[derive] Document use of Field trait

[joshlf]

Originally introduced in #2127, this trait is used to ensure that field
types which mention Self resolve correctly (always to the original
type, and never to generated utility types).

---

• 👉 [derive] Document use of Field trait #2865

[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 enhances the internal documentation of the zerocopy-derive macro by adding detailed comments explaining the purpose and mechanism of the Field trait. The Field trait is crucial for correctly resolving Self references within derived types, ensuring that field types always refer to the original type rather than any generated utility types. This improvement aims to provide clearer insights into the macro's type resolution logic for maintainers and future contributors.

Highlights

• Documentation: Added comprehensive comments within the derive_known_layout_inner function in zerocopy-derive/src/lib.rs to explain the critical role of the Field trait. These comments clarify how the trait is used to project field types and ensure correct Self hygiene, preventing Self from resolving to generated helper types instead of the original derive target type.

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[gemini-code-assist]

Code Review

This pull request adds documentation in the form of code comments to explain the use of the Field trait within the KnownLayout derive macro. The added comments clearly explain that the Field trait is used to solve Self hygiene issues in proc-macros, ensuring that Self in field types correctly refers to the original type being derived on, rather than any generated helper types. The changes are accurate, well-explained, and significantly improve the maintainability of this complex macro code. This is a great improvement, and I have no further suggestions.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.94%. Comparing base (b99b86d) to head (57a5b76).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2865   +/-   ##
=======================================
  Coverage   91.94%   91.94%           
=======================================
  Files          20       20           
  Lines        5824     5824           
=======================================
  Hits         5355     5355           
  Misses        469      469           

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