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 unit-aware handling across MIR building and simplification.

• Add From<!> for Operand and support for () in body! and rvalue! (e.g., tuple;) to construct empty tuples
• In InstSimplify, fold empty tuple aggregates (AggregateKind::Tuple with no operands) into RValue::Load(Constant::Unit)
• Update tests: new empty_tuple_to_unit and refreshed inline/pre-inlining snapshots to use () directly (removing temporary unit locals)
• Tidy lib.rs feature flags (reordered, remove unused)

^{Written by Cursor Bugbot for commit f0ef5be. 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-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 👈 (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

This stack of pull requests is managed by Graphite. Learn more about stacking.

[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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.14%. Comparing base (7e32c71) to head (f0ef5be).

Additional details and impacted files

@@                           Coverage Diff                           @@
##           bm/be-197-hashql-implement-inlining    #8237      +/-   ##
=======================================================================
+ Coverage                                76.02%   81.14%   +5.12%     
=======================================================================
  Files                                      288      191      -97     
  Lines                                    42866    28548   -14318     
  Branches                                  1059      773     -286     
=======================================================================
- Hits                                     32587    23166    -9421     
+ Misses                                    9916     5122    -4794     
+ Partials                                   363      260     -103     

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

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[codspeed-hq]

Merging this PR will not alter performance

✅ 17 untouched benchmarks

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_{Comparing bm/be-270-hashql-simplify-aggregate-to-unit-const (f0ef5be) with bm/be-197-hashql-implement-inlining (7e32c71)}

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