perf(parser): remove lexer lookahead

crates/oxc_parser/src/cursor.rs

@@ -61,48 +61,6 @@ impl<'a> ParserImpl<'a> {
         self.lexer.get_template_string(self.token.start())
     }
 
-    /// Peek next token, returns EOF for final peek
-    #[inline]
-    pub(crate) fn peek_token(&mut self) -> Token {
-        self.lexer.lookahead(1)
-    }
-
-    /// Peek next kind, returns EOF for final peek
-    #[inline]
-    #[expect(dead_code)]
-    pub(crate) fn peek_kind(&mut self) -> Kind {
-        self.peek_token().kind()
-    }
-
-    /// Peek at kind
-    #[inline]
-    pub(crate) fn peek_at(&mut self, kind: Kind) -> bool {
-        self.peek_token().kind() == kind
-    }
-
-    /// Peek nth token
-    #[inline]
-    pub(crate) fn nth(&mut self, n: u8) -> Token {
-        if n == 0 {
-            return self.cur_token();
-        }
-        self.lexer.lookahead(n)
-    }
-
-    /// Peek at nth kind
-    #[inline]
-    #[expect(dead_code)]
-    pub(crate) fn nth_at(&mut self, n: u8, kind: Kind) -> bool {
-        self.nth(n).kind() == kind
-    }
-
-    /// Peek nth kind
-    #[inline]
-    #[expect(dead_code)]
-    pub(crate) fn nth_kind(&mut self, n: u8) -> Kind {
-        self.nth(n).kind()
-    }
-
     /// Checks if the current index has token `Kind`
     #[inline]
     pub(crate) fn at(&self, kind: Kind) -> bool {
@@ -384,7 +342,13 @@ impl<'a> ParserImpl<'a> {
             if first {
                 first = false;
             } else {
-                if !trailing_separator && self.at(separator) && self.peek_at(close) {
+                if !trailing_separator
+                    && self.at(separator)
+                    && self.lookahead(|p| {
+                        p.bump_any();
+                        p.at(close)
+                    })
+                {
                     break;
                 }
                 self.expect(separator);

crates/oxc_parser/src/lexer/jsx.rs

@@ -116,9 +116,6 @@ impl Lexer<'_> {
         }
         self.consume_char();
 
-        // Clear the current lookahead `Minus` Token
-        self.lookahead.clear();
-
         // Consume bytes which are part of identifier tail
         let next_byte = byte_search! {
             lexer: self,

crates/oxc_parser/src/lexer/mod.rs

@@ -5,8 +5,6 @@
 //!     * [rustc](https://github.com/rust-lang/rust/blob/1.82.0/compiler/rustc_lexer/src)
 //!     * [v8](https://v8.dev/blog/scanner)
 
-use std::collections::VecDeque;
-
 use rustc_hash::FxHashMap;
 
 use oxc_allocator::Allocator;
@@ -59,12 +57,6 @@ pub enum LexerContext {
     JsxAttributeValue,
 }
 
-#[derive(Debug, Clone, Copy)]
-struct Lookahead<'a> {
-    position: SourcePosition<'a>,
-    token: Token,
-}
-
 pub struct Lexer<'a> {
     allocator: &'a Allocator,
 
@@ -77,8 +69,6 @@ pub struct Lexer<'a> {
 
     pub(crate) errors: Vec<OxcDiagnostic>,
 
-    lookahead: VecDeque<Lookahead<'a>>,
-
     context: LexerContext,
 
     pub(crate) trivia_builder: TriviaBuilder,
@@ -115,7 +105,6 @@ impl<'a> Lexer<'a> {
             source_type,
             token,
             errors: vec![],
-            lookahead: VecDeque::with_capacity(4), // 4 is the maximum lookahead for TypeScript
             context: LexerContext::Regular,
             trivia_builder: TriviaBuilder::default(),
             escaped_strings: FxHashMap::default(),
@@ -163,40 +152,6 @@ impl<'a> Lexer<'a> {
         self.errors.truncate(checkpoint.errors_pos);
         self.source.set_position(checkpoint.position);
         self.token = checkpoint.token;
-        self.lookahead.clear();
-    }
-
-    /// Find the nth lookahead token lazily
-    pub fn lookahead(&mut self, n: u8) -> Token {
-        let n = n as usize;
-        debug_assert!(n > 0);
-
-        if let Some(lookahead) = self.lookahead.get(n - 1) {
-            return lookahead.token;
-        }
-
-        let position = self.source.position();
-
-        if let Some(lookahead) = self.lookahead.back() {
-            self.source.set_position(lookahead.position);
-        }
-
-        for _i in self.lookahead.len()..n {
-            let kind = self.read_next_token();
-            let peeked = self.finish_next(kind);
-            self.lookahead.push_back(Lookahead { position: self.source.position(), token: peeked });
-        }
-
-        // Call to `finish_next` in loop above leaves `self.token = Token::default()`.
-        // Only circumstance in which `self.token` wouldn't have been default at start of this
-        // function is if we were at very start of file, before any tokens have been read, when
-        // `token.is_on_new_line` is `true`. But `lookahead` isn't called before the first token is
-        // read, so that's not possible. So no need to restore `self.token` here.
-        // It's already in same state as it was at start of this function.
-
-        self.source.set_position(position);
-
-        self.lookahead[n - 1].token
     }
 
     /// Set context
@@ -206,10 +161,6 @@ impl<'a> Lexer<'a> {
 
     /// Main entry point
     pub fn next_token(&mut self) -> Token {
-        if let Some(lookahead) = self.lookahead.pop_front() {
-            self.source.set_position(lookahead.position);
-            return lookahead.token;
-        }
         let kind = self.read_next_token();
         self.finish_next(kind)
     }

crates/oxc_parser/src/lexer/punctuation.rs

@@ -61,9 +61,6 @@ impl Lexer<'_> {
     pub(crate) fn re_lex_right_angle(&mut self) -> Token {
         self.token.set_start(self.offset());
         let kind = self.read_right_angle();
-        if kind != Kind::RAngle {
-            self.lookahead.clear();
-        }
         self.finish_next(kind)
     }
 

crates/oxc_parser/src/lexer/regex.rs

@@ -21,7 +21,6 @@ impl Lexer<'_> {
                 },
         );
         let (pattern_end, flags, flags_error) = self.read_regex();
-        self.lookahead.clear();
         let token = self.finish_next(Kind::RegExp);
         (token, pattern_end, flags, flags_error)
     }

crates/oxc_parser/src/lexer/template.rs

@@ -389,7 +389,6 @@ impl<'a> Lexer<'a> {
     pub(crate) fn next_template_substitution_tail(&mut self) -> Token {
         self.token.set_start(self.offset() - 1);
         let kind = self.read_template_literal(Kind::TemplateMiddle, Kind::TemplateTail);
-        self.lookahead.clear();
         self.finish_next(kind)
     }
 

crates/oxc_parser/src/lexer/typescript.rs

@@ -11,7 +11,6 @@ impl Lexer<'_> {
         self.token.set_start(self.offset() - offset);
         self.source.back(offset as usize - 1);
         let kind = Kind::LAngle;
-        self.lookahead.clear();
         self.finish_next(kind)
     }
 
@@ -25,7 +24,6 @@ impl Lexer<'_> {
         self.token.set_start(self.offset() - offset);
         self.source.back(offset as usize - 1);
         let kind = Kind::RAngle;
-        self.lookahead.clear();
         self.finish_next(kind)
     }
 }