable UFind.node](cx.sets.nodes) == 0u) {
+                ret ures_ok(typ);
+            }
+
             auto set_types = unify_sets(cx);
-            auto t2 = substitute(cx, set_types, t);
+            auto t2 = substitute(cx, set_types, typ);
             ret ures_ok(t2);
         }
         case (_) { ret ures; }