BE-270: HashQL: Optimize empty tuples to unit constants in MIR

[indietyp]

🌟 What is the purpose of this PR?

This PR optimizes empty tuple handling in the MIR by simplifying empty tuple aggregates to unit constants and fixing tuple type handling in the body builder macro.

🔍 What does this change?

• Added From<!> implementation for Operand<'_> to support empty tuples
• Fixed tuple type handling in the body builder macro to properly handle empty tuples
• Added support for empty tuples in the rvalue macro
• Added optimization in the instruction simplifier to convert empty tuple aggregates to unit constants
• Added a test case to verify empty tuple simplification
• Sorted feature flags in lib.rs for better organization
• Updated test outputs to reflect the new optimizations

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 specific test case empty_tuple_to_unit() to verify the optimization
• Existing test outputs have been updated to reflect the changes

❓ How to test this?

1. Run the MIR tests to verify that empty tuples are properly optimized
2. Check that the updated test outputs match the expected behavior

[cursor]

PR Summary

Introduces canonical handling of empty tuples as unit across MIR construction and optimization.

• Add From<!> for Operand to enable zero-arg tuple paths
• Extend body! type macro to support () and non-empty tuple arity distinctly
• Extend rvalue! to build empty tuples (tuple) and update RValueBuilder::tuple
• InstSimplify: simplify empty tuple aggregates (AggregateKind::Tuple with no operands) to Constant::Unit
• New test empty_tuple_to_unit(); update inline/pre-inlining snapshot outputs to use () directly in closures/applies
• Tidy lib.rs feature flags ordering

^{Written by Cursor Bugbot for commit a01405e. This will update automatically on new commits. Configure here.}

[indietyp]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• BE-254: 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 👈 (View in Graphite)
• 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
• 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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[augmentcode]

🤖 Augment PR Summary

Summary: Canonicalizes empty tuple construction in HashQL MIR and optimizes it into unit (()) constants to reduce redundant aggregates/locals.

Changes:

• Add `From` for `Operand` so empty-array tuple builders can type-check via `Into<Operand>`.
• Fix `body!` type macro to handle `()` explicitly and require 1+ elements for non-empty tuple types.
• Extend `rvalue!` with a `tuple;` form to build empty tuple aggregates.
• InstSimplify: rewrite empty tuple aggregates (`AggregateKind::Tuple` with no operands) to `Constant::Unit` loads.
• Add `empty_tuple_to_unit()` coverage and update MIR UI snapshots to reflect the simplified output.
• Minor tidy: reorder `#![feature(...)]` entries.

Technical Notes: This makes unit values flow as constants earlier, reducing MIR noise and improving downstream pass stability.

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[augmentcode]

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[codecov]

Codecov Report

❌ Patch coverage is 96.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 64.94%. Comparing base (c391344) to head (a01405e).

Files with missing lines	Patch %	Lines
...shql/mir/src/pass/transform/inst_simplify/tests.rs	94.11%	1 Missing ⚠️

Additional details and impacted files

@@                           Coverage Diff                           @@
##           bm/be-197-hashql-implement-inlining    #8237      +/-   ##
=======================================================================
+ Coverage                                64.93%   64.94%   +0.01%     
=======================================================================
  Files                                      735      735              
  Lines                                    62952    62982      +30     
  Branches                                  3676     3677       +1     
=======================================================================
+ Hits                                     40877    40906      +29     
- Misses                                   21596    21597       +1     
  Partials                                   479      479              

Flag	Coverage Δ
rust.hashql-compiletest	46.65% <ø> (ø)
rust.hashql-mir	89.69% <96.66%> (+0.02%)	⬆️

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[codspeed-hq]

CodSpeed Performance Report

Merging #8237 will not alter performance

_{Comparing bm/be-270-hashql-simplify-aggregate-to-unit-const (a01405e) with bm/be-197-hashql-implement-inlining (c391344)}

Summary

✅ 17 untouched