[WIP][derive] Use ReadOnly in is_bit_valid #2874

joshlf · 2026-01-07T23:08:52Z

Introduce `CastExact: Cast`, which denotes that a `Cast` exactly preserves the set of referent bytes. Add this bound to `SizeEq::CastFrom`, allowing `SizeEq` to be safe to implement. gherrit-pr-id: G57ec07c3841271440bbaf40cab04b942cbdbddb9

Previously, we allowed for `Src: Read<A, R>` and `Dst: Read<A, S>` for different `R` and `S` "reason" parameters. This is an unnecessary degree of freedom (two types are only ever `Read<A>` for a particular `A` for the same reason – either because `A = Exclusive` or because both types implement `Immutable`). This degree of freedom required more verbose type annotations by callers. gherrit-pr-id: Gbec926233303ce55c894f8ba65c939dd42690530

gherrit-pr-id: Gbe8d7edd150d80731c79815685c596ed88460ae7

gherrit-pr-id: G7691845b6b02e9f3d9578435d732bacfa6ca674f

gherrit-pr-id: G6cefafff5fb576d45dbb1a303780ca47c3ea03e5

gemini-code-assist · 2026-01-07T23:09:08Z

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 focuses on refactoring the zerocopy-derive crate to incorporate the ReadOnly wrapper type within the is_bit_valid derivation logic. The changes aim to improve type safety and consistency in pointer manipulation by explicitly marking read-only references and standardizing the use of WrappedProjection for field and variant projections, leading to a more robust and idiomatic implementation.

Highlights

Integration of ReadOnly Type: The derive_is_bit_valid function now explicitly uses the ReadOnly wrapper type for casting and projecting enum variants and raw enum structures, enhancing type safety by clearly indicating read-only access.
Transition to WrappedProjection: The Projection type parameter in various pointer casting operations has been consistently replaced with WrappedProjection across derive_is_bit_valid, derive_try_from_bytes_struct, and derive_try_from_bytes_union.
Refined Pointer Casting: Some unsafe blocks around cast_unchecked calls have been removed in favor of cast with ReadOnly, and field projection logic has been updated to use project_transmute_unchecked with WrappedProjection for struct and union fields.

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Code Review

This pull request refactors the is_bit_valid derive logic to use ReadOnly and a new WrappedProjection mechanism. This improves safety by removing an unsafe block in zerocopy-derive/src/enum.rs and makes the projection logic more consistent across the derive implementations. The changes look good, but there are a few places where the old code has been commented out. These should be removed to improve code clarity.

gemini-code-assist · 2026-01-07T23:10:05Z

zerocopy-derive/src/enum.rs

+                // let tag_ptr = raw_enum.reborrow().project::<
+                //     (),
+                //     { #zerocopy_crate::ident_id!(tag) }
+                // >()


This commented-out code appears to be from a previous implementation. Please remove it to improve code clarity.

gemini-code-assist · 2026-01-07T23:10:05Z

zerocopy-derive/src/enum.rs

+                WrappedProjection<_, _, { #zerocopy_crate::STRUCT_VARIANT_ID }, { #zerocopy_crate::ident_id!(variants) }>,
+                _,
+            >();
+            // let variants = raw_enum.project::<_, { #zerocopy_crate::ident_id!(variants) }>();


This commented-out line seems to be leftover from a previous implementation. Please remove it.

gemini-code-assist · 2026-01-07T23:10:05Z

zerocopy-derive/src/lib.rs

+                        // let field_candidate = candidate.reborrow().project::<
+                        //     _,
+                        //     { #zerocopy_crate::ident_id!(#field_names) }
+                        // >();


This commented-out code seems to be from a previous implementation. Please remove it to improve code clarity.

joshlf added 5 commits January 7, 2026 21:22

Make SizeEq safe, introduce CastExact

a6e9a9b

Introduce `CastExact: Cast`, which denotes that a `Cast` exactly preserves the set of referent bytes. Add this bound to `SizeEq::CastFrom`, allowing `SizeEq` to be safe to implement. gherrit-pr-id: G57ec07c3841271440bbaf40cab04b942cbdbddb9

[WIP] ReadOnly

ee75641

gherrit-pr-id: Gbe8d7edd150d80731c79815685c596ed88460ae7

[WIP] Use ReadOnly in is_bit_valid

a353963

gherrit-pr-id: G7691845b6b02e9f3d9578435d732bacfa6ca674f

[WIP][derive] Use ReadOnly in is_bit_valid

6a13458

gherrit-pr-id: G6cefafff5fb576d45dbb1a303780ca47c3ea03e5

This was referenced Jan 7, 2026

Make SizeEq safe, introduce CastExact #2872

Open

Simplify MutationCompatible impls #2867

Closed

[WIP] ReadOnly #2866

Open

[WIP] Use ReadOnly in is_bit_valid #2873

Open

gemini-code-assist bot reviewed Jan 7, 2026

View reviewed changes

joshlf force-pushed the G7691845b6b02e9f3d9578435d732bacfa6ca674f branch 14 times, most recently from e6fab2d to 6a4dcd8 Compare January 9, 2026 17:22

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[WIP][derive] Use ReadOnly in is_bit_valid #2874

[WIP][derive] Use ReadOnly in is_bit_valid #2874

joshlf commented Jan 7, 2026 •

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[WIP][derive] Use ReadOnly in is_bit_valid #2874

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joshlf commented Jan 7, 2026 •

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