fix(parser): correct precedence handling for private-in expression

[Boshen]

Summary

• Fix incorrect parsing of 1 + #a in b which was parsed as 1 + (#a in b) instead of (1 + #a) in b
• Add precedence check before parsing #a in as PrivateInExpression
• Report "Unexpected private identifier" error when precedence doesn't allow in operator

Details

The parser was greedily parsing #a in as a PrivateInExpression without checking operator precedence. For example, when parsing the RHS of + in 1 + #a in b, the in operator's precedence (Compare = 13) is lower than +'s precedence (Add = 15), so #a in should not be parsed as a unit.

The fix extracts private-in parsing into parse_private_in_expression and adds a check: if lhs_precedence >= Precedence::Compare, we reject the private identifier with a syntax error, matching V8 and Babel behavior.

Test plan

• cargo test -p oxc_parser passes
• Conformance improved: Negative Passed 1648 → 1649 for Babel tests
• invalid-private-followed-by-in-2 test case now correctly reports syntax error

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR fixes incorrect parsing of private-in expressions by adding proper precedence checking. Previously, 1 + #a in b was incorrectly parsed as 1 + (#a in b) instead of the correct (1 + #a) in b.

Changes:

• Extracts private-in parsing logic into a new parse_private_in_expression function with precedence validation
• Adds precedence check to reject private identifiers when the in operator's precedence is too low for the current context
• Introduces new unexpected_private_identifier diagnostic for more precise error reporting

Reviewed changes

Copilot reviewed 2 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
crates/oxc_parser/src/js/expression.rs	Adds parse_private_in_expression function with precedence check and refactors existing inline logic to use it
crates/oxc_parser/src/diagnostics.rs	Adds new unexpected_private_identifier error diagnostic
tasks/coverage/snapshots/parser_babel.snap	Shows improved test conformance (1648→1649 negative tests passed) and more precise error messages
tasks/coverage/snapshots/parser_test262.snap	Updates error messages to be more precise, pointing to specific private identifier issues
tasks/coverage/snapshots/parser_typescript.snap	Correctly identifies precedence-related syntax errors in TypeScript test cases

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[codspeed-hq]

Merging this PR will not alter performance

✅ 42 untouched benchmarks
⏩ 3 skipped benchmarks¹

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_{Comparing fix/parser-private-in-precedence (527fc94) with main (38e4b53)}

Footnotes

1. 3 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[Boshen]

Merge activity

• Jan 18, 7:46 AM UTC: The merge label '0-merge' was detected. This PR will be added to the Graphite merge queue once it meets the requirements.
• Jan 18, 7:46 AM UTC: Boshen added this pull request to the Graphite merge queue.
• Jan 18, 7:57 AM UTC: Merged by the Graphite merge queue.