[derive] Allow TryFromBytes on non-Immutable unions

[joshlf]

Makes progress on #5
Closes #1831
Closes #1832

---

• 👉 [derive] Allow TryFromBytes on non-Immutable unions #2876
• 　 [WIP] Use ReadOnly in is_bit_valid #2873

Latest Update: v54 — Compare vs v53

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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 flexibility of the zerocopy crate by removing the Immutable trait requirement for deriving TryFromBytes, FromZeros, and FromBytes for unions. This change allows unions containing types like UnsafeCell, which are not inherently Immutable, to correctly implement these traits, broadening the utility of the derive macros without compromising memory safety.

Highlights

• Removed Immutable Trait Bound for Unions: The Immutable trait bound has been removed for TryFromBytes, FromZeros, and FromBytes derives on unions, allowing these derives to be used with unions containing non-Immutable types.
• Updated Safety Justification: The safety comment for TryFromBytes on unions has been updated to clarify that safety is ensured by ReadOnly<Self>: Immutable, which holds unconditionally, rather than Self: Immutable.
• New Test Cases for UnsafeCell Unions: New test cases have been added for unions containing UnsafeCell to validate the updated derive behavior for FromBytes, FromZeros, and TryFromBytes.

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[gemini-code-assist]

Code Review

This pull request correctly removes the Immutable bound requirement for fields of unions when deriving TryFromBytes, FromZeros, and FromBytes. This is a significant improvement that allows unions containing types with interior mutability (like UnsafeCell) to use these derives. The implementation ensures safety by using a ReadOnly wrapper during validity checks, and the corresponding safety comments are updated accordingly. The change is supported by new tests that verify the new functionality. The implementation is sound and the code quality is high.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.82%. Comparing base (d2d392a) to head (619328b).

Additional details and impacted files

@@                              Coverage Diff                              @@
##           G7691845b6b02e9f3d9578435d732bacfa6ca674f    #2876      +/-   ##
=============================================================================
- Coverage                                      91.93%   90.82%   -1.12%     
=============================================================================
  Files                                             19       20       +1     
  Lines                                           5856     5928      +72     
=============================================================================
  Hits                                            5384     5384              
- Misses                                           472      544      +72     

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