perf(ast): optimize JSXIdentifier::is_reference (#5344)

crates/oxc_ast/src/ast_impl/jsx.rs

@@ -24,8 +24,20 @@ impl<'a> JSXIdentifier<'a> {
     ///
     /// References begin with a capital letter, `_` or `$`.
     /// <https://babeljs.io/repl#?code_lz=DwMQ9mAED0B8DcAoYAzCMHIPpqnJwAJLhkkA&presets=react>
+    // `name.chars().next().unwrap()` cannot panic because name is never an empty string.
+    #[allow(clippy::missing_panics_doc)]
     pub fn is_reference(&self) -> bool {
-        self.name.chars().next().map_or(false, |c| c.is_uppercase() || c == '_' || c == '$')
+        // The identifier has already been checked to be valid, so when first char is ASCII, it can only
+        // be `a-z`, `A-Z`, `_` or `$`. But compiler doesn't know that, so we can help it create faster
+        // code by taking that invariant into account.
+        // `b < b'a'` matches `A-Z`, `_` and `$`.
+        // Use a fast path for common case of ASCII characters, to avoid the more expensive
+        // `char::is_uppercase` in most cases.
+        let name = self.name.as_str();
+        match name.as_bytes()[0] {
+            b if b.is_ascii() => b < b'a',
+            _ => name.chars().next().unwrap().is_uppercase(),
+        }
     }
 }