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refactor(parser): remove lookahead usage in parsing arrow function expressions #11220

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Merged
graphite-app merged 1 commit into from
May 22, 2025
Merged

refactor(parser): remove lookahead usage in parsing arrow function expressions #11220

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123 changes: 65 additions & 58 deletions crates/oxc_parser/src/js/arrow.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -61,22 +61,19 @@ impl<'a> ParserImpl<'a> {
}

fn is_parenthesized_arrow_function_expression_worker(&mut self) -> Tristate {
let mut offset = 0;

if self.at(Kind::Async) {
let second_token = self.peek_token();
let second = second_token.kind();
if second_token.is_on_new_line() {
self.bump(Kind::Async);
if self.cur_token().is_on_new_line() {
return Tristate::False;
}
if second != Kind::LParen && second != Kind::LAngle {
if !self.at(Kind::LParen) && !self.at(Kind::LAngle) {
return Tristate::False;
}
offset = 1;
}

let first = self.nth_kind(offset);
let second = self.nth_kind(offset + 1);
let first = self.cur_kind();
self.bump_any();
let second = self.cur_kind();

match first {
Kind::LParen => {
Expand All @@ -86,73 +83,80 @@ impl<'a> ParserImpl<'a> {
// The last one is not actually an arrow function,
// but this is probably what the user intended.
Kind::RParen => {
let third = self.nth_kind(offset + 2);
return match third {
self.bump_any();
let third = self.cur_kind();
match third {
Kind::Colon if self.is_ts => Tristate::Maybe,
Kind::Arrow | Kind::LCurly => Tristate::True,
_ => Tristate::False,
};
}
}
// If encounter "([" or "({", this could be the start of a binding pattern.
// Examples:
// ([ x ]) => { }
// ({ x }) => { }
// ([ x ])
// ({ x })
Kind::LBrack | Kind::LCurly => {
return Tristate::Maybe;
}
Kind::LBrack | Kind::LCurly => Tristate::Maybe,
// Simple case: "(..."
// This is an arrow function with a rest parameter.
Kind::Dot3 => {
return match self.nth_kind(offset + 1) {
self.bump_any();
let third = self.cur_kind();
match third {
// '(...ident' is a lambda
Kind::Ident => Tristate::True,
// '(...null' is not a lambda
kind if kind.is_literal() => Tristate::False,
_ => Tristate::Maybe,
};
}
_ => {}
}
let third = self.nth_kind(offset + 2);

// Check for "(xxx yyy", where xxx is a modifier and yyy is an identifier. This
// isn't actually allowed, but we want to treat it as a lambda so we can provide
// a good error message.
if second.is_modifier_kind()
&& second != Kind::Async
&& third.is_binding_identifier()
{
if third == Kind::As {
return Tristate::False; // https://github.com/microsoft/TypeScript/issues/44466
}
}
return Tristate::True;
}
_ => {
self.bump_any();
let third = self.cur_kind();

// Check for "(xxx yyy", where xxx is a modifier and yyy is an identifier. This
// isn't actually allowed, but we want to treat it as a lambda so we can provide
// a good error message.
if second.is_modifier_kind()
&& second != Kind::Async
&& third.is_binding_identifier()
{
if third == Kind::As {
return Tristate::False; // https://github.com/microsoft/TypeScript/issues/44466
}
return Tristate::True;
}

// If we had "(" followed by something that's not an identifier,
// then this definitely doesn't look like a lambda. "this" is not
// valid, but we want to parse it and then give a semantic error.
if !second.is_binding_identifier() && second != Kind::This {
return Tristate::False;
}
// If we had "(" followed by something that's not an identifier,
// then this definitely doesn't look like a lambda. "this" is not
// valid, but we want to parse it and then give a semantic error.
if !second.is_binding_identifier() && second != Kind::This {
return Tristate::False;
}

match third {
// If we have something like "(a:", then we must have a
// type-annotated parameter in an arrow function expression.
Kind::Colon => Tristate::True,
// If we have "(a?:" or "(a?," or "(a?=" or "(a?)" then it is definitely a lambda.
Kind::Question => {
let fourth = self.nth_kind(offset + 3);
if matches!(fourth, Kind::Colon | Kind::Comma | Kind::Eq | Kind::RParen) {
return Tristate::True;
match third {
// If we have something like "(a:", then we must have a
// type-annotated parameter in an arrow function expression.
Kind::Colon => Tristate::True,
// If we have "(a?:" or "(a?," or "(a?=" or "(a?)" then it is definitely a lambda.
Kind::Question => {
self.bump_any();
let fourth = self.cur_kind();
if matches!(
fourth,
Kind::Colon | Kind::Comma | Kind::Eq | Kind::RParen
) {
return Tristate::True;
}
Tristate::False
}
// If we have "(a," or "(a=" or "(a)" this *could* be an arrow function
Kind::Comma | Kind::Eq | Kind::RParen => Tristate::Maybe,
// It is definitely not an arrow function
_ => Tristate::False,
}
Tristate::False
}
// If we have "(a," or "(a=" or "(a)" this *could* be an arrow function
Kind::Comma | Kind::Eq | Kind::RParen => Tristate::Maybe,
// It is definitely not an arrow function
_ => Tristate::False,
}
}
Kind::LAngle => {
Expand All @@ -166,13 +170,16 @@ impl<'a> ParserImpl<'a> {
if self.source_type.is_jsx() {
// <const Ident extends Ident>
// ^^^^^ Optional
offset += if second == Kind::Const { 3 } else { 2 };
return match self.nth_kind(offset) {
self.bump(Kind::Const);
self.bump_any();
let third = self.cur_kind();
return match third {
Kind::Extends => {
let third = self.nth_kind(offset + 1);
if matches!(third, Kind::Eq | Kind::RAngle | Kind::Slash) {
self.bump_any();
let fourth = self.cur_kind();
if matches!(fourth, Kind::Eq | Kind::RAngle | Kind::Slash) {
Tristate::False
} else if third.is_binding_identifier() {
} else if fourth.is_binding_identifier() {
Tristate::Maybe
} else {
Tristate::True
Expand Down
7 changes: 4 additions & 3 deletions tasks/coverage/snapshots/parser_babel.snap
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Original file line number Diff line number Diff line change
Expand Up @@ -8912,10 +8912,11 @@ Expect to Parse: tasks/coverage/babel/packages/babel-parser/test/fixtures/typesc
· ───
╰────

× Unexpected token
╭─[babel/packages/babel-parser/test/fixtures/esprima/es2015-arrow-function/rest-without-arrow/input.js:1:2]
× Expected `=>` but found `+`
╭─[babel/packages/babel-parser/test/fixtures/esprima/es2015-arrow-function/rest-without-arrow/input.js:1:8]
1 │ (...a) + 1
· ───
· ┬
· ╰── `=>` expected
╰────

× Cannot assign to 'eval' in strict mode
Expand Down
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