BE-262: HashQL: Do not intern empty slices

[indietyp]

🌟 What is the purpose of this PR?

Add Interned::empty() for zero-length slices and use IdProducer in InternMap

The previous version of this PR was about switching the allocator for interning, I found that there was no benefit to do so, hence another optimization: canonical empty slices (something that needed to be done either way for correctness) has been implemented. Giving a ~5-12% boost in places that mostly deal with empty collections, such as MIR passes.

🔍 What does this change?

• Adds Interned::empty() method that returns a canonical empty interned slice
• Replaces AtomicU32 with IdProducer<T::Id> in InternMap for ID generation
• Initializes LocalLock instances with fast_hash_map() instead of using default()
• Optimizes intern_slice to return the canonical empty slice for empty inputs
• Adds a test for the stable empty slice functionality
• Updates the test:miri script to include the new stable_empty_slice test

Pre-Merge Checklist 🚀

🚢 Has this modified a publishable library?

This PR:

• does not modify any publishable blocks or libraries, or modifications do not need publishing

📜 Does this require a change to the docs?

The changes in this PR:

• are internal and do not require a docs change

🕸️ Does this require a change to the Turbo Graph?

The changes in this PR:

• do not affect the execution graph

🛡 What tests cover this?

• Added a new test stable_empty_slice to verify that empty slices share the same address
• Existing tests cover the IdProducer changes

❓ How to test this?

1. Run cargo test to verify all tests pass
2. Run cargo miri nextest run -- stable_empty_slice to verify the new empty slice functionality
3. Verify that interning empty slices returns the same reference regardless of element type

[cursor]

PR Summary

Introduces canonical empty interned slices and simplifies ID generation and map initialization.

• Adds Interned::empty() for slices and updates InternSet::intern_slice to return the canonical empty slice for empty inputs
• Replaces AtomicU32 with IdProducer<T::Id> in InternMap and updates next_id() accordingly
• Initializes LocalLock maps with fast_hash_map() in InternMap and InternSet
• Adds stable_empty_slice test and includes it in the test:miri script

^{Written by Cursor Bugbot for commit 752a68c. This will update automatically on new commits. Configure here.}

[indietyp]

• BE-273: HashQL: Profile and benchmark MIR interpreter #8251
• BE-272: HashQL: Add numeric addition and subtraction operators to MIR #8250
• BE-257: HashQL: Implement MIR interpreter with runtime value representation #8245
• BE-271: HashQL: Implement PostInline pass #8240
• BE-269: HashQL: Move PreInline out into Canonicalization pass #8239
• BE-268: HashQL: Rename PreInlining to PreInline for consistency #8238
• BE-270: HashQL: Optimize empty tuples to unit constants in MIR #8237
• BE-197: Implement MIR inline pass for HashQL #8236
• BE-267: Use bump scopes instead of full resets across transformation passes #8234
• BE-266: HashQL: Implement PreInlining pass for MIR optimization #8233
• BE-265: HashQL: Make CopyPropagation aware of local re-assignments #8231
• BE-264: HashQL: Rename SROA to ForwardSubstitution for better semantic clarity #8230
• BE-263: HashQL: add body! macro for declarative MIR construction in tests #8229
• BE-261: HashQL: Implement Administrative Reduction pass for MIR #8227
• BE-262: HashQL: Do not intern empty slices #8223 👈 (View in Graphite)
• BE-260: HashQL: Switch to bump allocator with scoping support #8218
• BE-259: HashQL: Add benchmarks for type system operations #8216
• BE-227: HashQL: Implement call graph analysis for MIR #8214
• BE-258: HashQL: Add Changed enum to track MIR transformation pass modifications #8210
• BE-255: HashQL: Implement Copy Propagation (CP) pass for MIR #8207
• BE-231: HashQL: Add InstSimplify pass for MIR instruction simplification and constant folding #8206
• BE-254: Add CodSpeed benchmarking for HashQL MIR transformation passes #8204
• main

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.36%. Comparing base (bf829ac) to head (752a68c).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8223      +/-   ##
==========================================
+ Coverage   59.24%   59.36%   +0.12%     
==========================================
  Files        1191     1195       +4     
  Lines      113363   113729     +366     
  Branches     4981     4985       +4     
==========================================
+ Hits        67159    67520     +361     
- Misses      45428    45433       +5     
  Partials      776      776              

Flag	Coverage Δ
rust.hash-graph-api	2.89% <ø> (ø)
rust.hashql-compiletest	46.65% <ø> (ø)

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[codspeed-hq]

Merging this PR will improve performance by 13.31%

⚡ 1 improved benchmark
✅ 13 untouched benchmarks
🗄️ 15 archived benchmarks run¹

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	diamond	7.2 µs	6.4 µs	+13.31%

---

_{Comparing bm/be-262-hashql-use-heap-to-store-interners (752a68c) with bm/be-260-hashql-switch-internal-allocator-implementation-to-scoping (82890e0)²}

Footnotes

1. 15 benchmarks were run, but are now archived. If they were deleted in another branch, consider rebasing to remove them from the report. Instead if they were added back, click here to restore them. ↩

2. No successful run was found on bm/be-260-hashql-switch-internal-allocator-implementation-to-scoping (226e70c) during the generation of this report, so 5e19b4d was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

[augmentcode]

🤖 Augment PR Summary

Summary: Refactors HashQL interning collections to allocate their backing hash tables in the arena Heap and to centralize sequential ID generation.

Changes:

• InternMap: replace the manual AtomicU32 counter with IdProducer<T::Id> and generate IDs via IdProducer::next()
• InternMap: build inner/lookup via fast_hash_map_in / fast_hash_map_with_capacity_in, and carry &Heap as the map allocator parameter
• InternSet: allocate the internal map via the same *_in helpers and update the map type to include the allocator parameter

Technical Notes: This keeps hash map bucket allocations scoped to the Heap lifetime and removes duplicated atomic counter logic in favor of the shared ID utility.

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[augmentcode]

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