Clarify semantics of size-preserving transmutes

[joshlf]

Makes progress on #2701, #1940, #1852

---

• 　 Permit shrinking transmutes in transmute machinery #2912
• 　 Make SizeEq safe #2907
• 👉 Clarify semantics of size-preserving transmutes #2911

Latest Update: v6 — Compare vs v5

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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 clarifies the semantic rules and safety guarantees for size-preserving pointer transmutes within the pointer::transmute module. The changes aim to provide a more precise understanding of how these transmutes operate, particularly by detailing the role of the SizeEq trait and its associated CastFrom type, and by tightening the conditions for validity to prevent unsafe shrinking or overwriting operations.

Highlights

• Clarified Transmute Mechanism: The documentation now explicitly states that size-preserving transmutes are performed using <Dst as SizeEq<Src>>::CastFrom::project, providing a more precise definition of the underlying mechanism.
• Refined Validity Conditions: The 'Reverse transmutation' precondition has been enhanced to clearly ban shrinking or overwriting transmutes by specifying that the set of DV-valid referents of dst must be a subset of the SV-valid referents of src.
• Simplified Safety Proofs: The internal documentation explaining the soundness proof for transmutes has been refactored and simplified, making it more concise and directly referencing the invariants of the SizeEq trait.
• Improved SizeEq Trait Documentation: The SizeEq trait now includes a comprehensive description of its purpose, explaining its role in enabling size-preserving casts or conversions. Its safety requirements have also been simplified to focus solely on the CastFrom associated type's responsibility to preserve referent size.

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[gemini-code-assist]

Code Review

This pull request provides excellent clarifications to the documentation and safety comments related to size-preserving transmutes in src/pointer/transmute.rs. The changes significantly improve the clarity and correctness of the semantics, especially around the SizeEq trait and the conditions for sound transmutations. The refactoring of the safety proofs makes them more concise and easier to follow. Overall, these are high-quality improvements that enhance the maintainability and understandability of the code.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.90%. Comparing base (6aae3de) to head (eb8f348).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2911   +/-   ##
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  Coverage   91.90%   91.90%           
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  Files          20       20           
  Lines        5877     5879    +2     
=======================================
+ Hits         5401     5403    +2     
  Misses        476      476           

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