[WIP] Remove MutationCompatible, Read #2878

joshlf · 2026-01-09T00:00:05Z

MutationCompatible introduces unnecessary indirection and makes many
type signatures needlessly complex. In this commit, we flatten the trait
hierarchy so that TryTransmuteFromPtr has more impls, and
MutationCompatible is removed entirely.

We also remove the Read trait for a similar reason.

Latest Update: v17 — Compare vs v16

📚 Full Patch History

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gemini-code-assist · 2026-01-09T00:00:31Z

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 undertakes a significant internal refactoring of the zerocopy crate's trait system. The core objective is to simplify the codebase by eliminating the MutationCompatible and Read traits, which were identified as sources of excessive indirection and complexity. By flattening the TryTransmuteFromPtr trait hierarchy and streamlining type parameters in various pointer manipulation functions and macros, the changes aim to enhance the clarity, maintainability, and overall design of the crate's low-level memory safety mechanisms.

Highlights

Trait Removal: The MutationCompatible and Read traits have been entirely removed from the codebase, as they introduced unnecessary indirection and complexity in type signatures.
Trait Hierarchy Flattening: The TryTransmuteFromPtr trait hierarchy has been flattened, leading to more direct implementations and simplified type bounds in various functions and methods.
Code Simplification: Numerous Ptr methods (e.g., recall_validity, transmute, cast) and associated macros have been simplified by removing redundant type parameters and complex trait bounds, improving readability and maintainability.
Updated zerocopy-derive Macros: The zerocopy-derive macros have been updated to align with the removed traits, specifically in cast calls and InvariantsEq implementations, ensuring consistency across the crate.
Safety Documentation Updates: New TODO comments have been added to update safety documentation for TryTransmuteFromPtr and to consider unifying Ptr::cast and Ptr::transmute.

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gemini-code-assist

Code Review

This pull request is a significant and welcome refactoring that removes the MutationCompatible and Read traits, simplifying the trait hierarchy and making type signatures less complex. The changes are extensive, touching many parts of the codebase, but they appear to be consistent and correct. The introduction of new TryTransmuteFromPtr impls to replace the old traits is a good approach. I've noticed you've added several TODO comments to track necessary updates to safety documentation, which is great for a work-in-progress PR. I've raised a few review comments to formalize the need to address these TODOs, particularly for the new unsafe implementations, to ensure they are fully justified before this PR is merged.

gemini-code-assist · 2026-01-09T00:02:26Z

src/pointer/ptr.rs

                    unsafe { Ptr::from_inner(inner) })
                })?;

+            // TODO: Update this safety comment for `TryTransmuteFromPtr`.


This TODO is important. The following safety comment is outdated as it refers to the Read trait which has been removed. Please update the safety justification to reflect the new TryTransmuteFromPtr bounds.

gemini-code-assist · 2026-01-09T00:02:26Z

src/pointer/transmute.rs

-// TODO: Maybe use the same reason for both? There shouldn't be any situation in
-// which the reasons are different (one is BecauseExclusive and the other is
-// BecauseImmutable).
+// SAFETY: TODO


Please fill in the safety comment for this unsafe impl. The justification should explain why this implementation of TryTransmuteFromPtr is sound for Exclusive aliasing, considering the preconditions and postconditions of the trait.

gemini-code-assist · 2026-01-09T00:02:26Z

src/pointer/transmute.rs

-// SAFETY: `Src: Read<A, _>` and `Dst: Read<A, _>`.
-unsafe impl<Src: ?Sized, Dst: ?Sized, A: Aliasing, SV: Validity, DV: Validity, R>
-    MutationCompatible<Src, A, SV, DV, (BecauseRead, R)> for Dst
+// SAFETY: TODO


Please fill in the safety comment for this unsafe impl. The justification should explain why this implementation of TryTransmuteFromPtr is sound for BecauseInvariantsEq, considering the InvariantsEq and TransmuteFrom bounds.

codecov-commenter · 2026-01-09T00:04:41Z

Codecov Report

❌ Patch coverage is 96.07843% with 2 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (G27dd847426e3572e87b20bd64642c9fcb0e51ec8@b685399). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
src/util/macros.rs	0.00%	1 Missing ⚠️
src/wrappers.rs	66.66%	1 Missing ⚠️

Additional details and impacted files

@@                             Coverage Diff                              @@
##             G27dd847426e3572e87b20bd64642c9fcb0e51ec8    #2878   +/-   ##
============================================================================
  Coverage                                             ?   91.44%           
============================================================================
  Files                                                ?       20           
  Lines                                                ?     5912           
  Branches                                             ?        0           
============================================================================
  Hits                                                 ?     5406           
  Misses                                               ?      506           
  Partials                                             ?        0

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`MutationCompatible` introduces unnecessary indirection and makes many type signatures needlessly complex. In this commit, we flatten the trait hierarchy so that `TryTransmuteFromPtr` has more impls, and `MutationCompatible` is removed entirely. We also remove the `Read` trait for a similar reason. gherrit-pr-id: G8a3e5c48698ed092b1175cfb42f6f85fb3fb6a34

This was referenced Jan 9, 2026

Rename SizeEq to SizeCompat, make it safe #2872

Open

[WIP] ReadOnly, CastExact #2866

Open

[WIP] Use ReadOnly in is_bit_valid #2873

Open

[derive] Refactor is_bit_valid tests #2875

Closed

[derive] Allow TryFromBytes on non-Immutable unions #2876

Open

joshlf force-pushed the G8a3e5c48698ed092b1175cfb42f6f85fb3fb6a34 branch from 847fe34 to 70fb472 Compare January 9, 2026 00:01

joshlf changed the title ~~[WIP] Remove MutationCompatible~~ [WIP] Remove MutationCompatible, Read Jan 9, 2026

gemini-code-assist bot reviewed Jan 9, 2026

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joshlf force-pushed the G27dd847426e3572e87b20bd64642c9fcb0e51ec8 branch from 050ea06 to 2972d07 Compare January 9, 2026 16:57

joshlf force-pushed the G8a3e5c48698ed092b1175cfb42f6f85fb3fb6a34 branch from 70fb472 to 3c218ea Compare January 9, 2026 16:57

joshlf mentioned this pull request Jan 9, 2026

[derive] Refactor is_bit_valid tests #2879

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joshlf force-pushed the G27dd847426e3572e87b20bd64642c9fcb0e51ec8 branch 2 times, most recently from f5919bc to 9562030 Compare January 9, 2026 17:22

joshlf force-pushed the G8a3e5c48698ed092b1175cfb42f6f85fb3fb6a34 branch from 3c218ea to 741e5f8 Compare January 15, 2026 23:32

joshlf force-pushed the G27dd847426e3572e87b20bd64642c9fcb0e51ec8 branch from 9562030 to 625fa9b Compare January 15, 2026 23:32

joshlf force-pushed the G8a3e5c48698ed092b1175cfb42f6f85fb3fb6a34 branch from 741e5f8 to 91658ad Compare January 16, 2026 00:02

joshlf force-pushed the G27dd847426e3572e87b20bd64642c9fcb0e51ec8 branch 2 times, most recently from 74741ec to a8fdf22 Compare January 16, 2026 00:29

joshlf force-pushed the G8a3e5c48698ed092b1175cfb42f6f85fb3fb6a34 branch 2 times, most recently from 256fa34 to 7a4c315 Compare January 16, 2026 00:35

joshlf force-pushed the G27dd847426e3572e87b20bd64642c9fcb0e51ec8 branch from a8fdf22 to 4314268 Compare January 16, 2026 00:35

joshlf mentioned this pull request Jan 16, 2026

Tighten impl of CastUnsized #2887

Open

joshlf force-pushed the G8a3e5c48698ed092b1175cfb42f6f85fb3fb6a34 branch from 7a4c315 to 88add0a Compare January 16, 2026 00:41

joshlf force-pushed the G27dd847426e3572e87b20bd64642c9fcb0e51ec8 branch from 4314268 to 10d1c38 Compare January 16, 2026 00:41

joshlf force-pushed the G8a3e5c48698ed092b1175cfb42f6f85fb3fb6a34 branch from 88add0a to 4a75a90 Compare January 16, 2026 00:54

joshlf force-pushed the G27dd847426e3572e87b20bd64642c9fcb0e51ec8 branch 2 times, most recently from 6cde490 to 152c38f Compare January 16, 2026 01:01

joshlf force-pushed the G8a3e5c48698ed092b1175cfb42f6f85fb3fb6a34 branch from 4a75a90 to 1f5ac66 Compare January 16, 2026 01:01

joshlf force-pushed the G8a3e5c48698ed092b1175cfb42f6f85fb3fb6a34 branch from 1f5ac66 to d3c8fb4 Compare January 16, 2026 01:14

joshlf force-pushed the G27dd847426e3572e87b20bd64642c9fcb0e51ec8 branch from 152c38f to 4959305 Compare January 16, 2026 01:14

joshlf force-pushed the G8a3e5c48698ed092b1175cfb42f6f85fb3fb6a34 branch from d3c8fb4 to b201ea1 Compare January 16, 2026 19:00

joshlf force-pushed the G27dd847426e3572e87b20bd64642c9fcb0e51ec8 branch from 4959305 to 6f92cae Compare January 16, 2026 19:00

joshlf force-pushed the G8a3e5c48698ed092b1175cfb42f6f85fb3fb6a34 branch from b201ea1 to 62478e2 Compare January 16, 2026 20:37

joshlf force-pushed the G27dd847426e3572e87b20bd64642c9fcb0e51ec8 branch from 6f92cae to 3fb9013 Compare January 16, 2026 20:37

joshlf force-pushed the G8a3e5c48698ed092b1175cfb42f6f85fb3fb6a34 branch from 62478e2 to e6bfe16 Compare January 16, 2026 20:57

joshlf force-pushed the G27dd847426e3572e87b20bd64642c9fcb0e51ec8 branch from 3fb9013 to b685399 Compare January 16, 2026 20:57

joshlf mentioned this pull request Jan 16, 2026

[cast] Add Project::pointer_inner #2894

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[WIP] Remove MutationCompatible, Read #2878

[WIP] Remove MutationCompatible, Read #2878

joshlf commented Jan 9, 2026 •

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