[wip] Introduce Ptr::project_map

[jswrenn]

Enables piecewise initialization.

---

• 👉 [wip] Introduce Ptr::project_map #2984
• 　 [WIP] Introduce compound Ptr validity #2966

[gemini-code-assist]

Summary of Changes

Hello @jswrenn, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a significant enhancement to pointer manipulation by adding a project_map method to the Ptr type. This new functionality is designed to enable more flexible and controlled piecewise initialization of data structures. It achieves this by allowing developers to project a field, apply a mapping function, and then safely assume the validity of the original pointer. Supporting this, new Idx and Map types have been introduced to refine how tuple fields are identified and how validity components are managed within the type system.

Highlights

• New Ptr::project_map method: Introduced a new project_map method on the Ptr struct, which allows mapping a projected field and then assuming the validity of the original pointer. This is intended to facilitate piecewise initialization.
• Introduction of Idx struct: A new Idx struct was added in src/lib.rs to serve as a field visibility token for public tuple fields, replacing the previous use of () in HasField and ProjectField implementations for tuples.
• New Map trait for validity components: A Map trait was introduced in src/pointer/invariant.rs to provide a mechanism for replacing a validity component within a type's invariants, which is utilized by the new project_map method.
• Tuple HasField and ProjectField updates: The macro for implementing HasField and ProjectField for tuples in src/impls.rs was updated to use the new crate::Idx struct for field identification and to implement the new crate::invariant::Map trait.

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Changelog

• src/impls.rs
  • Implemented crate::invariant::Map for tuples, allowing validity components to be replaced.
  • Updated crate::HasField and crate::ProjectField implementations for tuples to use crate::Idx<{ $CurrI }> instead of () for field identification.
  • Adjusted comments to reflect the change from () to crate::Idx in HasField and ProjectField implementations.
• src/lib.rs
  • Introduced the Idx<const INDEX: u16> struct, serving as a field visibility token for public tuple fields.
• src/pointer/invariant.rs
  • Added the Map<Replacement: Validity, Field> trait, which defines a mechanism to replace a validity component in a type's invariants.
• src/pointer/ptr.rs
  • Added the project_map method to the Ptr struct, enabling the mapping of projected fields and subsequent assumption of the original pointer's validity.

Activity

• The pull request is currently marked as a Work In Progress ([wip]), indicating that further changes or refinements are expected.
• The author, jswrenn, has linked this PR to related issues or pull requests: [wip] Introduce Ptr::project_map #2984 and [WIP] Introduce compound Ptr validity #2966, suggesting it's part of a larger effort.
• The primary goal of this PR is to enable piecewise initialization.
• A // SAFETY: TODO comment is present in the newly added project_map function, indicating that the safety invariants for this new method still need to be fully addressed and documented.

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[gemini-code-assist]

Code Review

This pull request introduces Ptr::project_map to enable piecewise initialization of data structures. This is a valuable addition. The supporting machinery, including the invariant::Map trait and its implementation for tuples, seems correct.

However, the implementation of project_map itself in src/pointer/ptr.rs has a critical soundness issue. It uses unsafe { self.assume_validity() } without proper justification, and the logic seems to incorrectly handle the result of the mapping closure. As indicated by the // SAFETY: TODO comment, this is likely a work in progress, but it's crucial to address this before merging.

[gemini-code-assist]

This implementation has a critical soundness issue.

The map closure is called on a reborrow of self, and its resulting Ptr is immediately discarded (assigned to _mapped). This means any type-level proof of a validity change is lost.

Subsequently, unsafe { self.assume_validity() } is called without justification. Since map operated on a temporary borrow, it cannot logically change the validity of the original self. This unsafe block is unsound as written and, as the // SAFETY: TODO comment indicates, lacks the required safety proof.

This could lead to Ptrs with incorrect validity invariants, causing undefined behavior. The map closure's returned Ptr must be used to prove that the operation is sound before assume_validity can be justified.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 27 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.55%. Comparing base (5902593) to head (7a3d360).

Files with missing lines	Patch %	Lines
src/pointer/ptr.rs	0.00%	27 Missing ⚠️

Additional details and impacted files

@@                              Coverage Diff                              @@
##           G06edd27f8b77b5ad480ca94749098bb4b0808342    #2984      +/-   ##
=============================================================================
- Coverage                                      91.96%   91.55%   -0.41%     
=============================================================================
  Files                                             19       19              
  Lines                                           6033     6060      +27     
=============================================================================
  Hits                                            5548     5548              
- Misses                                           485      512      +27     

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