From a711ff43357f4f929891827cf597f7377802fbf5 Mon Sep 17 00:00:00 2001
From: overlookmotel <557937+overlookmotel@users.noreply.github.com>
Date: Wed, 14 May 2025 11:20:21 +0000
Subject: [PATCH] perf(parser): make `Kind::Eof` (default) 0 (#11014)

Follow-on after #10933. Make `u8` value of `Kind::Eof` (`Kind::default()`) be 0. This makes `Token::default()` a little cheaper, as all bits of default `Token` are 0.
---
 crates/oxc_parser/src/lexer/kind.rs  |  4 ++--
 crates/oxc_parser/src/lexer/token.rs | 20 +++++++++++++-------
 2 files changed, 15 insertions(+), 9 deletions(-)

diff --git a/crates/oxc_parser/src/lexer/kind.rs b/crates/oxc_parser/src/lexer/kind.rs
index f178bb2469954..57c9a0fa8614d 100644
--- a/crates/oxc_parser/src/lexer/kind.rs
+++ b/crates/oxc_parser/src/lexer/kind.rs
@@ -9,9 +9,9 @@ use std::fmt::{self, Display};
 #[repr(u8)]
 #[non_exhaustive]
 pub enum Kind {
-    Undetermined,
     #[default]
-    Eof,
+    Eof = 0,
+    Undetermined,
     Skip, // Whitespace, line breaks, comments
     // 12.5 Hashbang Comments
     HashbangComment,
diff --git a/crates/oxc_parser/src/lexer/token.rs b/crates/oxc_parser/src/lexer/token.rs
index 181af88bcb05c..072e85b2adbe7 100644
--- a/crates/oxc_parser/src/lexer/token.rs
+++ b/crates/oxc_parser/src/lexer/token.rs
@@ -37,12 +37,18 @@ const HAS_SEPARATOR_FLAG: u128 = 1 << HAS_SEPARATOR_SHIFT;
 pub struct Token(u128);
 
 impl Default for Token {
+    #[inline]
     fn default() -> Self {
-        let mut token = Self(0);
-        // `Kind::default()` is `Kind::Eof`. `Kind::Eof as u8` needs to be set.
-        // Assuming `Kind::Eof` will be 1 after `#[repr(u8)]` (Undetermined = 0, Eof = 1)
-        token.set_kind(Kind::default());
-        token
+        // `Kind::default()` is `Kind::Eof`. So `0` is equivalent to:
+        // start: 0,
+        // end: 0,
+        // kind: Kind::default(),
+        // is_on_new_line: false,
+        // escaped: false,
+        // lone_surrogates: false,
+        // has_separator: false,
+        const _: () = assert!(Kind::Eof as u8 == 0);
+        Self(0)
     }
 }
 
@@ -57,8 +63,8 @@ impl Token {
 
     #[inline]
     pub fn kind(&self) -> Kind {
-        // SAFETY: `Kind` is `#[repr(u8)]`. Only `Token::set_kind` alters these bits,
-        // and it sets them to the `u8` value of an existing `Kind`.
+        // SAFETY: `Kind` is `#[repr(u8)]`. Only `Token::default` and `Token::set_kind` set these bits,
+        // and they set them to the `u8` value of an existing `Kind`.
         // So transmuting these bits back to `Kind` must produce a valid `Kind`.
         unsafe { mem::transmute::<u8, Kind>(((self.0 >> KIND_SHIFT) & KIND_MASK) as u8) }
     }