Make validate_downcastable_from unsafe#6851

coolreader18

Meant to put this as part of #6231

Summary by CodeRabbit

Refactor
- Improved type validation safety mechanisms within the object system to enforce explicit unsafe boundaries around type downcast operations, ensuring more secure type checking during runtime.

_{✏️ Tip: You can customize this high-level summary in your review settings.}

coderabbitai

📝 Walkthrough

Walkthrough

The PR refactors RustPython's object downcasting mechanism by making validate_downcastable_from unsafe across the codebase and optimizing PyObject::downcastable with a type-ID fast path. The trait method downcastable_from is removed and replaced with instance method dispatch via downcastable.

Changes

Cohort / File(s)	Summary
Payload Trait Updates `crates/vm/src/object/payload.rs`	Removed trait method `downcastable_from`, changed `validate_downcastable_from` signature to `unsafe fn`, and updated `try_downcast_from` to use `obj.downcastable::<Self>()` for delegation
Core Downcast Optimization `crates/vm/src/object/core.rs`	Refactored `PyObject::downcastable<T>` to perform type-ID comparison before calling unsafe `T::validate_downcastable_from(self)` for a fast-path optimization
Derive Macro Implementations `crates/derive-impl/src/pyclass.rs`, `crates/derive-impl/src/pystructseq.rs`	Updated `validate_downcastable_from` method signature in PyPayload implementations from `fn` to `unsafe fn`
Builtin Type Implementation `crates/vm/src/builtins/str.rs`	Changed `PyUtf8Str::validate_downcastable_from` signature to `unsafe fn`

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~12 minutes

Possibly related PRs

Wtf8-compatible fixes #5985: Modifies PyObject::downcastable with type-ID fast path and updates PyPayload validation handling in similar pattern
overridable validate_downcastable_from #6393: Changes PyPayload downcast validation logic with validate_downcastable_from and updates downcastable_from usage patterns
Separate Debug from PyPayload #6320: Modifies the PyPayload trait definition in crates/vm/src/object/payload.rs affecting trait structure and constraints

Poem

🐰 Unsafe functions now guard the path,
Type IDs dash through fast,
Downcasting logic, sleek and lean,
Faster validation's never been seen! ✨

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'Make validate_downcastable_from unsafe' clearly and specifically describes the primary change across all modified files: converting validate_downcastable_from from a safe function to an unsafe function.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

✨ Finishing touches

📝 Generate docstrings

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

_{Comment @coderabbitai help to get the list of available commands and usage tips.}

github-actions

Code has been automatically formatted

The code in this PR has been formatted using:

ruff format
Please pull the latest changes before pushing again:

git pull origin validate_downcastable_from-unsafe

youknowone

please check the clippy. ( safety docs)

youknowone requested changes Jan 24, 2026

View reviewed changes

Make validate_downcastable_from unsafe

d8e999c

coolreader18 force-pushed the validate_downcastable_from-unsafe branch from 42b66a8 to d8e999c Compare January 25, 2026 18:32

youknowone enabled auto-merge (squash) January 25, 2026 22:48

youknowone approved these changes Jan 26, 2026

View reviewed changes

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Make validate_downcastable_from unsafe#6851