Simplify MutationCompatible impls

[joshlf]

Previously, we allowed for Src: Read<A, R> and Dst: Read<A, S> for
different R and S "reason" parameters. This is an unnecessary degree
of freedom (two types are only ever Read<A> for a particular A for
the same reason – either because A = Exclusive or because both types
implement Immutable). This degree of freedom required more verbose
type annotations by callers.

---

• 　 [WIP] Use ReadOnly in is_bit_valid #2873
• 　 [WIP] ReadOnly #2866
• 👉 Simplify MutationCompatible impls #2867
• 　 Make SizeEq safe, introduce CastExact #2872

Latest Update: v9 — Compare vs v8

📚 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 streamlines the type system within the zerocopy library by simplifying the MutationCompatible trait's implementation for BecauseRead. By eliminating an redundant type parameter, the change reduces the complexity of type annotations required by callers and enhances the overall clarity and consistency of the library's internal type logic. This ultimately makes the library more ergonomic and easier to work with.

Highlights

• Type System Simplification: The MutationCompatible trait implementation for BecauseRead has been simplified by removing an unnecessary S type parameter, reducing the "degree of freedom" in type annotations.
• Reduced Type Verbosity: Calls to recall_validity and transmute methods now require less verbose type annotations, improving code readability and maintainability.
• Trait Bound Consistency: Added [u8]: Read<A, R> trait bounds to Ptr methods (as_bytes and try_cast_into_no_leftover) to ensure consistent Read trait implementation across types after the simplification.
• Test Updates: Existing test cases in zerocopy-derive were updated to reflect the new, simplified type signatures, ensuring correctness.

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 92.03%. Comparing base (a6e9a9b) to head (27689d2).

Files with missing lines	Patch %	Lines
src/util/macros.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@                            Coverage Diff                             @@
##           G57ec07c3841271440bbaf40cab04b942cbdbddb9    #2867   +/-   ##
==========================================================================
  Coverage                                      92.03%   92.03%           
==========================================================================
  Files                                             20       20           
  Lines                                           5874     5874           
==========================================================================
  Hits                                            5406     5406           
  Misses                                           468      468           

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[gemini-code-assist]

Code Review

This pull request simplifies the MutationCompatible trait implementations by removing an unnecessary degree of freedom in the "reason" parameters. This change is propagated across the codebase, simplifying type annotations at various call sites. The changes are sound and improve the maintainability of the code. I have no major concerns.