From 4d0618ee3014b96287bb6914296b78bd474b0290 Mon Sep 17 00:00:00 2001
From: Nicholas Nethercote <nnethercote@mozilla.com>
Date: Thu, 24 Nov 2016 21:10:08 +1100
Subject: [PATCH] Avoid more unnecessary mk_ty calls in Ty::super_fold_with.

This speeds up several rustc-benchmarks by 1--5%.
---
 src/librustc/ty/structural_impls.rs | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/src/librustc/ty/structural_impls.rs b/src/librustc/ty/structural_impls.rs
index e73be23a42c..877da7ee3b5 100644
--- a/src/librustc/ty/structural_impls.rs
+++ b/src/librustc/ty/structural_impls.rs
@@ -481,7 +481,12 @@ impl<'tcx> TypeFoldable<'tcx> for Ty<'tcx> {
             ty::TyUint(_) | ty::TyFloat(_) | ty::TyError | ty::TyInfer(_) |
             ty::TyParam(..) | ty::TyNever => return self
         };
-        folder.tcx().mk_ty(sty)
+
+        if self.sty == sty {
+            self
+        } else {
+            folder.tcx().mk_ty(sty)
+        }
     }
 
     fn fold_with<'gcx: 'tcx, F: TypeFolder<'gcx, 'tcx>>(&self, folder: &mut F) -> Self {