Introduce code generation mechanism

[jswrenn]

---

This PR is on branch fields.

• Introduce code generation mechanism #2844
• Add HasField::project; simplify is_bit_valid #2843
• Introduce HasField trait #2842

[gemini-code-assist]

Summary of Changes

Hello @jswrenn, 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 establishes a code generation framework within the project, specifically to automate the creation of boilerplate trait implementations. The primary application demonstrated is the automatic derivation of the Immutable trait for Rust tuples of varying arities, which previously required manual implementation. This change streamlines the process of extending trait support to common Rust types and refactors the internal derive macro logic for improved flexibility.

Highlights

• New Code Generation Mechanism: A new code generation system has been introduced, primarily through the addition of zerocopy-derive/src/codegen.rs, to automate the creation of boilerplate code.
• Immutable Trait Implementations for Tuples: The new code generation mechanism is immediately utilized to automatically generate Immutable trait implementations for Rust tuples ranging from 1 to 26 elements, located in the new src/generated.rs file.
• Refactored ImplBlockBuilder API: The ImplBlockBuilder in zerocopy-derive has been refactored with a new from_derive_input constructor, simplifying its usage and making it more flexible to handle both named types (structs, enums, unions) and unnamed types (tuples) for trait implementations.

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[gemini-code-assist]

Code Review

This pull request introduces a code generation mechanism to implement Immutable for tuples, which is a great addition. The implementation is cleverly done by refactoring ImplBlockBuilder to be more generic and reusing it within a test function that generates the code. The refactoring across zerocopy-derive to use the new ImplBlockBuilder::from_derive_input constructor and consolidate logic for structs, enums, and unions is a significant improvement, making the code cleaner and more maintainable.

I have one suggestion to improve the developer experience around the code generation test. Otherwise, the changes look solid and well-executed.

[gemini-code-assist]

The current implementation of codegen as a test has the side effect of modifying the source tree whenever cargo test is run. This can be surprising for developers and lead to a dirty worktree.

A more robust pattern is to make the test verify that the generated code is up-to-date, and only update it when an environment variable is set. This makes cargo test a verification step, and updating the code an explicit action. This also improves CI, as it can fail if generated code is not checked in.

I've also replaced unwrap() with expect() to provide more helpful error messages.

fn codegen() {
    let path = {
        use std::path::PathBuf;
        let mut path = PathBuf::from(env!("CARGO_MANIFEST_DIR"));
        assert!(path.pop());
        path.push("src");
        path.push("generated.rs");
        path
    };

    let generated = [HEADER, &tuples()].map(ToString::to_string).join("\n");

    if std::env::var("ZEROCOPY_UPDATE_CODEGEN").is_ok() {
        std::fs::write(&path, generated.as_bytes())
            .expect("failed to write generated code to src/generated.rs");
    } else {
        let existing = std::fs::read_to_string(&path)
            .expect("failed to read existing generated code from src/generated.rs");
        if existing != generated {
            panic!("\n\nGenerated code is out of date. Run `ZEROCOPY_UPDATE_CODEGEN=1 cargo test` to update.\n\n");
        }
    }
}

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.05%. Comparing base (ba063c7) to head (807fb77).

Additional details and impacted files

@@                            Coverage Diff                             @@
##           G9e7039c715e1ec53e6d860909bb0d618898fd46b    #2844   +/-   ##
==========================================================================
  Coverage                                      92.05%   92.05%           
==========================================================================
  Files                                             20       20           
  Lines                                           5812     5812           
==========================================================================
  Hits                                            5350     5350           
  Misses                                           462      462           

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