Introduce ProjectField trait

[jswrenn]

This trait powers invariant-respecting projection over Ptrs.

---

• 　 Rename invariant::Valid to invariant::Safe #2913
• 👉 Introduce ProjectField trait #2886

Latest Update: v23 — Compare vs v22

📚 Full Patch History

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[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 the ProjectField trait, a foundational component for enabling safe and invariant-respecting field projection within the system. This trait provides a structured way to access fields of Ptr types while upholding data integrity and safety guarantees, likely serving as an internal mechanism for future derive macros or similar functionalities.

Highlights

• New ProjectField Trait: Introduced a new unsafe trait ProjectField designed for invariant-respecting projection over Ptrs. This trait is marked doc(hidden) and includes a method only_derive_is_allowed_to_implement_this_trait, indicating its intended use primarily by derive macros for internal mechanisms.
• Trait Definition: The ProjectField trait defines associated types Invariants (for the projected field's pointer's invariants) and Error (for fallible projection), along with a project function that safely projects a field from a Ptr.
• Safety Invariants: The trait explicitly states safety requirements for its implementations and for the project function, ensuring that the returned pointer refers to a non-strict subset of the original Ptr's bytes and maintains provenance.

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[gemini-code-assist]

Code Review

This pull request introduces a new unsafe trait ProjectField to facilitate invariant-respecting projections over Ptrs. The new trait is well-defined and documented. My review includes a suggestion to correct a minor inconsistency in the documentation for the project method, where the parameter name used in the comments differs from the one in the function signature. Aligning these will improve clarity, which is crucial for an unsafe trait.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 58.62069% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.72%. Comparing base (c358fd5) to head (2974513).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/impls.rs	0.00%	12 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2886      +/-   ##
==========================================
- Coverage   91.90%   91.72%   -0.19%     
==========================================
  Files          20       20              
  Lines        5883     5910      +27     
==========================================
+ Hits         5407     5421      +14     
- Misses        476      489      +13     

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[joshlf]

/gemini review

[gemini-code-assist]

Code Review

The pull request introduces the ProjectField trait, which is a significant and positive change for invariant-respecting projection over Ptrs. This refactoring allows for more granular control and safety guarantees when projecting fields, especially with fallible projections. The introduction of the into_inner! macro for infallible Result unwrapping is also a good addition.

[joshlf]

Please resolve remaining comments before merging.