[derive] Fix IntoBytes on repr(C) DST

[joshlf]

Closes #2835

[gemini-code-assist]

Summary of Changes

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This pull request addresses a critical bug in the zerocopy crate's derive macros, specifically fixing the IntoBytes implementation for repr(C) structs that incorporate Dynamically Sized Types (DSTs). The changes involve refining the internal macro logic and code generation to accurately account for the memory layout of these complex types, thereby ensuring correct trait derivation and preventing compilation failures. The resolution is validated by a new, targeted test case.

Highlights

• Fix for IntoBytes derive on repr(C) DSTs: This pull request resolves an issue where the IntoBytes derive macro would fail for structs marked with #[repr(C)] that contain Dynamically Sized Types (DSTs).
• Macro syntax adjustment for type handling: The internal macros, specifically repr_c_struct_has_padding!, have been updated to correctly process types by wrapping them in parentheses, which improves how slice and unpadded types are handled during layout calculations.
• New test case for DSTs in repr(C) structs: A dedicated test file (issue_2835.rs) has been added to validate the fix, ensuring that #[derive(IntoBytes)] correctly compiles for #[repr(C)] structs containing DSTs like [u8].

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[gemini-code-assist]

Code Review

This pull request provides a solid fix for an issue with deriving IntoBytes on repr(C) DSTs. The core change, which involves updating the repr_c_struct_has_padding! macro to use DstLayout::for_slice, is correct and well-implemented. The associated changes to macro syntax for disambiguation are a standard and effective technique. All modifications across the affected files are consistent, and the inclusion of a new regression test for issue #2835 is excellent practice. The code quality is high, and I have no further suggestions for improvement.

[joshlf]

@jswrenn lmk if you can think of something here – it'd be great to be able to merge these with the following branches rather than just add them so we have branch duplication. I tried to move the parentheses up to the top branch's match, but that didn't work (that said, I didn't try that hard to get it to work). This is very minor, so we can just leave it, but it'd be nice to merge these if it turns out there's something easy we can do.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.14%. Comparing base (f7c9c51) to head (04babb0).
⚠️ Report is 10 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2836   +/-   ##
=======================================
  Coverage   88.14%   88.14%           
=======================================
  Files          20       20           
  Lines        5450     5450           
=======================================
  Hits         4804     4804           
  Misses        646      646           

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