perf(parser): remove bounds check for getting strings from lexer

[camchenry]

One profile of the parser showed that this function took up 1% of the time on binder.ts. It seemed like this is a case where we could possibly remove the bounds checks, since our tokens are guaranteed to be contained within the source string. Since this is called really often, it's worth optimizing.

+1% on all parser benchmarks:

[camchenry]

• perf(parser): remove bounds check for getting strings from lexer #16135 👈 (View in Graphite)
• feat(parser): add diagnostic for JSX identifiers with hyphens #16133
• feat(parser): add diagnostic for expected ident after optional chain #16132
• feat(parser): improve diagnostic for unexpected optional declarations #16131
• main

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[codspeed-hq]

CodSpeed Performance Report

Merging #16135 will not alter performance

_{Comparing 11-25-perf_parser_remove_bounds_check_for_getting_strings_from_lexer (45740bd) with main (17a8caa)}

Summary

✅ 42 untouched
⏩ 3 skipped¹

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. ↩

[Copilot]

Pull request overview

This PR optimizes the get_string method in the lexer by replacing safe string slicing operations with unsafe get_unchecked to eliminate bounds checking. The optimization targets a hot path that profiling showed took 1% of parsing time on binder.ts, achieving a +1% performance improvement across all parser benchmarks.

Key Changes:

• Refactored string extraction to compute adjusted start/end positions before slicing
• Replaced safe slice operations with unsafe { get_unchecked(start..end) }
• Added comprehensive debug assertions to validate safety invariants

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