[WIP][derive] Factor out unsafe impl block building

[joshlf]

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• 👉 [WIP][derive] Factor out unsafe impl block building #2893
• 　 [derive] Factor out core_reexport path generation #2899

Latest Update: v21 — Compare vs v20

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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 zerocopy-derive crate by extracting the logic for building unsafe impl blocks into a dedicated ImplBlockBuilder struct. This change aims to improve the modularity and maintainability of the derive macro implementation by centralizing the generation of common code structures, making the macro code cleaner and more reusable.

Highlights

• New ImplBlockBuilder Struct: Introduced a new ImplBlockBuilder struct in zerocopy-derive/src/util.rs to encapsulate the logic for constructing unsafe impl blocks, centralizing this common code generation pattern.
• Builder Initialization: The ImplBlockBuilder includes a from_derive_input constructor, simplifying its instantiation by taking a Ctx, DeriveInput, and trait Path to extract necessary generic parameters.
• TokenStream Generation: A build method was added to ImplBlockBuilder which generates the proc_macro2::TokenStream representing the complete unsafe impl block, including handling of impl_generics, trait_name, self_name, self_generics, and where_clause.
• Dependency Updates: Updated imports in zerocopy-derive/src/util.rs to include quote macro and additional syn types such as ImplGenerics, TypeGenerics, and WhereClause to support the new builder.

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[gemini-code-assist]

Code Review

This pull request begins the work of refactoring the unsafe impl block generation into a dedicated ImplBlockBuilder. The new builder is a good step towards simplifying the derive macro logic.

My review includes a couple of suggestions for the new ImplBlockBuilder:

• A high-severity comment regarding the type used for trait generics, which appears to be incorrect and could lead to issues. I've also suggested making the builder more flexible by allowing trait generics to be set.
• A medium-severity style suggestion to make the build method more idiomatic and concise.

Overall, this is a promising start to the refactoring effort.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.90%. Comparing base (c97f53b) to head (2e540b1).

Additional details and impacted files

@@                            Coverage Diff                             @@
##           G9c0c3d07b59c78d561a16ab42bfe6eacaadd7156    #2893   +/-   ##
==========================================================================
  Coverage                                      91.90%   91.90%           
==========================================================================
  Files                                             20       20           
  Lines                                           5877     5877           
==========================================================================
  Hits                                            5401     5401           
  Misses                                           476      476           

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