[WIP] Use ReadOnly in is_bit_valid #2873

joshlf · 2026-01-07T21:28:02Z

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gemini-code-assist · 2026-01-07T21:28:30Z

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 introduces a significant refactoring to consistently use ReadOnly views when performing bit validity checks across the codebase. By updating the Maybe type alias and modifying various macros and trait implementations, the change ensures that is_bit_valid functions operate on immutable data, enhancing type safety and correctness. This also streamlines the implementation of validity checks for types like UnsafeCell and atomic types, making the system more robust.

Highlights

is_bit_valid Refactoring: The is_bit_valid function for UnsafeCell<T> and within the impl_for_transmute_from! macro has been simplified to directly use candidate.transmute() instead of into_exclusive_or_pme() and get_mut().
Maybe Type Alias Update: The Maybe type alias, which defines the argument type for TryFromBytes::is_bit_valid, now uses ReadOnly<T> instead of T, ensuring that bit validity checks operate on immutable views.
Macro Enhancements for ReadOnly: The impl_size_eq! and unsafe_impl_for_transparent_wrapper! macros have been extended to correctly handle ReadOnly types, including new SizeEq implementations and casting mechanisms.
Atomic Type Trait Refinement: The macros for implementing traits for atomic types have been split into impl_layout_traits_for_atomics! and impl_validity_traits_for_atomics!, providing a clearer separation of concerns for layout and validity traits, and incorporating the new impl_size_eq! syntax.

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gemini-code-assist

Code Review

This pull request introduces a ReadOnly wrapper to the Maybe type alias, which is used in is_bit_valid checks. This is a significant safety improvement, as it enforces at the type level that these checks do not perform mutations. The changes correctly propagate this new approach through various macros and implementations, and the refactoring of some macros for better clarity is appreciated. My review focuses on cleaning up some leftover commented-out code from this refactoring effort.

gemini-code-assist · 2026-01-07T21:29:41Z

src/impls.rs

+        // // The only way to implement this function is using an exclusive-aliased
+        // // pointer. `UnsafeCell`s cannot be read via shared-aliased pointers
+        // // (other than by using `unsafe` code, which we can't use since we can't
+        // // guarantee how our users are accessing or modifying the `UnsafeCell`).
+        // //
+        // // `is_bit_valid` is documented as panicking or failing to monomorphize
+        // // if called with a shared-aliased pointer on a type containing an
+        // // `UnsafeCell`. In practice, it will always be a monomorphization error.
+        // // Since `is_bit_valid` is `#[doc(hidden)]` and only called directly
+        // // from this crate, we only need to worry about our own code incorrectly
+        // // calling `UnsafeCell::is_bit_valid`. The post-monomorphization error
+        // // makes it easier to test that this is truly the case, and also means
+        // // that if we make a mistake, it will cause downstream code to fail to
+        // // compile, which will immediately surface the mistake and give us a
+        // // chance to fix it quickly.
+        // let c = candidate.into_exclusive_or_pme();
+
+        // // SAFETY: Since `UnsafeCell<T>` and `T` have the same layout and bit
+        // // validity, `UnsafeCell<T>` is bit-valid exactly when its wrapped `T`
+        // // is. Thus, this is a sound implementation of
+        // // `UnsafeCell::is_bit_valid`.
+        // T::is_bit_valid(c.get_mut())


This large block of commented-out code appears to be the old implementation. To improve readability and maintainability, it would be great to remove it before this pull request is merged.

gemini-code-assist · 2026-01-07T21:29:41Z

src/pointer/ptr.rs

+    // /// `Ptr<'a, T, (_, _, _)>` → `Ptr<'a, ReadOnly<T>, (_, _, _)>`
+    // impl<'a, T, I> Ptr<'a, T, I>
+    // where
+    //     T: ?Sized,
+    //     I: Invariants,
+    // {
+    //     /// TODO
+    //     pub(crate) fn into_read_only<R>(
+    //         self,
+    //     ) -> Ptr<'a, crate::ReadOnly<T>, (I::Aliasing, I::Alignment, I::Validity)>
+    //     where
+    //         T: Read<I::Aliasing, R>,
+    //     {
+    //         let ro = self.transmute::<_, _, (_, _)>();
+    //         unsafe { ro.assume_alignment() }
+    //     }
+    // }


This commented-out implementation of into_read_only seems to be unused. To keep the codebase clean, it would be best to remove it if it's not part of this change.

gemini-code-assist · 2026-01-07T21:29:41Z

src/util/macro_util.rs

    let ptr: Ptr<'_, Dst, _> = ptr.cast::<_, crate::pointer::cast::CastSized, _>();

-    if Dst::is_bit_valid(ptr.forget_aligned()) {
+    if Dst::is_bit_valid(ptr.transmute::<_, _, (_, (_, BecauseExclusive))>().forget_aligned()) {


The call to .forget_aligned() here seems redundant. The transmute() method on Ptr already returns a Ptr with Unaligned alignment, so forget_aligned() is a no-op in this context. Removing it would make the code slightly cleaner.

Suggested change

if Dst::is_bit_valid(ptr.transmute::<_, _, (_, (_, BecauseExclusive))>().forget_aligned()) {

if Dst::is_bit_valid(ptr.transmute::<_, _, (_, (_, BecauseExclusive))>()) {

gemini-code-assist · 2026-01-07T21:29:42Z

src/util/macros.rs

        fn is_bit_valid<A: crate::pointer::invariant::Reference>(candidate: Maybe<'_, Self, A>) -> bool {
-            let c: Maybe<'_, Self, crate::pointer::invariant::Exclusive> = candidate.into_exclusive_or_pme();
-            let c: Maybe<'_, $repr, _> = c.transmute::<_, _, (_, (_, BecauseExclusive))>();
+            // let c: Maybe<'_, Self, crate::pointer::invariant::Exclusive> = candidate.into_exclusive_or_pme();


This commented-out line can be removed to clean up the code.

gemini-code-assist · 2026-01-07T21:29:42Z

src/wrappers.rs

+// enum BecauseReadOnly {}
+
+// /// Denotes that `src: Ptr<Src, (A, _, SV)>` and `dst: Ptr<Self, (A, _, DV)>`,
+// /// referencing the same referent at the same time, cannot be used by safe code
+// /// to break library safety invariants of `Src` or `Self`.
+// ///
+// /// # Safety
+// ///
+// /// At least one of the following must hold:
+// /// - `Src: Read<A, _>` and `Self: Read<A, _>`
+// /// - `Self: InvariantsEq<Src>`, and, for some `V`:
+// ///   - `Dst: TransmuteFrom<Src, V, V>`
+// ///   - `Src: TransmuteFrom<Dst, V, V>`
+
+// // SAFETY: TODO
+// unsafe impl<T: ?Sized, A: Aliasing, V> MutationCompatible<T, A, V, V, BecauseReadOnly>
+//     for ReadOnly<T>
+// {
+// }


This block of commented-out code seems to be a work-in-progress idea. If it's not intended to be part of this pull request, it would be best to remove it to avoid cluttering the codebase.

codecov-commenter · 2026-01-08T14:23:59Z

Codecov Report

❌ Patch coverage is 94.59459% with 4 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (Gbe8d7edd150d80731c79815685c596ed88460ae7@3c09949). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
src/macros.rs	0.00%	2 Missing ⚠️
src/util/macros.rs	66.66%	2 Missing ⚠️

Additional details and impacted files

@@                             Coverage Diff                              @@
##             Gbe8d7edd150d80731c79815685c596ed88460ae7    #2873   +/-   ##
============================================================================
  Coverage                                             ?   91.51%           
============================================================================
  Files                                                ?       20           
  Lines                                                ?     5919           
  Branches                                             ?        0           
============================================================================
  Hits                                                 ?     5417           
  Misses                                               ?      502           
  Partials                                             ?        0

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gherrit-pr-id: G7691845b6b02e9f3d9578435d732bacfa6ca674f

This was referenced Jan 7, 2026

Make SizeEq safe, introduce CastExact #2872

Open

Simplify MutationCompatible impls #2867

Closed

[WIP] ReadOnly #2866

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gemini-code-assist bot reviewed Jan 7, 2026

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joshlf force-pushed the G7691845b6b02e9f3d9578435d732bacfa6ca674f branch from 81ada6e to a353963 Compare January 7, 2026 23:08

joshlf force-pushed the Gbe8d7edd150d80731c79815685c596ed88460ae7 branch from ad978ce to ee75641 Compare January 7, 2026 23:08

joshlf mentioned this pull request Jan 7, 2026

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

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joshlf force-pushed the G7691845b6b02e9f3d9578435d732bacfa6ca674f branch 3 times, most recently from 09a6d35 to 6ac888a Compare January 8, 2026 14:21

joshlf force-pushed the Gbe8d7edd150d80731c79815685c596ed88460ae7 branch from ee75641 to 6539cee Compare January 8, 2026 15:26

joshlf force-pushed the G7691845b6b02e9f3d9578435d732bacfa6ca674f branch from 6ac888a to 1b4834d Compare January 8, 2026 15:26

joshlf force-pushed the Gbe8d7edd150d80731c79815685c596ed88460ae7 branch from 6539cee to e2f1bc3 Compare January 8, 2026 15:36

joshlf force-pushed the G7691845b6b02e9f3d9578435d732bacfa6ca674f branch 2 times, most recently from c83b71f to db5ec29 Compare January 8, 2026 15:58

joshlf force-pushed the Gbe8d7edd150d80731c79815685c596ed88460ae7 branch from e2f1bc3 to 45ff87c Compare January 8, 2026 15:58

joshlf force-pushed the G7691845b6b02e9f3d9578435d732bacfa6ca674f branch from db5ec29 to 845c5e7 Compare January 8, 2026 16:54

This was referenced Jan 8, 2026

[derive] Refactor is_bit_valid tests #2875

Closed

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

Open

joshlf force-pushed the Gbe8d7edd150d80731c79815685c596ed88460ae7 branch from 45ff87c to d1f62db Compare January 8, 2026 19:34

joshlf force-pushed the G7691845b6b02e9f3d9578435d732bacfa6ca674f branch 3 times, most recently from b6642a7 to d0f57ea Compare January 8, 2026 19:50

joshlf force-pushed the Gbe8d7edd150d80731c79815685c596ed88460ae7 branch from d1f62db to 65f15b8 Compare January 8, 2026 19:50

joshlf mentioned this pull request Jan 8, 2026

Simplify MutationCompatible impls #2877

Merged

joshlf force-pushed the Gbe8d7edd150d80731c79815685c596ed88460ae7 branch from 65f15b8 to 318b671 Compare January 9, 2026 00:00

joshlf force-pushed the G7691845b6b02e9f3d9578435d732bacfa6ca674f branch from d0f57ea to 6cbe1ca Compare January 9, 2026 00:00

joshlf mentioned this pull request Jan 9, 2026

[WIP] Remove MutationCompatible, Read #2878

Open

joshlf force-pushed the Gbe8d7edd150d80731c79815685c596ed88460ae7 branch from 318b671 to 9230a53 Compare January 9, 2026 16:57

joshlf force-pushed the G7691845b6b02e9f3d9578435d732bacfa6ca674f branch from 6cbe1ca to 5cf70a2 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 G7691845b6b02e9f3d9578435d732bacfa6ca674f branch from 5cf70a2 to e6fab2d Compare January 9, 2026 17:04

[WIP] Use ReadOnly in is_bit_valid

6a4dcd8

gherrit-pr-id: G7691845b6b02e9f3d9578435d732bacfa6ca674f

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

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[WIP] Use ReadOnly in is_bit_valid #2873

[WIP] Use ReadOnly in is_bit_valid #2873

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	if Dst::is_bit_valid(ptr.transmute::<_, _, (_, (_, BecauseExclusive))>().forget_aligned()) {
	if Dst::is_bit_valid(ptr.transmute::<_, _, (_, (_, BecauseExclusive))>()) {

[WIP] Use ReadOnly in is_bit_valid #2873

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[WIP] Use ReadOnly in is_bit_valid #2873

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