From 99d02fb40fd339255ed08596ebeb41e9b8a09d45 Mon Sep 17 00:00:00 2001
From: Nicholas Nethercote <n.nethercote@gmail.com>
Date: Thu, 7 Nov 2024 16:09:39 +1100
Subject: [PATCH] Optimize `check_keyword_case`.

`to_lowercase` allocates, but `eq_ignore_ascii_case` doesn't. This path
is hot enough that this makes a small but noticeable difference in
benchmarking.
---
 compiler/rustc_parse/src/parser/mod.rs | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/compiler/rustc_parse/src/parser/mod.rs b/compiler/rustc_parse/src/parser/mod.rs
index 50a8b6542df..042ee96bbe8 100644
--- a/compiler/rustc_parse/src/parser/mod.rs
+++ b/compiler/rustc_parse/src/parser/mod.rs
@@ -641,9 +641,10 @@ impl<'a> Parser<'a> {
             return true;
         }
 
+        // Do an ASCII case-insensitive match, because all keywords are ASCII.
         if case == Case::Insensitive
             && let Some((ident, IdentIsRaw::No)) = self.token.ident()
-            && ident.as_str().to_lowercase() == kw.as_str().to_lowercase()
+            && ident.as_str().eq_ignore_ascii_case(kw.as_str())
         {
             true
         } else {