[transmute] Encode cast in TryTransmuteFromPtr params

[joshlf]

This prepares us for a future change in which some impls will only be
sound for certain cast types. It also allows us to simplify some APIs
and implementations on Ptr.

Makes progress on #2701, #1940, #1852

---

• 　 Permit shrinking transmutes in transmute machinery #2912
• 👉 [transmute] Encode cast in TryTransmuteFromPtr params #2921

Latest Update: v20 — Compare vs v19

📚 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 introduces a significant refinement to the pointer transmutation traits by adding a CastExact type parameter. This parameter explicitly defines the casting mechanism used during transmutations, paving the way for future safety improvements where certain transmutations might only be valid with specific cast types. The changes involve updating trait definitions, method signatures, and their corresponding implementations to leverage this new type-level information, enhancing the precision and safety guarantees of pointer operations.

Highlights

• Trait Parameterization: The TryTransmuteFromPtr and TransmuteFromPtr traits now include a new type parameter C, which specifies the exact casting mechanism (CastExact) to be used during pointer transmutations.
• Enhanced Type Safety: This change allows for more granular control over the soundness of transmutations, enabling future implementations to enforce specific casting requirements.
• Codebase Alignment: Various Ptr methods (transmute, recall_validity, try_into_valid) and TryTransmuteFromPtr implementations have been updated to incorporate and utilize this new C type parameter, ensuring consistency across the codebase.

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 95.23810% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 91.31%. Comparing base (1a21b6d) to head (2bb0c47).

Files with missing lines	Patch %	Lines
src/lib.rs	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2921      +/-   ##
==========================================
- Coverage   91.33%   91.31%   -0.02%     
==========================================
  Files          20       20              
  Lines        5918     5897      -21     
==========================================
- Hits         5405     5385      -20     
+ Misses        513      512       -1     

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[gemini-code-assist]

Code Review

This pull request refactors the TransmuteFromPtr and TryTransmuteFromPtr traits to explicitly include the cast type as a generic parameter. This change enhances type safety and provides more granular control over pointer transmutations, which is crucial for future soundness checks. The documentation and associated unsafe impl blocks have been updated to reflect these changes. All modifications appear to be correct and align with the stated objective of preparing for future soundness-dependent cast types.