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.

---

• 　 [derive] Allow TryFromBytes on non-Immutable unions #2876
• 　 [derive] Refactor is_bit_valid tests #2875
• 　 [WIP] Use ReadOnly in is_bit_valid #2873
• 　 [WIP] ReadOnly, CastExact #2866
• 　 Rename SizeEq to SizeCompat, make it safe #2872
• 👉 Simplify MutationCompatible impls #2877

[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 refactors the internal type system, specifically simplifying the MutationCompatible trait's implementation by unifying the 'reason' type parameter for Read trait bounds. This change eliminates an unnecessary degree of freedom in type definitions, resulting in more concise and clearer type annotations throughout the codebase. The modifications primarily affect how types are constrained and cast, leading to a more streamlined and maintainable type system.

Highlights

• Type Parameter Simplification: The MutationCompatible trait implementation has been simplified by removing an unnecessary degree of freedom in its type parameters. Previously, Src and Dst types could implement Read<A, R> and Read<A, S> with different 'reason' parameters R and S. This change enforces that both Src and Dst must implement Read<A, R> for the same reason R, as this was implicitly true in practice.
• Reduced Type Annotation Verbosity: This simplification directly leads to less verbose type annotations for callers, as the redundant S type parameter is no longer required in the MutationCompatible trait's reason tuple, changing (BecauseRead, (R, S)) to (BecauseRead, R).
• Explicit Trait Bounds for [u8]: Explicit [u8]: Read<A, R> trait bounds were added to as_bytes and cast_into_no_leftover methods in src/pointer/ptr.rs. This addresses a current limitation where Rust's type system doesn't automatically infer that T: Read<A, R> implies [u8]: Read<A, R].
• Consistent Type Parameter Usage: Across the codebase, various recall_validity, cast_for_sized, and transmute calls have been updated to reflect the simplified type parameter structure, changing instances of (_, (_, (BecauseExclusive, BecauseExclusive))) to (_, (_, BecauseExclusive)).
• Test Case Updates: Corresponding test cases in zerocopy-derive/tests/struct_try_from_bytes.rs and zerocopy-derive/tests/union_try_from_bytes.rs have been updated to align with the new, simplified type parameter usage in cast calls.

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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 91.90%. Comparing base (1ed124b) to head (b85ccfe).

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

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2877   +/-   ##
=======================================
  Coverage   91.90%   91.90%           
=======================================
  Files          20       20           
  Lines        5878     5878           
=======================================
  Hits         5402     5402           
  Misses        476      476           

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[gemini-code-assist]

Code Review

This pull request simplifies the MutationCompatible trait implementation by removing an unnecessary degree of freedom in its generic parameters. Previously, it allowed source and destination types to have different "reason" parameters for the Read trait, which is now constrained to be the same. This change simplifies the trait and makes type annotations at call sites less verbose. I've reviewed the changes, and they are consistent and correctly propagated throughout the codebase, including updates to function signatures and test cases. The addition of the [u8]: Read<I::Aliasing, R> bound where necessary is a correct consequence of this simplification. The refactoring is well-executed and improves the code's clarity.