[project] Support projecting enum tags

[joshlf]

---

• 　 Simplify try_transmute! (and friends) internals #2938
• 　 [derive] Allow TryFromBytes on non-Immutable unions #2876
• 　 Use ReadOnly in TryFromBytes::is_bit_valid #2873
• 👉 [project] Support projecting enum tags #2940

Latest Update: v16 — Compare vs v15

📚 Full Patch History

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[gemini-code-assist]

Summary of Changes

Hello @joshlf, 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 significantly enhances the zerocopy library by integrating robust support for projecting enum tags. This foundational change allows the library to understand and work with the discriminant of enums, which is crucial for safely and correctly accessing enum variants in a zero-copy manner. This enables more advanced and type-safe operations on data structures that include enums, improving the overall utility and correctness of the library for complex data layouts.

Highlights

• Enum Tag Projection Support: Introduced new associated types (Tag, ProjectToTag) and a method (project_tag) to the HasField and ProjectField traits, enabling the projection of an enum's discriminant (tag) for safe variant access.
• Default Tag Implementation: Provided a default Tag = () and ProjectToTag = CastToUnit for non-enum types, ensuring backward compatibility and a consistent interface across all types.
• CastToUnit Utility: Added a new CastToUnit type and its implementations to facilitate casting any type to (), which is used for the default tag projection for non-enum types.
• Derive Macro Updates: Modified the zerocopy-derive macros to automatically implement the new Tag and ProjectToTag associated types for both enums (using a generated ___ZerocopyTag) and other types, streamlining integration.
• Refined Projectability Checks: Updated the core project method to leverage the new project_tag for more accurate and type-safe checks of field projectability, especially for enum variants.

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[gemini-code-assist]

Code Review

This pull request introduces support for projecting enum tags, which is a significant new feature. It involves extending the HasField and ProjectField traits with Tag and ProjectToTag associated types, and modifying the Ptr::project logic to leverage these new types. New utility types like CastToUnit and internal ___ZerocopyTag enums are also introduced to facilitate this. While the core logic seems aligned with the feature's goal, there are some critical issues related to safety justifications and a potential breaking change in tuple support, as well as a medium-severity issue regarding the visibility of an internal enum.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 66.66667% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.33%. Comparing base (1876c9d) to head (a98e1d0).

Files with missing lines	Patch %	Lines
src/impls.rs	0.00%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2940      +/-   ##
==========================================
- Coverage   92.44%   92.33%   -0.12%     
==========================================
  Files          19       19              
  Lines        5825     5843      +18     
==========================================
+ Hits         5385     5395      +10     
- Misses        440      448       +8     

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[joshlf]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces support for projecting enum tags, which is a significant and well-implemented feature. The changes consistently update the HasField trait with Tag and ProjectToTag associated types, and provide appropriate implementations across the codebase, including for tuples and generated enum variants. The new CastToUnit type simplifies handling non-enum types. The safety comments are thorough and reference relevant Rust documentation, which is excellent for maintaining soundness. The modifications to is_projectable and related logic correctly integrate the new tag projection mechanism.