perf(parser): fast path for no syntax errors when checking modifiers

[overlookmotel]

check_modifier contained a large chunk of code with nested matches - lots of branches, all to handle cases which should never happen (syntax errors).

Instead, do a fast check for validity using only a couple of operations and a small lookup table. Only if that fast check fails, hand over to a 2nd function illegal_modifier_error which does the slow work of figuring out what error to raise and raising it. In normal operation, that path should never be taken, so mark it #[cold] and #[inline(never)].

This also replaces an unpredictable branch with a very predictable one.

Small perf improvement on some parser benchmarks (+0.2%).

[overlookmotel]

• perf(parser): defer calculation of Modifier spans #20823 : 2 dependent PRs (#20824 , #20826 )
• perf(parser): remove branch from parse_modifiers #20821
• perf(parser): reduce Kind to ModifierKind conversions in eat_modifiers_before_declaration #20820
• refactor(parser): shorten code handling illegal modifiers #20781
• perf(parser): fast path for no syntax errors when checking modifiers #20748 👈 (View in Graphite)
• perf(parser): remove a lookahead from eat_modifiers_before_declaration #20755
• refactor(parser): nest modifiers types in modules #20754
• refactor(parser): re-order modifiers code #20753
• perf(parser): store Modifiers on stack #20742
• perf(parser): add Modifiers::get method #20741
• refactor(parser): rename ModifierKinds::empty to none #20740
• refactor(parser): rename ModifierFlags to ModifierKinds #20739
• refactor(parser): simplify ModifierFlags #20738
• refactor(parser): import from std at top level #20737
• refactor(parser): remove Modifier::is_static() #20736
• refactor(parser): remove dead code from modifier parsing #20735
• main

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[codspeed-hq]

Merging this PR will not alter performance

✅ 48 untouched benchmarks
⏩ 8 skipped benchmarks¹

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_{Comparing om/03-23-perf_parser_fast_path_for_no_syntax_errors_when_checking_modifiers (1e19687) with main (1a370a6)}

Footnotes

1. 8 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

Improves parser modifier validation performance by adding a fast-path check that avoids the branch-heavy error-construction logic in the common (no syntax errors) case.

Changes:

• Refactors check_modifier to first run a fast legality check (is_illegal_modifier) and only fall back to error construction when needed.
• Adds a compile-time-generated lookup table to decide illegal modifier ordering via a single table lookup + bitset intersection.
• Adds a unit test to ensure is_illegal_modifier stays consistent with illegal_modifier_error.

[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated 1 comment.

[camc314]

Merge activity

• Mar 29, 3:28 PM UTC: The merge label '0-merge' was detected. This PR will be added to the Graphite merge queue once it meets the requirements.
• Mar 29, 3:28 PM UTC: camc314 added this pull request to the Graphite merge queue.
• Mar 29, 3:33 PM UTC: Merged by the Graphite merge queue.