Optimize check_keyword_case.

nnethercote · nnethercote · commit 99d02fb40fd3 · 2024-11-13T08:43:47.000+11:00
`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.
diff --git 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 {

Original file line number	Diff line number	Diff line change
`@@ -641,9 +641,10 @@ impl<'a> Parser<'a> {`
`641`	`641`	`return true;`
`642`	`642`	`}`
`643`	`643`
	`644`	`+ // Do an ASCII case-insensitive match, because all keywords are ASCII.`
`644`	`645`	`if case == Case::Insensitive`
`645`	`646`	`&& let Some((ident, IdentIsRaw::No)) = self.token.ident()`
`646`		`- && ident.as_str().to_lowercase() == kw.as_str().to_lowercase()`
	`647`	`+ && ident.as_str().eq_ignore_ascii_case(kw.as_str())`
`647`	`648`	`{`
`648`	`649`	`true`
`649`	`650`	`} else {`