Simplify projection methods

[joshlf]

Remove high-level HasField::project_inner, and rename
HasField::project_raw to HasField::project.

The new plan is for pointer::cast::Project and HasField to only have
project methods which project from PtrInner<Src> to *mut Dst, and
for callers who want PtrInner -> PtrInner or Ptr -> Ptr to use
inherent methods on PtrInner and Ptr. Compared to a default-impl'd
method on HasField, this reduces indirection and reduces pressure on
the optimizer.

Reverts #2884, #2885

---

• 　 [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
• 👉 Simplify projection methods #2895

Latest Update: v9 — Compare vs v8

📚 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 refactors the projection mechanisms within the zerocopy crate, focusing on simplifying the HasField and pointer::cast::Project traits. By removing a redundant high-level projection method and standardizing the primary project method to return raw mutable pointers, the change aims to create a more direct and efficient API. This approach delegates more complex pointer transformations to inherent methods on PtrInner and Ptr, thereby reducing indirection and potentially improving optimizer performance.

Highlights

• API Simplification: The high-level HasField::project_inner method has been removed to streamline the API.
• Method Renaming: The HasField::project_raw method has been renamed to HasField::project for consistency and clarity.
• Standardized Projection Output: Both pointer::cast::Project and HasField traits now exclusively feature project methods that return raw mutable pointers (*mut Dst), simplifying the core projection mechanism.
• Improved Performance and Reduced Indirection: Higher-level pointer transformations (e.g., PtrInner -> PtrInner or Ptr -> Ptr) are now handled by inherent methods on PtrInner and Ptr, reducing indirection and optimizing code generation.
• Reversion of Previous Changes: This pull request reverts changes introduced in Rename HasField::project to HasField::project_raw #2884 and Introduce HasField::project_inner #2885, indicating a refinement of the projection strategy.

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[gemini-code-assist]

Code Review

This pull request simplifies projection methods by removing the high-level HasField::project_inner method and renaming HasField::project_raw to HasField::project. The logic from project_inner is now part of inherent methods on PtrInner and Ptr, which reduces indirection. I have reviewed the changes across all modified files, including the trait definitions, implementations, and derive macros. The refactoring has been applied consistently and correctly. The renames are complete, and the removal of project_inner is compensated by the new logic in PtrInner::project. The changes are sound and improve the API's clarity. I have not found any issues.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.90%. Comparing base (709b5b2) to head (a8b0bbc).

Files with missing lines	Patch %	Lines
src/impls.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2895      +/-   ##
==========================================
+ Coverage   91.82%   91.90%   +0.07%     
==========================================
  Files          20       20              
  Lines        5883     5878       -5     
==========================================
  Hits         5402     5402              
+ Misses        481      476       -5     

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