Make SizeEq safe

[joshlf]

Make SizeEq safe by moving its safety invariant to a new CastExact
trait which now bounds the associated SizeEq::CastFrom type. This
permits us to write safety proofs in fewer places, since there are
certain CastFrom impls which we can re-use for many SizeEq impls.

Makes progress on #2701, #1940, #1852

---

• 　 Permit shrinking transmutes in transmute machinery #2912
• 👉 Make SizeEq safe #2907

Latest Update: v18 — Compare vs v17

📚 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 refactors the SizeEq trait in the codebase to enhance its safety and maintainability. By introducing a new CastExact trait, the responsibility for guaranteeing size-preserving casts is explicitly delegated, allowing SizeEq to become a regular trait. This change centralizes safety invariants, simplifies future safety proofs, and improves the overall robustness of pointer casting mechanisms.

Highlights

• Refactored SizeEq Trait: The SizeEq trait is no longer unsafe, with its safety invariants moved to a new CastExact trait, making SizeEq a regular trait and centralizing safety guarantees.
• Introduced CastExact Trait: A new unsafe trait CastExact is added, which explicitly guarantees that a pointer projection preserves the exact set of referent bytes, improving the clarity and enforceability of safety invariants.
• Improved Safety Proofs: By bounding SizeEq::CastFrom with CastExact, safety proofs are now centralized and can be reused across multiple implementations, reducing the surface area for potential errors and simplifying reasoning about correctness.
• New CastSizedExact Type: A new CastSizedExact enum is introduced for statically-sized exact casts, ensuring compile-time size equality and providing a dedicated mechanism for such operations.
• Renamed Macros and Functions: Several macros (impl_size_eq!, assert_size_eq!, unsafe_with_size_eq!) and test functions (test_size_eq) have been renamed to _compat variants to better reflect the new semantics of size compatibility handled by CastExact.

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[gemini-code-assist]

Code Review

This pull request refactors the SizeEq trait to be safe by introducing a new CastExact trait, which now carries the safety invariant. This is a positive change that improves the clarity and localization of unsafe code guarantees. The changes involve renaming macros, updating trait implementations, and adjusting documentation to reflect the new design. The UI test output files have also been updated to match the macro renames.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.90%. Comparing base (d6fd7f3) to head (4cb7174).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2907   +/-   ##
=======================================
  Coverage   91.90%   91.90%           
=======================================
  Files          20       20           
  Lines        5879     5883    +4     
=======================================
+ Hits         5403     5407    +4     
  Misses        476      476           

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