perf(parser): skip checkpoint for infer T extends U constraint in disallow context

[Boshen]

What

parse_constraint_of_infer_type always took a parser-level checkpoint() to speculatively parse the extends U constraint of an infer T extends U type, rewinding when a trailing ? meant the extends actually belonged to an enclosing conditional type (infer T extends U ? A : B).

Why the checkpoint was usually wasted

infer normally appears in a conditional type's extends clause, which is parsed under DisallowConditionalTypes (see parse_ts_type). In that context a trailing ? cannot reinterpret the extends, so the old post-parse check short-circuited to Some(constraint) through the ||:

if self.ctx.has_disallow_conditional_types() || !self.at(Kind::Question) {
    Some(constraint)   // <- always taken in the common case
} else {
    self.rewind(checkpoint);
    None
}

So the checkpoint was taken on every infer T extends U but never actually used.

Change

Guard on has_disallow_conditional_types() up front and parse the constraint directly when it is set, taking the checkpoint only on the genuinely-speculative path — the rare case where infer is parsed in a conditional-allowed position and a trailing ? reinterprets the extends.

Behavior-preserving:

• cargo coverage -- parser — test262 100%, snapshots unchanged.

🤖 Generated with Claude Code

[Boshen]

Merge activity

• Jun 8, 5:47 PM UTC: The merge label '0-merge' was detected. This PR will be added to the Graphite merge queue once it meets the requirements.
• Jun 8, 5:47 PM UTC: Boshen added this pull request to the Graphite merge queue.
• Jun 8, 5:53 PM UTC: Merged by the Graphite merge queue.

[codspeed-hq]

Merging this PR will not alter performance

✅ 57 untouched benchmarks
⏩ 14 skipped benchmarks¹

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_{Comparing perf/infer-constraint-skip-checkpoint (c5fc3bb) with main (7464dce)}

Footnotes

1. 14 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. ↩