fix(allocator): remove unsound Send impl and tighten Sync requirement for HashMap

[overlookmotel]

Same as #13041.

It's unsound for oxc_allocator::HashMap to be Send because if you can move a HashMap to another thread, you can call insert on it and it may allocate in the arena. This is unsound because another thread may be simultaneously be doing the same with another HashMap, and Bump is not thread-safe.

Remove the Send impl on HashMap.

However, we do want HashMap to be Sync. There's no reason why you shouldn't have 2 &HashMap references on different threads, as &HashMap doesn't allow mutating the HashMap in any way.

Tighten up the trait bounds so HashMap<K, V> is Sync only if K and V both are.

This is not yet completely sound. There are a couple of holes I'm aware of:

1. allocator method which allows obtaining a &Bump from a &HashMap.
2. clone which allocates into arena, taking only an immutable &self reference.

This PR closes those holes as much as possible without a major refactor. It's not completely sound, but it's at least now quite difficult to commit UB - it'd be unlikely to happen by accident.

So, not perfect, but at least there's now only a crack in the window, rather than the front door swinging wide open.

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The reason these 2 impls were added originally was as a quick fix to make oxc_semantic::Scoping Send and Sync. That is addressed in the next PR in this stack.

[overlookmotel]

• refactor(allocator): replace AtomicUsize with Cell<usize> in AllocationStats #13043
• refactor(semantic): implement Send and Sync for ScopingCell #13042
• fix(allocator): remove unsound Send impl and tighten Sync requirement for HashMap #13203 👈 (View in Graphite)
• fix(allocator): remove unsound Send impl and tighten Sync requirements for Vec #13041
• fix(allocator): remove Vec::bump method #13039
• fix(allocator): remove Clone impl from Vec #13040
• fix(allocator): remove unsound impl Sync for Allocator #13033
• fix(linter): prevent unsound use of Allocator across threads #13032
• main

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[codspeed-hq]

CodSpeed Instrumentation Performance Report

Merging #13203 will not alter performance

_{Comparing 08-18-fix_allocator_remove_unsound_send_impl_and_tighten_sync_requirement_for_hashmap_ (f490d27) with main (163dbea)¹}

Summary

✅ 34 untouched benchmarks

Footnotes

1. No successful run was found on 08-12-fix_allocator_remove_unsound_send_impl_and_tighten_sync_requirements_for_vec_ (0815a89) during the generation of this report, so main (163dbea) was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

[graphite-app]

Merge activity

• Aug 19, 9:35 PM UTC: overlookmotel added this pull request to the Graphite merge queue.
• Aug 19, 9:45 PM UTC: Merged by the Graphite merge queue.