perf(parser): store Modifiers on stack

[overlookmotel]

Modifiers represents a set of 0 or more modifier keywords (public, private, static etc), up to a maximum of 15 possible modifiers.

Previously it was stored as:

pub struct Modifiers<'a> {
    /// Set of `Modifier`s, each containing `Span` of modifier keyword + modifier kind.
    modifiers: Option<Vec<'a, Modifier>>,
    /// Bitfield of which modifiers are present.
    kinds: ModifierKinds,
}

This is inefficient in a few ways:

1. Modifiers is not part of the AST, only temporary data used during parsing. Using a Vec<'a, Modifier> results in this temporary data taking up space in the AST arena, which hangs around unused for the whole life of the AST.
2. modifiers Vec can contains duplicates. This is pointless because using the same modifier twice is always a syntax error, and this is already handled by check_modifier.
3. Modifier contains the Span of the modifier keyword. The length of modifier keywords is statically known from their kind (e.g. public is always 6 bytes). Storing just the start offsets of keywords is sufficient.

This PR switches to a more efficient representation:

pub struct Modifiers {
    /// Start offset for each modifier, indexed by `ModifierKind` discriminant.
    offsets: [MaybeUninit<u32>; 15],
    /// Bitfield of which modifiers are present.
    kinds: ModifierKinds,
}

This type contains all its data inline, and can be stored on the stack (64 bytes) - versus previous 32 bytes on stack + 16 bytes for each modifier in arena. Despite the larger footprint on stack, the use of MaybeUninit means that constructing an empty Modifiers requires less memory ops:

• Now: 2-byte write to set kinds = ModifierKinds::none().
• Previously: 2-byte write for kinds + 8-byte write to set modifiers = None.

Offsets are only written to offsets when a modifier is added. The kinds bitfield indicates which entries in offsets are present and initialized. All methods check kinds before accessing offsets, to avoid reading uninitialized memory (which would be UB).

Small positive impact on some benchmarks (+0.2%). To be honest, it's more of a "the principle of the matter" thing - temporary data should not be stored in the arena. This stack was also an experiment in getting Claude to create a stack of PRs unattended from an initial step-by-step plan.

[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
• 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 👈 (View in Graphite)
• 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

✅ 53 untouched benchmarks
⏩ 3 skipped benchmarks¹

---

_{Comparing om/03-23-perf_parser_store_modifiers_on_stack (4fc398f) with om/03-23-perf_parser_add_modifiers_get_method (126134b)}

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

Refactors Modifiers (temporary parser-only state for TS/JS modifier keywords) to store modifier presence and keyword start offsets inline on the stack, avoiding arena allocations and duplicate storage while reconstructing Spans on demand.

Changes:

• Replaces Modifiers<'a>’s Option<Vec<'a, Modifier>> storage with a fixed-size [MaybeUninit<u32>; 15] offsets array plus a ModifierKinds bitfield.
• Updates parser call sites to use the new Modifiers API (no lifetime, new_single, updated iter/get semantics).
• Adjusts invalid-modifier reporting to preserve source-order diagnostics while iterating modifiers in discriminant order.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tasks/track_memory_allocations/allocs_parser.snap	Updates allocation snapshot numbers consistent with reduced arena usage.
crates/oxc_parser/src/modifiers.rs	Implements new stack-based Modifiers representation and updates modifier parsing/validation logic.
crates/oxc_parser/src/js/statement.rs	Switches const enum modifier construction to Modifiers::new_single.
crates/oxc_parser/src/js/module.rs	Switches export default abstract class modifier construction to Modifiers::new_single.
crates/oxc_parser/src/js/function.rs	Updates function parsing signatures to accept &Modifiers (no lifetime).
crates/oxc_parser/src/js/object.rs	Updates method-getter/setter parsing signature to accept &Modifiers (no lifetime).
crates/oxc_parser/src/js/class.rs	Updates class parsing signatures/usages to accept &Modifiers (no lifetime).
crates/oxc_parser/src/ts/statement.rs	Updates TS declaration parsing signatures to accept &Modifiers (no lifetime).
crates/oxc_parser/src/ts/types.rs	Updates TS signature parsing signatures to accept &Modifiers (no lifetime).

[graphite-app]

Merge activity

• Mar 26, 1:12 PM UTC: camc314 added this pull request to the Graphite merge queue.
• Mar 26, 1:13 PM UTC: The Graphite merge queue removed this pull request due to downstack failures on PR #20739.
• Mar 26, 1:13 PM UTC: The Graphite merge queue removed this pull request due to downstack failures on PR #20739.
• Mar 26, 1:29 PM UTC: camc314 added this pull request to the Graphite merge queue.
• Mar 26, 1:33 PM UTC: The Graphite merge queue removed this pull request due to downstack failures on PR #20740.
• Mar 26, 1:33 PM UTC: The Graphite merge queue removed this pull request due to downstack failures on PR #20740.
• Mar 26, 1:34 PM UTC: The merge label '0-merge' was removed. This PR will no longer be merged by the Graphite merge queue
• Mar 26, 1:37 PM UTC: camc314 added this pull request to the Graphite merge queue.
• Mar 26, 1:50 PM UTC: Merged by the Graphite merge queue.