BE-481: HashQL: Reify null and empty tuple as unit constants

[indietyp]

🌟 What is the purpose of this PR?

During MIR reification, null primitive values and empty tuples were not being lowered to the canonical unit constant (). This meant that null literals and #tuple: [] expressions were handled inconsistently compared to other unit-like values. This PR ensures both are promoted to Constant::Unit during reification, aligning them with the existing treatment of unit expressions.

🔍 What does this change?

• Primitive::Null is now matched before the general Primitive arm in both atom.rs and rvalue.rs, lowering it directly to Constant::Unit instead of falling through to the opaque constant path.
• Empty tuples (Tuple { fields } where fields.is_empty()) are now lowered to Constant::Unit in rvalue.rs rather than being constructed as a zero-field aggregate.
• Existing snapshot tests for filter-aggressive have been updated to reflect that null comparisons now display as == () rather than == null in MIR output.

🛡 What tests cover this?

• New UI test reify/null-value verifies that a null literal is reified as a unit constant.
• New UI test reify/null-value-in-binary verifies that null values used in binary expressions (e.g. equality comparisons) are correctly promoted to ().
• New UI test reify/empty-tuple verifies that an empty tuple expression is reified as a unit constant.

❓ How to test this?

1. Run the MIR UI tests: cargo test -p hashql-mir
2. Confirm all new and existing snapshot tests pass.

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

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Project	Deployment	Actions	Updated (UTC)
hash	Ready	Preview, Comment	Jun 23, 2026 10:00am
petrinaut	Ready	Preview	Jun 23, 2026 10:00am

1 Skipped Deployment

Project	Deployment	Actions	Updated (UTC)
hashdotdesign-tokens	Ignored	Preview	Jun 23, 2026 10:00am

[cursor]

PR Summary

Low Risk
Scoped to internal MIR lowering and test output; behavior change is intentional normalization of null/empty-tuple to unit, with no auth or external API surface touched.

Overview
MIR reification now treats null literals and empty #tuple values as the canonical unit constant () instead of opaque primitives or zero-field tuple aggregates.

In reify/atom.rs and reify/rvalue.rs, Primitive::Null is handled in a dedicated arm that lowers to Constant::Unit (operand/load paths). Empty tuples short-circuit to the same unit constant rather than building a tuple aggregate. MIR snapshots reflect this—e.g. comparisons against null show as == () in filter-aggressive. New UI reify tests cover standalone null, null in binary ==, and empty tuples.

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[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-614: HashQL: Generate synthetic closure bodies for intrinsics used as first-class values #8895
• BE-615: HashQL: Introduce the concept of synthetic closures in the MIR #8894
• BE-607: HashQL: Refactor ModuleRegistry into builder/immutable split with allocator-generic stdlib construction #8887
• BE-538: HashQL: Permission system integration #8882
• BE-71: HashQL: Consolidate AST special-form expansion stages into one #8870
• BE-481: HashQL: Reify null and empty tuple as unit constants #8853 👈 (View in Graphite)
• BE-537, BE-537: hgraph: REST /hashql endpoint with E2E tests #8827
• BE-592: HashQL: Remove old eval graph path and core value types #8841
• BE-590: hgraph: Refactor entity query handlers into entity/query submodule #8840
• BE-587, BE-588: HashQL: Introduce CodeGenerationContext/CodeExecutionContext split and eval-stage Interner #8839
• BE-581, BE-589: HashQL: Introduce pass::lower/pass::place pipeline facades and fix cached constructor closure bug #8838
• BE-585, BE-591: HashQL: Add MIR interpreter test suite and refactor diagnostic categories #8837
• BE-583, BE-584, BE-586: HashQL: Add HeapPool, improve spans, and propagate Critical severity through diagnostics #8836
• BE-582: HashQL: Change opaque types from invariant to covariant #8835
• main

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

[Copilot]

Pull request overview

This PR standardizes MIR reification so that null literals and empty tuple literals are lowered to the canonical unit constant Constant::Unit (()), aligning them with existing unit-expression handling and making MIR output consistent.

Changes:

• Reify Primitive::Null directly into Constant::Unit in both atom and rvalue lowering.
• Reify empty tuple literals (#tuple: []) into Constant::Unit instead of a zero-field tuple aggregate.
• Add new MIR UI tests for null and empty tuple reification, and update an existing snapshot to reflect == () comparisons.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
libs/@local/hashql/mir/src/reify/rvalue.rs	Lowers null primitives and empty tuple data nodes to Constant::Unit during rvalue reification.
libs/@local/hashql/mir/src/reify/atom.rs	Lowers null primitives to Constant::Unit when reifying atom operands.
libs/@local/hashql/mir/tests/ui/reify/null-value.jsonc	Adds UI test input asserting null literal is promoted to unit.
libs/@local/hashql/mir/tests/ui/reify/null-value.stdout	Adds expected MIR output showing return () for null.
libs/@local/hashql/mir/tests/ui/reify/null-value-in-binary.jsonc	Adds UI test input asserting null promotion inside a binary expression.
libs/@local/hashql/mir/tests/ui/reify/null-value-in-binary.stdout	Adds expected MIR output showing null operands lowered to () before comparison.
libs/@local/hashql/mir/tests/ui/reify/empty-tuple.jsonc	Adds UI test input asserting empty tuple literal is promoted to unit.
libs/@local/hashql/mir/tests/ui/reify/empty-tuple.stdout	Adds expected MIR output showing empty tuple lowered to ().
libs/@local/hashql/mir/tests/ui/pass/inline/filter-aggressive.stdout	Updates snapshot output to reflect null comparisons now printing as == ().

---

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[codspeed-hq]

Merging this PR will not alter performance

✅ 24 untouched benchmarks
⏩ 56 skipped benchmarks¹

---

_{Comparing bm/be-481-hashql-lower-null-to-constantunit-during-reification (41cf466) with bm/be-537-hashql-remove-old-backend-wire-up-hashql-in-the-api (211e26c)}

Footnotes

1. 56 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 56.77%. Comparing base (211e26c) to head (41cf466).

Additional details and impacted files

@@                                      Coverage Diff                                       @@
##           bm/be-537-hashql-remove-old-backend-wire-up-hashql-in-the-api    #8853   +/-   ##
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  Coverage                                                          56.77%   56.77%           
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  Files                                                                390      390           
  Lines                                                              63060    63060           
  Branches                                                            1060     1060           
==============================================================================================
  Hits                                                               35802    35802           
  Misses                                                             26882    26882           
  Partials                                                             376      376           

Flag	Coverage Δ
rust.hashql-compiletest	28.40% <ø> (ø)

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[Copilot]

Pull request overview

Copilot reviewed 9 out of 9 changed files in this pull request and generated no new comments.

[Copilot]

Pull request overview

Copilot reviewed 9 out of 9 changed files in this pull request and generated no new comments.