Introduce `HasField` trait #2842

jswrenn · 2025-12-03T22:00:34Z

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.

This PR is on branch fields.

gemini-code-assist · 2025-12-03T22:00:59Z

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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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.

zerocopy-derive/src/lib.rs

codecov-commenter · 2025-12-04T15:28:08Z

Codecov Report

❌ Patch coverage is 0% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.05%. Comparing base (60f4102) to head (bced22a).

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

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

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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. [scoped trait pattern]: https://jack.wrenn.fyi/blog/private-trait-impls/ gherrit-pr-id: G2238bc341570838db412d880017b3f0c25ac09fa

joshlf · 2025-12-04T17:57:22Z

zerocopy-derive/src/enum.rs

+                    // Since Rust does not presently support explicit visibility
+                    // modifiers on enum fields, any public type is suitable
+                    // here; we use `()`.
+                    field: parse_quote!(()),


Feel free to disregard, but it might be worth considering whether we could put this in a const or static or something to avoid recomputing it for every field.

joshlf · 2025-12-04T18:01:23Z

zerocopy-derive/src/lib.rs

-                FieldBounds::TRAILING_SELF
-            };
+            let require_trait_bound_on_field_types =
+                if matches!(self_bounds, SelfBounds::All(&[Trait::Sized])) {


Is this a breaking change?

joshlf · 2025-12-04T18:02:49Z

zerocopy-derive/src/lib.rs

+
+    if fields.is_empty() {
+        return quote! {};
+    }


Style uber-nit; feel free to disregard.

Suggested change

if fields.is_empty() {

return quote! {};

}

if fields.is_empty() {

return quote! {};

}

zerocopy-derive/src/lib.rs

joshlf · 2025-12-04T18:04:54Z

zerocopy-derive/src/lib.rs

+    });
+
+    let has_fields = fields.iter().map(|(_, ident, ty)| {
+        let field_token = Ident::new(&format!("ẕ{}", ident), ident.span());


Same here re: __Zerocopy_ prefix.

joshlf · 2025-12-04T18:05:56Z

zerocopy-derive/src/lib.rs

+            ast,
+            data,
+            Trait::HasField {
+                variant_id: parse_quote!({ #zerocopy_crate::ident_id!(0) }),


If this is for a variant, why is it always 0? Shouldn't it be the ID of the variant in question?

joshlf · 2025-12-04T18:09:59Z

zerocopy-derive/tests/ui-nightly/privacy.rs

@@ -0,0 +1,102 @@
+// Copyright 2019 The Fuchsia Authors


Suggested change

// Copyright 2019 The Fuchsia Authors

// Copyright 2025 The Fuchsia Authors

joshlf · 2025-12-04T18:10:33Z

zerocopy-derive/tests/ui-nightly/privacy.rs

+// These tests cause errors which are generated by a later compilation pass than
+// the other errors we generate, and so if they're compiled in the same file,
+// the compiler will never get to that pass, and so we won't get the errors.


That comment only makes sense in the late_compile_pass.rs test IMO. Is it true for this file too?

gemini-code-assist bot reviewed Dec 3, 2025

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zerocopy-derive/src/lib.rs Outdated Show resolved Hide resolved

jswrenn force-pushed the G2238bc341570838db412d880017b3f0c25ac09fa branch from 9526d5d to 2b6d45e Compare December 4, 2025 15:24

jswrenn force-pushed the G2238bc341570838db412d880017b3f0c25ac09fa branch from 2b6d45e to e5dae4c Compare December 4, 2025 17:27

jswrenn changed the title ~~introduce HasField~~ Introduce HasField trait Dec 4, 2025

jswrenn force-pushed the G2238bc341570838db412d880017b3f0c25ac09fa branch 5 times, most recently from cff59b0 to e738a09 Compare December 4, 2025 17:44

jswrenn force-pushed the G2238bc341570838db412d880017b3f0c25ac09fa branch from e738a09 to bced22a Compare December 4, 2025 17:47

joshlf requested changes Dec 4, 2025

View reviewed changes

This was referenced Dec 4, 2025

Add HasField::project; simplify is_bit_valid #2843

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Introduce `HasField` trait #2842

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	// Copyright 2019 The Fuchsia Authors
	// Copyright 2025 The Fuchsia Authors

Introduce HasField trait #2842

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Introduce `HasField` trait #2842

Introduce `HasField` trait #2842

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