Improve object traversal for heap types and mro#6976

youknowone

Fix heap type instance traversal to include type reference (enables correct cycle detection for instance ↔ type references)
Enable mro traversal in PyType (was previously disabled)

Summary by CodeRabbit

Refactor
- Improved garbage collection traversal for type objects to ensure proper memory management of type hierarchies and heap-allocated types.

- Fix heap type instance traversal to include type reference (enables correct cycle detection for instance ↔ type references) - Enable mro traversal in PyType (was previously disabled)

coderabbitai

📝 Walkthrough

Walkthrough

This PR enhances garbage collection tracing in the RustPython VM by re-enabling MRO traversal in PyType and adding explicit heap-type reference tracing in PyInner object traversal. These changes ensure complete marking of type relationships during GC cycles.

Changes

Cohort / File(s)	Summary
GC Traversal Enablement `crates/vm/src/builtins/type.rs`	Re-enabled MRO traversal in PyType's traverse implementation by uncommenting `self.mro.traverse(tracer_fn)`, ensuring all MRO entries are visited during GC tracing.
Heap Type Reference Tracing `crates/vm/src/object/traverse_object.rs`	Added explicit tracing of heap-type references in both PyInner and PyInner paths. When a type is a heap type, the type object is now converted to PyObject and traced before dict/slots traversal continues.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~22 minutes

Possibly related PRs

make mro representation fit to CPython #5866: Directly modifies PyType traverse/mro behavior related to this PR's MRO tracing enablement.
traverse and clear #6780: Also modifies object traversal logic in traverse_object.rs, rewiring traversal hooks that interact with these changes.

Poem

🐰 Hop, hop, the MRO flows,
No more gaps in traces that the garbage knows,
Heap types shine with reference bright,
GC cycles now collect all in sight! ✨

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 66.67% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'Improve object traversal for heap types and mro' directly and accurately summarizes the main changes: enabling MRO traversal in PyType and improving heap type instance traversal to include type references.

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✨ Finishing touches

📝 Generate docstrings

🧪 Generate unit tests (beta)

Create PR with unit tests
Post copyable unit tests in a comment

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Improve object traversal for heap types and mro …

f358d86

- Fix heap type instance traversal to include type reference (enables correct cycle detection for instance ↔ type references) - Enable mro traversal in PyType (was previously disabled)

youknowone marked this pull request as ready for review February 3, 2026 08:10

youknowone deleted the traverse-improvements branch February 3, 2026 08:54

coderabbitai Bot mentioned this pull request Feb 3, 2026

Add GC infrastructure: tracking bits, tp_clear #6977

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Improve object traversal for heap types and mro#6976