[pointer] Add generic projection/cast framework #2860
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Summary of ChangesHello @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 introduces a foundational generic framework for pointer projection and casting within the Highlights
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Code Review
This pull request introduces a significant and well-designed refactoring of the pointer casting and projection framework by adding the Project and Cast traits. This unification simplifies the codebase and makes the casting logic more extensible and type-driven. The changes across the files are consistent with this new design.
My review focuses on the correctness and safety of these new abstractions. I've found a few places where // SAFETY: TODO comments indicate missing safety justifications for unsafe blocks. It's important to fill these in to ensure the soundness of the library. Most notably, I've identified one InvariantsEq implementation in zerocopy-derive/src/enum.rs that appears to be incorrect and could lead to unsoundness. Please address these points to finalize this excellent refactoring.
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Code Review
This pull request introduces a generic framework for pointer projections and casts using the new Project and Cast traits. This is a significant and valuable refactoring that unifies various casting and projection mechanisms throughout the codebase, making them more composable and robust. The changes are extensive, touching many core parts of the pointer manipulation logic.
While the overall direction is excellent, I've found several critical issues related to TODOs in SAFETY comments, particularly concerning potential soundness holes with InvariantsEq implementations for enums and a missing safety justification for a new public project method. These must be addressed to ensure the soundness guarantees of zerocopy are upheld. I've also pointed out a few other places where TODOs in safety comments should be replaced with proper justifications.
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| // [1] Per | ||
| // https://doc.rust-lang.org/1.92.0/reference/type-layout.html#reprc-unions, | ||
| // it's not *technically* guaranteed that non-maximally- | ||
| // sized fields are at offset 0, but it's clear that this is | ||
| // the intention of `repr(C)` unions. It says: |
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For repr(C) unions, this guarantees zero offsets:
Fields might have a non-zero offset (except when the C representation is used); in that case the bits starting at the offset of the fields are read.
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Code Review
This pull request introduces a powerful and generic framework for pointer projections and casts with the Project and Cast traits. This is a significant improvement that unifies various casting mechanisms throughout the codebase, reducing boilerplate and increasing clarity. The use of transitive projections is particularly elegant. The changes are extensive and touch core parts of the library, and the refactoring appears to be very well thought out. I've found a couple of places where safety comments need to be addressed to fully justify the soundness of the new additions.
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Add `unsafe trait Project` and `unsafe trait Cast: Project`. `Cast` is implemented for any address-preserving cast, while `Project` generalizes to conversions which may not preserve the address of the referent (ie, field projections). Use these (mostly `Cast`, but some `Project`) to unify `PtrInner`/`Ptr` casts, field projections, and `SizeEq` casts. Replace a good amount of unsafe derive-generated code with uses of this machinery. Makes progress on #196 Closes #2856 gherrit-pr-id: Gdeb4f5a282b778a82175703218739a93074d0cc4
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Code Review
This is a significant and well-executed refactoring that introduces a generic framework for pointer projections and casts through the new Project and Cast traits. This change successfully unifies various casting mechanisms throughout the codebase, replacing a good amount of unsafe derive-generated code with this new, more robust machinery. The new abstractions are thoughtfully designed and improve the overall structure and safety of pointer manipulations. I have one minor suggestion for code style.
3 participants
Add
unsafe trait Projectandunsafe trait Cast: Project.Castisimplemented for any address-preserving cast, while
Projectgeneralizesto conversions which may not preserve the address of the referent (ie,
field projections). Use these (mostly
Cast, but someProject) tounify
PtrInner/Ptrcasts, field projections, andSizeEqcasts.Replace a good amount of unsafe derive-generated code with uses of this
machinery.
Makes progress on #196
Closes #2856
Latest Update: v16 — Compare vs v15
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