refactor(parser): reduce unnecessary backtracking in hot paths (#12708)

Author: Copilot

crates/oxc_parser/src/js/arrow.rs

@@ -192,20 +192,19 @@ impl<'a> ParserImpl<'a> {
     }
 
     fn is_un_parenthesized_async_arrow_function_worker(&mut self) -> bool {
-        let checkpoint = self.checkpoint();
-        self.bump(Kind::Async);
-        // If the "async" is followed by "=>" token then it is not a beginning of an async arrow-function
-        // but instead a simple arrow-function which will be parsed inside "parseAssignmentExpressionOrHigher"
-        if !self.cur_token().is_on_new_line() && self.cur_kind().is_binding_identifier() {
-            // Arrow before newline is checked in `parse_simple_arrow_function_expression`
-            self.bump_any();
-            if self.at(Kind::Arrow) {
-                self.rewind(checkpoint);
-                return true;
+        // Use lookahead to avoid checkpoint/rewind
+        self.lookahead(|parser| {
+            parser.bump(Kind::Async);
+            // If the "async" is followed by "=>" token then it is not a beginning of an async arrow-function
+            // but instead a simple arrow-function which will be parsed inside "parseAssignmentExpressionOrHigher"
+            if !parser.cur_token().is_on_new_line() && parser.cur_kind().is_binding_identifier() {
+                // Arrow before newline is checked in `parse_simple_arrow_function_expression`
+                parser.bump_any();
+                parser.at(Kind::Arrow)
+            } else {
+                false
             }
-        }
-        self.rewind(checkpoint);
-        false
+        })
     }
 
     pub(crate) fn parse_simple_arrow_function_expression(

crates/oxc_parser/src/js/expression.rs

@@ -731,24 +731,26 @@ impl<'a> ParserImpl<'a> {
             let mut question_dot = false;
             let is_property_access = if allow_optional_chain && self.at(Kind::QuestionDot) {
                 // ?.
-                let checkpoint = self.checkpoint();
-                self.bump_any();
-                let kind = self.cur_kind();
-                let is_identifier_or_keyword = kind.is_identifier_or_keyword();
-                if kind == Kind::LBrack
-                    || is_identifier_or_keyword
-                    || kind.is_template_start_of_tagged_template()
+                // Fast check to avoid checkpoint/rewind in common cases
+                let next_kind = self.lexer.peek_token().kind();
+                if next_kind == Kind::LBrack
+                    || next_kind.is_identifier_or_keyword()
+                    || next_kind.is_template_start_of_tagged_template()
                 {
+                    // This is likely a valid optional chain, proceed with normal parsing
+                    self.bump_any(); // consume ?.
+                    let kind = self.cur_kind();
+                    let is_identifier_or_keyword = kind.is_identifier_or_keyword();
                     // ?.[
                     // ?.something
                     // ?.template`...`
                     *in_optional_chain = true;
                     question_dot = true;
                     is_identifier_or_keyword
                 } else {
+                    // This is not a valid optional chain pattern, don't consume ?.
                     // Should be a cold branch here, as most real-world optional chaining will look like
                     // `?.something` or `?.[expr]`
-                    self.rewind(checkpoint);
                     false
                 }
             } else {

crates/oxc_parser/src/ts/types.rs

@@ -430,14 +430,15 @@ impl<'a> ParserImpl<'a> {
             Kind::LParen => self.parse_parenthesized_type(),
             Kind::Import => TSType::TSImportType(self.parse_ts_import_type()),
             Kind::Asserts => {
-                let checkpoint = self.checkpoint();
-                let asserts_start_span = self.start_span();
-                self.bump_any(); // bump `asserts`
-
-                if self.is_token_identifier_or_keyword_on_same_line() {
+                // Use lookahead to check if this is an asserts type predicate
+                if self.lookahead(|parser| {
+                    parser.bump(Kind::Asserts);
+                    parser.is_token_identifier_or_keyword_on_same_line()
+                }) {
+                    let asserts_start_span = self.start_span();
+                    self.bump_any(); // bump `asserts`
                     self.parse_asserts_type_predicate(asserts_start_span)
                 } else {
-                    self.rewind(checkpoint);
                     self.parse_type_reference()
                 }
             }