Introduce `ProjectField` trait #2886

jswrenn · 2026-01-14T19:03:40Z

This trait powers invariant-respecting projection over Ptrs.

Latest Update: v21 — Compare vs v20

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gemini-code-assist · 2026-01-14T19:03:56Z

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 the ProjectField trait, a foundational component for enabling safe and invariant-respecting field projection within the system. This trait provides a structured way to access fields of Ptr types while upholding data integrity and safety guarantees, likely serving as an internal mechanism for future derive macros or similar functionalities.

Highlights

New ProjectField Trait: Introduced a new unsafe trait ProjectField designed for invariant-respecting projection over Ptrs. This trait is marked doc(hidden) and includes a method only_derive_is_allowed_to_implement_this_trait, indicating its intended use primarily by derive macros for internal mechanisms.
Trait Definition: The ProjectField trait defines associated types Invariants (for the projected field's pointer's invariants) and Error (for fallible projection), along with a project function that safely projects a field from a Ptr.
Safety Invariants: The trait explicitly states safety requirements for its implementations and for the project function, ensuring that the returned pointer refers to a non-strict subset of the original Ptr's bytes and maintains provenance.

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gemini-code-assist

Code Review

This pull request introduces a new unsafe trait ProjectField to facilitate invariant-respecting projections over Ptrs. The new trait is well-defined and documented. My review includes a suggestion to correct a minor inconsistency in the documentation for the project method, where the parameter name used in the comments differs from the one in the function signature. Aligning these will improve clarity, which is crucial for an unsafe trait.

src/lib.rs

codecov-commenter · 2026-01-14T19:29:05Z

Codecov Report

❌ Patch coverage is 50.00000% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.71%. Comparing base (5c67d2c) to head (7f5262d).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
src/impls.rs	0.00%	12 Missing ⚠️
src/pointer/ptr.rs	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2886      +/-   ##
==========================================
- Coverage   91.90%   91.71%   -0.19%     
==========================================
  Files          20       20              
  Lines        5877     5903      +26     
==========================================
+ Hits         5401     5414      +13     
- Misses        476      489      +13

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src/lib.rs

src/util/macro_util.rs

src/lib.rs

src/pointer/ptr.rs

zerocopy-derive/src/lib.rs

src/lib.rs

zerocopy-derive/src/lib.rs

joshlf · 2026-01-23T16:36:17Z

zerocopy-derive/src/derive/try_from_bytes.rs

-                unsafe { #core::ptr::addr_of_mut!((*slf).#ident) }
+                unsafe { #zerocopy_crate::util::macro_util::core_reexport::ptr::addr_of_mut!((*slf).#ident) }


Revert this?

joshlf · 2026-01-23T16:36:44Z

zerocopy-derive/src/derive/try_from_bytes.rs

+    let fields_preserve_alignment = StructUnionRepr::from_attrs(&ctx.ast.attrs)
+        .map(|repr| repr.get_packed().is_none())
+        .unwrap();


Move this to the else if matches!(&ctx.ast.data, Data::Struct(..)) branch below since it's only used there?

joshlf · 2026-01-23T16:47:05Z

/gemini review

This trait powers invariant-respecting projection over `Ptr`s. gherrit-pr-id: Gb6fa34cec5080caffa0980ed1d3e1ae273550f44

gemini-code-assist

Code Review

The pull request introduces the ProjectField trait, which is a significant and positive change for invariant-respecting projection over Ptrs. This refactoring allows for more granular control and safety guarantees when projecting fields, especially with fallible projections. The introduction of the into_inner! macro for infallible Result unwrapping is also a good addition.

gemini-code-assist · 2026-01-23T16:50:45Z

zerocopy-derive/src/derive/try_from_bytes.rs

+                    invariants: parse_quote!((Aliasing, Alignment, #zerocopy_crate::invariant::Initialized)),
+                },


The ProjectField implementation for enum fields uses (Aliasing, Alignment, #zerocopy_crate::invariant::Initialized) for type Invariants. This means that projecting an enum field will always result in an Initialized pointer, even if the original Ptr had Valid invariants and the field itself is Valid. This could lead to a loss of the stronger Valid invariant, which might be an unnecessary restriction or potentially incorrect if Valid could be preserved.

Suggested change

invariants: parse_quote!((Aliasing, Alignment, #zerocopy_crate::invariant::Initialized)),

},

invariants: parse_quote!((Aliasing, Alignment, I::Validity)),

gemini-code-assist · 2026-01-23T16:50:45Z

zerocopy-derive/src/derive/try_from_bytes.rs

+                // `Initialized`, so too is the validity of its fields.
+                type Invariants = (Aliasing, #alignment, #zerocopy_crate::invariant::Initialized);


Similar to the enum field projection, the ProjectField implementation for struct fields also uses (Aliasing, #alignment, #zerocopy_crate::invariant::Initialized) for type Invariants. This has the same potential issue of losing the Valid invariant if the original Ptr was Valid and the field is also Valid. The invariant propagation should ideally preserve the strongest possible validity.

Suggested change

// `Initialized`, so too is the validity of its fields.

type Invariants = (Aliasing, #alignment, #zerocopy_crate::invariant::Initialized);

type Invariants = (Aliasing, #alignment, I::Validity);

src/impls.rs

src/lib.rs

gemini-code-assist · 2026-01-23T16:50:45Z

src/pointer/ptr.rs

            unsafe { self.cast_unchecked::<_, C>() }
        }

        // FIXME(#196): Support all validity invariants (not just those that are


The FIXME(#196) comment regarding supporting all validity invariants is still present. With the introduction of the ProjectField trait, it might be a good time to re-evaluate if this new abstraction helps address or changes the scope of this long-standing issue.

Please fix.

joshlf · 2026-01-23T16:53:07Z

src/lib.rs

+    /// Is the given field projectable from `ptr`?
+    ///
+    /// This method must be overriden if the field's projectability depends on
+    /// the value of the bytes in `ptr`.


Update these docs (and maybe the name of the name of the method) given the new signature (which returns Ptr in the Ok variant)?

joshlf

Please resolve remaining comments before merging.

This was referenced Jan 14, 2026

Rename HasField::project to HasField::project_raw #2884

Merged

Introduce HasField::project_inner #2885

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jswrenn force-pushed the Gb6fa34cec5080caffa0980ed1d3e1ae273550f44 branch from 7261c75 to 30859f4 Compare January 14, 2026 19:14

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jswrenn force-pushed the G90165d85418ed6203ef3caaa77662dd3b313e030 branch from dc4cfc8 to dbdce80 Compare January 16, 2026 18:18

jswrenn force-pushed the Gb6fa34cec5080caffa0980ed1d3e1ae273550f44 branch 2 times, most recently from 4c71ea4 to 7850e9f Compare January 16, 2026 19:23

jswrenn force-pushed the G90165d85418ed6203ef3caaa77662dd3b313e030 branch from dbdce80 to 16c5cd9 Compare January 16, 2026 19:23

Base automatically changed from G90165d85418ed6203ef3caaa77662dd3b313e030 to main January 16, 2026 20:35

jswrenn force-pushed the Gb6fa34cec5080caffa0980ed1d3e1ae273550f44 branch 9 times, most recently from e9aba20 to 3389089 Compare January 21, 2026 17:41

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jswrenn force-pushed the Gb6fa34cec5080caffa0980ed1d3e1ae273550f44 branch from 3389089 to 0ea7f5e Compare January 21, 2026 18:37

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joshlf mentioned this pull request Jan 21, 2026

Blanket impl ProjectField for struct and union types #2902

Open

jswrenn force-pushed the Gb6fa34cec5080caffa0980ed1d3e1ae273550f44 branch 3 times, most recently from 9ee7b9e to 86ed3f5 Compare January 22, 2026 20:53

jswrenn mentioned this pull request Jan 22, 2026

Rename invariant::Valid to invariant::Safe #2913

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jswrenn force-pushed the Gb6fa34cec5080caffa0980ed1d3e1ae273550f44 branch from 86ed3f5 to 0208eba Compare January 23, 2026 12:39

jswrenn requested a review from joshlf January 23, 2026 12:41

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Introduce ProjectField trait

7f5262d

This trait powers invariant-respecting projection over `Ptr`s. gherrit-pr-id: Gb6fa34cec5080caffa0980ed1d3e1ae273550f44

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jswrenn force-pushed the Gb6fa34cec5080caffa0980ed1d3e1ae273550f44 branch from 0208eba to 7f5262d Compare January 23, 2026 16:54

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		unsafe { #core::ptr::addr_of_mut!((*slf).#ident) }
		unsafe { #zerocopy_crate::util::macro_util::core_reexport::ptr::addr_of_mut!((*slf).#ident) }

		invariants: parse_quote!((Aliasing, Alignment, #zerocopy_crate::invariant::Initialized)),
		},

		// `Initialized`, so too is the validity of its fields.
		type Invariants = (Aliasing, #alignment, #zerocopy_crate::invariant::Initialized);

Introduce ProjectField trait #2886

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Introduce ProjectField trait #2886

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Introduce `ProjectField` trait #2886

Introduce `ProjectField` trait #2886

jswrenn commented Jan 14, 2026 •

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