[WIP] Use ReadOnly in is_bit_valid #2873

joshlf · 2026-01-07T21:28:02Z

Latest Update: v44 — Compare vs v43

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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.
✅ Project coverage is 91.44%. Comparing base (fc38019) to head (0c1b442).

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                                      91.39%   91.44%   +0.05%     
=============================================================================
  Files                                             20       20              
  Lines                                           5912     5916       +4     
=============================================================================
+ Hits                                            5403     5410       +7     
+ Misses                                           509      506       -3

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gherrit-pr-id: G7691845b6b02e9f3d9578435d732bacfa6ca674f

This was referenced Jan 7, 2026

Rename SizeEq to SizeCompat, make it safe #2872

Open

Simplify MutationCompatible impls #2867

Closed

[WIP] ReadOnly, CastExact #2866

Open

gemini-code-assist bot reviewed Jan 7, 2026

View reviewed changes

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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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

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joshlf mentioned this pull request Jan 8, 2026

Simplify MutationCompatible impls #2877

Merged

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joshlf mentioned this pull request Jan 9, 2026

[WIP] Remove MutationCompatible, Read #2878

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joshlf mentioned this pull request Jan 16, 2026

Tighten impl of CastUnsized #2887

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joshlf mentioned this pull request Jan 16, 2026

[cast] Add Project::pointer_inner #2894

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Simplify projection methods #2895

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[WIP] Use ReadOnly in is_bit_valid

0c1b442

gherrit-pr-id: G7691845b6b02e9f3d9578435d732bacfa6ca674f

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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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