perf(parser): construct completed tokens without intermediate writes (#28291)

Build completed tokens directly from the scratch start and flags, avoiding intermediate kind/end writes.

AI-assisted implementation.

crates/oxc_parser/src/lexer/mod.rs

@@ -82,6 +82,8 @@ pub struct Lexer<'a, C: Config> {
     source_type: SourceType,
 
     /// The token currently being built.
+    /// Only its start and flags are written here; its kind and end remain zero until
+    /// `Token::finish` constructs the completed token.
     ///
     /// Between lexer calls, `token` is always `Token::default()`. Every method which produces a token
     /// ends in `finish_next_inner` or `finish_re_lex`, and both reset it.
@@ -314,9 +316,7 @@ impl<'a, C: Config> Lexer<'a, C> {
     #[expect(clippy::inline_always)]
     #[inline(always)]
     fn finish_next_inner(&mut self, kind: Kind, mode: FinishTokenMode) -> Token {
-        self.token.set_kind(kind);
-        self.token.set_end(self.offset());
-        let token = self.token;
+        let token = self.token.finish(kind, self.offset());
         if self.config.tokens() {
             match mode {
                 FinishTokenMode::Push => {
@@ -342,9 +342,7 @@ impl<'a, C: Config> Lexer<'a, C> {
     /// Finish a re-lexed token used only for parser disambiguation.
     /// This must not mutate the externally collected token stream.
     fn finish_re_lex(&mut self, kind: Kind) -> Token {
-        self.token.set_kind(kind);
-        self.token.set_end(self.offset());
-        let token = self.token;
+        let token = self.token.finish(kind, self.offset());
         self.token = Token::default();
         token
     }

crates/oxc_parser/src/lexer/token.rs

@@ -82,6 +82,14 @@ impl fmt::Debug for Token {
 }
 
 impl Token {
+    /// Finish the lexer's scratch token without writing its fields back to memory first.
+    pub(super) fn finish(self, kind: Kind, end: u32) -> Self {
+        debug_assert_eq!(self.kind(), Kind::Eof);
+        debug_assert_eq!(self.end(), 0);
+        debug_assert!(end >= self.start());
+        Self(self.0 | (u128::from(end) << END_SHIFT) | ((kind as u128) << KIND_SHIFT))
+    }
+
     #[inline]
     pub(super) fn new_on_new_line() -> Self {
         // Start with a default token, then set the flag