[project] Distinguish structs/union/enums

[joshlf]

For the HasField VARIANT_ID and FIELD_ID constants, use i128
instead of u128. For structs and unions, set VARIANT_ID = -1 and
VARIANT_ID = -2 (respectively) instead of 0. This ensures that we will
never accidentally confuse the first (ie, 0th) variant of an enum with a
struct or union. It also paves the way for us to define methods which
only support a subset of structs/unions/enums.

Makes progress on #196, #2856

---

• [pointer] Add generic projection/cast framework #2854
• [project] Distinguish structs/union/enums #2857

Latest Update: v7 — Compare vs v6

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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 refines the internal type identification mechanism within the zerocopy crate by transitioning to signed 128-bit integers for variant and field IDs. By assigning a distinct negative identifier (-1) to structs and unions, the change eliminates potential ambiguities with enum variant IDs and establishes a clearer, more robust type distinction. This enhancement not only improves the current type safety but also enables more specialized type-handling capabilities for future development.

Highlights

• Type System Refinement: The VARIANT_ID and FIELD_ID constants used in the HasField trait have been changed from u128 (unsigned 128-bit integer) to i128 (signed 128-bit integer) across the codebase.
• Struct/Union Identification: A new constant, STRUCT_UNION_VARIANT_ID, has been introduced and set to -1. This value is now used to uniquely identify structs and unions within the HasField trait implementations, replacing the previous use of 0.
• Ambiguity Resolution: This change prevents accidental confusion between the 0th variant of an enum and a struct or union, as 0 can now exclusively represent the first variant of an enum, while -1 is reserved for structs and unions.
• Future Extensibility: The modification paves the way for defining methods that specifically target and support only struct/union types, without interfering with enum-specific logic.

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[gemini-code-assist]

Code Review

This pull request is a solid improvement to the library's design. Changing VARIANT_ID and FIELD_ID in the HasField trait to use i128 and reserving -1 as the VARIANT_ID for structs and unions is a smart move to prevent type confusion with enum variants. The changes are consistently applied throughout the codebase, including the derive macros. I've reviewed the implementation, and it appears correct and well-executed.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 25.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.94%. Comparing base (77f1c47) to head (1c5cebf).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/util/macro_util.rs	0.00%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2857   +/-   ##
=======================================
  Coverage   91.94%   91.94%           
=======================================
  Files          20       20           
  Lines        5824     5824           
=======================================
  Hits         5355     5355           
  Misses        469      469           

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