Introduce ProjectField trait

[jswrenn]

This trait powers invariant-respecting projection over Ptrs.

---

• Introduce ProjectField trait #2886
• Introduce HasField::project_inner #2885
• Rename HasField::project to HasField::project_raw #2884

Latest Update: v2 — Compare vs v1

📚 Full Patch History

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Version	Base	v1
v2	vs Base	vs v1
v1	vs Base

[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.

[gemini-code-assist]

The documentation for project refers to a parameter named slf, but the actual parameter is named ptr. To avoid confusion, especially in a safety-critical unsafe trait, the documentation should be updated to use the correct parameter name ptr.

    /// Projects from `ptr` to the field.
    ///
    /// # Safety
    ///
    /// The returned pointer refers to a non-strict subset of the bytes of
    /// `ptr`'s referent, and has the same provenance as `ptr`.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (G90165d85418ed6203ef3caaa77662dd3b313e030@c6f7382). Learn more about missing BASE report.

Additional details and impacted files

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##             G90165d85418ed6203ef3caaa77662dd3b313e030    #2886   +/-   ##
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