refactor(deserialize): exact allocation for arrays and maps

[Conaclos]

Summary

This PR ensures that we allocate exactly the amount of memory needed for Vec when we deserialize a JSON array (assuming that every element is deserialized).
This makes sure that converting a Vec into a boxed slice doesn't re-allcoate.

I had to add impl ExactSizeIterator for iterators in biome_rowan.

• AstSeparatedListElementsIterator can implement ExactSizeIterator because it builds on SyntaxSlots that implements ExactSizeIterator and no elements from SyntaxSlots is skipped.
• AstSeparatedListNodesIterator can implement ExactSizeIterator because it builds on AstSeparatedListElementsIterator that implements ExactSizeIterator and it just maps iterated elements in its Ietartor::next function.

Note: we could certainly improve the implementation of Ietrator for AstSeparatedListElementsIterator and AstSeparatedListNodesIterator by implementing methods like last, ... This is out of scope of this PR.

I changed the signature of Deserializable::visit_array and Deserializable::visit_map to accept ExactSizeIterator instead of Iterator.
This is a backward compatible change because an implementor can still use impl Iterator (we have a few places where it is still the case).

Test Plan

CI should still be green.

Docs

No change.

[changeset-bot]

⚠️ No Changeset found

Latest commit: 2e1bdb2

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[codspeed-hq]

CodSpeed Performance Report

Merging #8040 will not alter performance

_{Comparing conaclos/deserialize-exact-allocation (2e1bdb2) with main (78011b1)}

Summary

✅ 53 untouched
⏩ 85 skipped¹

Footnotes

1. 85 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. ↩

[coderabbitai]

Walkthrough

Trait methods visit_array and visit_map in crates/biome_deserialize now accept ExactSizeIterator instead of Iterator. Implementations for Vec, smallvec::SmallVec, HashSet, BTreeSet, indexmap::IndexSet, HashMap, BTreeMap and indexmap::IndexMap were updated to match, with several constructors now pre‑allocating and extending. In crates/biome_rowan, two AST iterators gained precise size_hint implementations and ExactSizeIterator conformance.

Pre-merge checks and finishing touches

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 36.36% which is insufficient. The required threshold is 80.00%.	You can run @coderabbitai generate docstrings to improve docstring coverage.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately summarises the main change: refactoring deserialization to use exact allocation for arrays and maps via ExactSizeIterator.
Description check	✅ Passed	The description is comprehensive and directly related to the changeset, explaining the motivation, implementation details, and rationale for API signature changes.

✨ Finishing touches

• 📝 Docstrings were successfully generated.

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch conaclos/deserialize-exact-allocation

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[github-actions]

Parser conformance results on

js/262

Test result	main count	This PR count	Difference
Total	50977	50977	0
Passed	49764	49764	0
Failed	1171	1171	0
Panics	42	42	0
Coverage	97.62%	97.62%	0.00%

jsx/babel

Test result	main count	This PR count	Difference
Total	40	40	0
Passed	37	37	0
Failed	3	3	0
Panics	0	0	0
Coverage	92.50%	92.50%	0.00%

symbols/microsoft

Test result	main count	This PR count	Difference
Total	6320	6320	0
Passed	2106	2106	0
Failed	4214	4214	0
Panics	0	0	0
Coverage	33.32%	33.32%	0.00%

ts/babel

Test result	main count	This PR count	Difference
Total	835	835	0
Passed	742	742	0
Failed	93	93	0
Panics	0	0	0
Coverage	88.86%	88.86%	0.00%

ts/microsoft

Test result	main count	This PR count	Difference
Total	18813	18813	0
Passed	14065	14065	0
Failed	4747	4747	0
Panics	1	1	0
Coverage	74.76%	74.76%	0.00%

[ematipico]

We could implement #8040 (review) while we're at it

[coderabbitai]

Warning

Docstrings generation - IN PROGRESS

Generating docstrings for this pull request

[arendjr]

Had overlooked this PR, but that's some nice work!