perf(lexer): only check for hashbang at start of file

[overlookmotel]

Small optimization to lexer. A hashbang can only appear at very start of file, so only check for hashbang when getting first token. This streamlines the byte handler for #, because a # anywhere else can only be a private identifier.

Note: self.token.set_is_on_new_line(true); in read_hashbang_comment is not required, because it's always true already.

[overlookmotel]

• perf(lexer): only check for hashbang at start of file #12521 👈 (View in Graphite)
• main

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[codspeed-hq]

CodSpeed Instrumentation Performance Report

Merging #12521 will improve performances by 8.47%

_{Comparing 07-25-perf_lexer_only_check_for_hashbang_at_start_of_file (47a565f) with main (c72f49e)}

Summary

⚡ 4 improvements
✅ 30 untouched benchmarks

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
⚡	lexer[RadixUIAdoptionSection.jsx]	20.8 µs	20 µs	+3.91%
⚡	lexer[binder.ts]	930.2 µs	870.5 µs	+6.85%
⚡	lexer[cal.com.tsx]	5.8 ms	5.3 ms	+8.47%
⚡	lexer[react.development.js]	384 µs	357.1 µs	+7.54%

[camc314]

nice 💪

[overlookmotel]

Hmm. I'm not sure that benchmark is right. First version made no difference at all. I may have made a mistake.

[Copilot]

Pull Request Overview

This PR optimizes the lexer by restricting hashbang comment detection to only the first token of a file. Since hashbang comments can only appear at the very start of a file, this eliminates unnecessary checks for every # character encountered during lexing.

• Adds a dedicated first_token() method that specifically checks for hashbang comments
• Simplifies the # byte handler to only handle private identifiers
• Updates parser and benchmark code to use the new first_token() method

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tasks/benchmark/benches/lexer.rs	Updates benchmark to use first_token() method and adds EOF check
crates/oxc_parser/src/lib.rs	Replaces bump_any() with first_token() call in parser initialization
crates/oxc_parser/src/lexer/mod.rs	Adds new first_token() method and inlines read_next_token()
crates/oxc_parser/src/lexer/comment.rs	Makes read_hashbang_comment() unsafe and removes unnecessary line setting
crates/oxc_parser/src/lexer/byte_handlers.rs	Simplifies # handler to only process private identifiers

[Boshen]

Merge activity

• Aug 10, 7:32 AM UTC: The merge label '0-merge' was detected. This PR will be added to the Graphite merge queue once it meets the requirements.
• Aug 10, 7:35 AM UTC: Boshen added this pull request to the Graphite merge queue.
• Aug 10, 7:42 AM UTC: Merged by the Graphite merge queue.