fix(engine): preserve map and set iterator indices after clear (#5523)

[MayankSharma-2812]

This Pull Request fixes/closes #5523.

Background

Per ECMAScript specifications (§24.1.3.1 Map.prototype.clear and §24.2.3.2 Set.prototype.clear), calling .clear() on a Map or Set does not reset or shift the underlying entry sequence when an iterator or forEach loop is actively traversing the collection. Instead, existing entries are marked as empty. Any entries subsequently inserted into the collection must retain index positions after the cleared entries so that in-flight iterators can observe them.

Previously, OrderedMap::clear() and OrderedSet::clear() unconditionally called .clear() on the backing IndexMap / IndexSet. When subsequent insertions were performed, they were placed at index 0. Active iterators (whose next_index >= 1) skipped them and terminated prematurely with { value: undefined, done: true }.

Changes

• OrderedMap::clear(): When self.lock > 0 (indicating an active iterator or loop), preserve index offsets by populating existing slots with MapKey::Empty(i) tombstones instead of dropping the backing map. If self.lock == 0, standard .clear() is retained.
• OrderedSet::clear(): Mirrored the tombstone preservation pattern for OrderedSet when self.lock > 0.
• OrderedSet::is_empty(): Fixed a latent bug where is_empty() checked self.inner.len() == 0 instead of self.len() == 0, which caused false negatives when tombstones were present.
• Unit Tests:
  • Added regression test iterator_after_clear_and_set in core/engine/src/builtins/map/tests.rs.
  • Added regression test set_iterator_after_clear_and_add in core/engine/src/builtins/set/tests.rs.

Verification

• cargo test -p boa_engine iterator_after_clear passes.
• All 28 existing tests in builtins::map::tests and all 30 tests in builtins::set::tests pass.
• cargo clippy -p boa_engine -- -D warnings passed with 0 warnings.
• cargo fmt -- --check passed cleanly.

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	53,578	53,578	0
Passed	51,432	51,432	0
Ignored	1,648	1,648	0
Failed	498	498	0
Panics	0	0	0
Conformance	95.99%	95.99%	0.00%

Tested main commit: 69388e59f789ed0846a8d6aad1e4dc0c91b35816
Tested PR commit: 746a1fb0756b025019fe9499d12c12b15d6de182
Compare commits: 69388e5...746a1fb

[codecov]

Codecov Report

❌ Patch coverage is 94.73684% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 62.82%. Comparing base (6ddc2b4) to head (746a1fb).
⚠️ Report is 1054 commits behind head on main.

Files with missing lines	Patch %	Lines
core/engine/src/builtins/set/ordered_set.rs	90.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #5528       +/-   ##
===========================================
+ Coverage   47.24%   62.82%   +15.58%     
===========================================
  Files         476      536       +60     
  Lines       46892    60298    +13406     
===========================================
+ Hits        22154    37883    +15729     
+ Misses      24738    22415     -2323     

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