Introduce `ProjectField` trait #2886

jswrenn · 2026-01-14T19:03:40Z

This trait powers invariant-respecting projection over Ptrs.

Introduce ProjectField trait #2886

Latest Update: v7 — Compare vs v6

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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.

gemini-code-assist · 2026-01-14T19:05:11Z

src/lib.rs

+    /// Projects from `slf` to the field.
+    ///
+    /// # Safety
+    ///
+    /// The returned pointer refers to a non-strict subset of the bytes of
+    /// `slf`'s referent, and has the same provenance as `slf`.


The documentation for project refers to a parameter named slf, but the actual parameter is named ptr. To avoid confusion, especially in a safety-critical unsafe trait, the documentation should be updated to use the correct parameter name ptr.

/// Projects from `ptr` to the field. /// /// # Safety /// /// The returned pointer refers to a non-strict subset of the bytes of /// `ptr`'s referent, and has the same provenance as `ptr`.

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

Codecov Report

❌ Patch coverage is 0% with 26 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.41%. Comparing base (dc4cfc8) to head (105108b).

Files with missing lines	Patch %	Lines
src/lib.rs	0.00%	14 Missing ⚠️
src/impls.rs	0.00%	12 Missing ⚠️

Additional details and impacted files

@@                              Coverage Diff                              @@
##           G90165d85418ed6203ef3caaa77662dd3b313e030    #2886      +/-   ##
=============================================================================
- Coverage                                      91.82%   91.41%   -0.41%     
=============================================================================
  Files                                             20       20              
  Lines                                           5883     5909      +26     
=============================================================================
  Hits                                            5402     5402              
- Misses                                           481      507      +26

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joshlf · 2026-01-15T23:07:30Z

src/lib.rs

+
+    /// TODO
+    // SAFETY: Struct and union projection is infallible.
+    fn is_projectable<'a>(ptr: Ptr<'a, Self, I>) -> Result<Ptr<'a, Self, I>, Self::Error> {


Suggested change

fn is_projectable<'a>(ptr: Ptr<'a, Self, I>) -> Result<Ptr<'a, Self, I>, Self::Error> {

#[inline(always)]

fn is_projectable<'a>(ptr: Ptr<'a, Self, I>) -> Result<Ptr<'a, Self, I>, Self::Error> {

joshlf · 2026-01-15T23:09:34Z

src/lib.rs

+    /// TODO
+    // SAFETY: Struct and union projection is infallible.


Also TODO: Docs should mention that implementers MUST override this for enum types.

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

This was referenced Jan 14, 2026

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

Merged

Introduce HasField::project_inner #2885

Merged

gemini-code-assist bot reviewed Jan 14, 2026

View reviewed changes

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

jswrenn force-pushed the G90165d85418ed6203ef3caaa77662dd3b313e030 branch from bb6f84e to c6f7382 Compare January 14, 2026 19:14

jswrenn force-pushed the G90165d85418ed6203ef3caaa77662dd3b313e030 branch from c6f7382 to dc4cfc8 Compare January 15, 2026 16:02

jswrenn force-pushed the Gb6fa34cec5080caffa0980ed1d3e1ae273550f44 branch 2 times, most recently from c37d1c9 to 105108b Compare January 15, 2026 18:45

joshlf reviewed Jan 15, 2026

View reviewed changes

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

Introduce ProjectField trait

d48e645

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

jswrenn force-pushed the Gb6fa34cec5080caffa0980ed1d3e1ae273550f44 branch from 7850e9f to d48e645 Compare January 16, 2026 20:52

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Introduce `ProjectField` trait #2886

Introduce `ProjectField` trait #2886

jswrenn commented Jan 14, 2026 •

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	fn is_projectable<'a>(ptr: Ptr<'a, Self, I>) -> Result<Ptr<'a, Self, I>, Self::Error> {
	#[inline(always)]
	fn is_projectable<'a>(ptr: Ptr<'a, Self, I>) -> Result<Ptr<'a, Self, I>, Self::Error> {

		/// TODO
		// SAFETY: Struct and union projection is infallible.

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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