BE-481: HashQL: Reify null and empty tuple as unit constants by indietyp · Pull Request #8853 · hashintel/hash

indietyp · 2026-06-11T12:34:41Z

🌟 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?

Run the MIR UI tests: cargo test -p hashql-mir
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

vercel · 2026-06-11T12:34:46Z

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cursor · 2026-06-11T12:34:51Z

PR Summary

Low Risk
Scoped compiler lowering change in hashql-mir reify with snapshot/UI test coverage; no auth, data, or runtime API surface changes.

Overview
MIR reification now treats null literals and empty tuples (#tuple: []) as the canonical unit value (), instead of opaque primitives or zero-field tuple aggregates.

In reify/atom.rs and reify/rvalue.rs, Primitive::Null is handled in a dedicated match arm that lowers to Constant::Unit (operand load paths). Empty tuples short-circuit to the same unit constant rather than building a tuple aggregate. Snapshot output for graph filter inlining now shows comparisons like == () instead of == null.

New MIR UI tests cover standalone null, null in binary ==, and empty tuple reification.

^{Reviewed by Cursor Bugbot for commit 1bcc5ff. Bugbot is set up for automated code reviews on this repo. Configure here.}

indietyp · 2026-06-11T12:35:04Z

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
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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 · 2026-06-11T12:41:16Z

Merging this PR will not alter performance

✅ 24 untouched benchmarks
⏩ 56 skipped benchmarks¹

_{Comparing bm/be-481-hashql-lower-null-to-constantunit-during-reification (1bcc5ff) with main (3db2f69)}

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 · 2026-06-11T12:43:32Z

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.42%. Comparing base (3db2f69) to head (1bcc5ff).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #8853   +/-   ##
=======================================
  Coverage   59.42%   59.42%           
=======================================
  Files        1357     1357           
  Lines      131847   131851    +4     
  Branches     6018     6019    +1     
=======================================
+ Hits        78352    78356    +4     
  Misses      52565    52565           
  Partials      930      930

Flag	Coverage Δ
apps.hash-ai-worker-ts	`1.39% <ø> (ø)`
apps.hash-api	`6.39% <ø> (ø)`
local.hash-backend-utils	`2.81% <ø> (ø)`
local.hash-graph-sdk	`10.02% <ø> (ø)`
local.hash-isomorphic-utils	`0.18% <ø> (ø)`
rust.hash-graph-api	`7.46% <ø> (ø)`
rust.hashql-compiletest	`28.40% <ø> (ø)`
rust.hashql-eval	`79.47% <ø> (ø)`
rust.hashql-mir	`88.05% <100.00%> (+<0.01%)`	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

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