From 59e41edcc15ed07de604c61876ea091900f73649 Mon Sep 17 00:00:00 2001
From: Nicholas Nethercote <nnethercote@mozilla.com>
Date: Thu, 10 Oct 2019 13:06:12 +1100
Subject: [PATCH] Special-case `ReEmpty` in `expand_node()`.

This wins 6% on `unicode_normalization`, by avoiding a call to
`lub_concrete_regions()` and a `Region` equality test.
---
 src/librustc/infer/lexical_region_resolve/mod.rs | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/src/librustc/infer/lexical_region_resolve/mod.rs b/src/librustc/infer/lexical_region_resolve/mod.rs
index 84960d149d0f..94ec3b981a47 100644
--- a/src/librustc/infer/lexical_region_resolve/mod.rs
+++ b/src/librustc/infer/lexical_region_resolve/mod.rs
@@ -360,13 +360,21 @@ impl<'cx, 'tcx> LexicalResolver<'cx, 'tcx> {
         match *b_data {
             VarValue::Value(cur_region) => {
                 // Identical scopes can show up quite often, if the fixed point
-                // iteration converges slowly, skip them
+                // iteration converges slowly. Skip them. This is purely an
+                // optimization.
                 if let (ReScope(a_scope), ReScope(cur_scope)) = (a_region, cur_region) {
                     if a_scope == cur_scope {
                         return false;
                     }
                 }
 
+                // This is a specialized version of the `lub_concrete_regions`
+                // check below for a common case, here purely as an
+                // optimization.
+                if let ReEmpty = a_region {
+                    return false;
+                }
+
                 let mut lub = self.lub_concrete_regions(a_region, cur_region);
                 if lub == cur_region {
                     return false;