Use to_ascii_lowercase to avoid heap alloc in detect_confuse_type

Signed-off-by: xizheyin <[email protected]>

Author: xizheyin

compiler/rustc_errors/src/emitter.rs

@@ -3546,7 +3546,7 @@ pub fn detect_confusion_type(sm: &SourceMap, suggested: &str, sp: Span) -> Confu
 
         for (f, s) in iter::zip(found.chars(), suggested.chars()) {
             if f != s {
-                if f.to_lowercase().to_string() == s.to_lowercase().to_string() {
+                if f.to_ascii_lowercase() == s.to_ascii_lowercase() {
                     // Check for case differences (any character that differs only in case)
                     if ascii_confusables.contains(&f) || ascii_confusables.contains(&s) {
                         has_case_diff = true;