[layout] Support sized->unsized, sized->sized casts

[joshlf]

Makes progress on #2721

---

• 　  Support unsized try_transmute_{ref,mut}! #2944
• 　  Support sized-to-unsized transmute_{ref,mut}! #2943
• 👉 [layout] Support sized->unsized, sized->sized casts #2950

Latest Update: v9 — Compare vs v8

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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 enhances the pointer casting capabilities within the layout module by introducing support for Sized to Unsized and Sized to Sized type conversions. The core change involves a redesign of the CastParams mechanism to explicitly model and compute the necessary parameters for these diverse casting scenarios at compile time, ensuring type safety and correctness. This work directly addresses a long-standing FIXME and generalizes the internal casting infrastructure.

Highlights

• Generalized Pointer Casting: The Cast and CastExact traits now support a broader range of pointer metadata types by removing the PointerMetadata = usize constraint from KnownLayout, allowing for more flexible type conversions.
• Explicit Cast Type Handling: The CastParams structure has been refactored into a generic struct with an enum CastParamsInner to explicitly differentiate and handle UnsizedToUnsized, SizedToUnsized, and SizedToSized pointer casts, improving clarity and correctness.
• Enhanced Metadata Conversion: A new to_elem_count method has been added to the PointerMetadata trait, providing a standardized way to convert pointer metadata into an element count, which is crucial for the new casting logic and its various scenarios.

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[gemini-code-assist]

Code Review

This pull request is a significant and well-executed refactoring that adds support for sized-to-sized and sized-to-unsized casts, complementing the existing support for unsized-to-unsized casts. The changes primarily involve src/layout.rs, where the casting logic is generalized using a new CastParamsInner enum and associated functions. A new to_elem_count method has been added to the PointerMetadata trait in src/lib.rs to support this generalization. The implementation appears correct, robust, and handles the increased complexity effectively. The code is of high quality, and I have no specific comments for improvement.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 18.84058% with 56 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.84%. Comparing base (7dc98fd) to head (9504690).

Files with missing lines	Patch %	Lines
src/layout.rs	15.87%	53 Missing ⚠️
src/lib.rs	50.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2950      +/-   ##
==========================================
- Coverage   92.29%   91.84%   -0.46%     
==========================================
  Files          19       19              
  Lines        5868     5908      +40     
==========================================
+ Hits         5416     5426      +10     
- Misses        452      482      +30     

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[jswrenn]

Let's also try to eventually support unsized to sized casts where the unsized thing has a trailing element size of 0.

[joshlf]

Let's also try to eventually support unsized to sized casts where the unsized thing has a trailing element size of 0.

Filed #2954