Tighten impl of CastUnsized

[joshlf]

Migrate alignment check from unconditional to only applying to the slice
DST case. Improve detail of safety proof.

---

• 　 [WIP] Remove MutationCompatible, Read #2878
• 　 [derive] Allow TryFromBytes on non-Immutable unions #2876
• 　 [WIP] Use ReadOnly in is_bit_valid #2873
• 　 [WIP] ReadOnly, CastExact #2866
• 　 Rename SizeEq to SizeCompat, make it safe #2872
• 👉 Tighten impl of CastUnsized #2887

Latest Update: v10 — Compare vs v9

📚 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 refines the implementation of CastUnsized by making its alignment check more precise, applying it only when casting between slice DSTs. This change improves the correctness and clarity of the type casting mechanism, ensuring that safety assertions are accurately tailored to the specific layout properties of the types involved. Additionally, the documentation for the safety proof has been enhanced to provide a more comprehensive understanding of the underlying guarantees.

Highlights

• Refined Alignment Check: The alignment check within the static_assert! for CastUnsized has been migrated to apply specifically to the slice Dynamically Sized Type (DST) case, rather than being applied unconditionally. This ensures the check is relevant to the specific type of DST being cast.
• Improved Safety Proof Documentation: The safety comment for the unsafe impl<Src, Dst> Project<Src, Dst> for CastUnsized has been updated to provide a more detailed explanation of the conditions under which Src and Dst are considered compatible for casting, clarifying the rationale behind the static_assert!.

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[gemini-code-assist]

Code Review

This pull request tightens the implementation of CastUnsized by moving the alignment check to only apply to slice DSTs and improving the safety proof. The changes look good, but I've found a small discrepancy between the updated safety comment and the implementation for slice DSTs. The comment implies an equality check for alignment to preserve size, while the code uses a greater-than-or-equal check, which could lead to shrinking casts. My suggestion aims to align the code with the documented safety invariants.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.82%. Comparing base (48bf69f) to head (07d042b).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2887   +/-   ##
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  Coverage   91.82%   91.82%           
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  Files          20       20           
  Lines        5883     5883           
=======================================
  Hits         5402     5402           
  Misses        481      481           

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