Introduce HasField trait

[jswrenn]

The HasField trait permits privacy-preserving field projections
from structs, enums and unions.

HasField preserves privacy by leveraging the scoped trait pattern;
for each field f, HasField is implemented with the Field parameter
set to a type with identical visibility as f. In many useful contexts,
this Field parameter is inferable, and need not be actually reachable.
Consequently, HasField can be used to index a field from only the name
of a field. See zerocopy-derive/tests/ui-nightly/privacy.rs for
examples.

---

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

Latest Update: v2 — Compare vs v1

📚 Full Patch History

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Version	Base	v1
v2	vs Base	vs v1
v1	vs Base

[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 introduces a foundational HasField trait and its corresponding derive macro, significantly enhancing the zerocopy crate's capabilities for compile-time introspection of struct fields. This new mechanism provides a structured way to query field metadata such as types, offsets, and unique identifiers, which is essential for advanced compile-time analysis and manipulation of data structures. The changes also include several internal improvements to macro utilities, streamlining offset calculations and refining trait handling within the derive system.

Highlights

• New HasField Trait: Introduced an unsafe trait HasField that allows compile-time querying of a struct's field type, offset, and a unique FIELD_ID (a u128 hash). This trait is marked #[doc(hidden)] and is intended for internal derive macro usage.
• Derive Macro for HasField: A new derive macro has been implemented to automatically generate HasField implementations for struct fields. This involves creating unique, hidden enum types for each field and calculating their respective offsets and IDs.
• Field ID Hashing Mechanism: A new ident_id! macro and a hash_name const function were added to generate stable u128 identifiers for fields. This hashing is designed to be deterministic within a compilation and is crucial for the FIELD_ID constant in the HasField trait.
• Improved trailing_field_offset! Macro: The trailing_field_offset! macro has been refined to directly return a usize value instead of an Option<usize>, indicating a more robust and guaranteed calculation of field offsets. It also now uses fully qualified paths for internal constants.
• Internal Derive Macro Refinements: Various internal improvements were made to the derive macro system, including using matches! for enum comparisons, updating the Trait enum to include HasField with generic arguments, and adjusting trait bound normalization to handle cloning of Trait values.

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[gemini-code-assist]

Code Review

This pull request introduces the HasField trait, which allows compile-time reflection on struct fields. The implementation includes a new derive macro for HasField and a compile-time hashing function for field identifiers. The changes are well-structured, but I found a bug in the ToTokens implementation for the Trait enum that will cause compilation failures for existing derives. My review includes a suggestion to fix this.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.05%. Comparing base (bca589a) to head (bce4eeb).

Files with missing lines	Patch %	Lines
src/util/macro_util.rs	0.00%	18 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2842      +/-   ##
==========================================
- Coverage   92.33%   92.05%   -0.29%     
==========================================
  Files          20       20              
  Lines        5794     5812      +18     
==========================================
  Hits         5350     5350              
- Misses        444      462      +18     

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