1 files changed, 24 insertions, 26 deletions

diff --git a/rust/zerocopy/src/pointer/mod.rs b/rust/zerocopy/src/pointer/mod.rs
index d6eacc52febe..0ce543391530 100644
--- a/rust/zerocopy/src/pointer/mod.rs
+++ b/rust/zerocopy/src/pointer/mod.rs
@@ -25,9 +25,9 @@ pub use transmute::*;
 use crate::wrappers::ReadOnly;
 
 /// A shorthand for a maybe-valid, maybe-aligned reference. Used as the argument
-/// to [`TryFromBytes::is_bit_valid`].
+/// to [`TryFromBytes::is_safe`].
 ///
-/// [`TryFromBytes::is_bit_valid`]: crate::TryFromBytes::is_bit_valid
+/// [`TryFromBytes::is_safe`]: crate::TryFromBytes::is_safe
 pub type Maybe<'a, T, Alignment = invariant::Unaligned> =
     Ptr<'a, ReadOnly<T>, (invariant::Shared, Alignment, invariant::Initialized)>;
 
@@ -47,10 +47,7 @@ where
 pub mod cast {
     use core::{marker::PhantomData, mem};
 
-    use crate::{
-        layout::{SizeInfo, TrailingSliceLayout},
-        HasField, KnownLayout, PtrInner,
-    };
+    use crate::{layout::SizeInfo, HasField, KnownLayout, PtrInner};
 
     /// A pointer cast or projection.
     ///
@@ -182,11 +179,9 @@ pub mod cast {
 
     // SAFETY: By the `static_assert!`, `Src` and `Dst` are either:
     // - Both sized and equal in size
-    // - Both slice DSTs with the same trailing slice offset and element size
-    //   and with align_of::<Src>() == align_of::<Dst>(). These ensure that any
-    //   given pointer metadata encodes the same size for both `Src` and `Dst`
-    //   (note that the alignment is required as it affects the amount of
-    //   trailing padding). Thus, `project` preserves the set of referent bytes.
+    // - Both slice DSTs with the same trailing-slice offset, the same object
+    //   size for every metadata value, and with align_of::<Src>() ==
+    //   align_of::<Dst>(). Thus, `project` preserves the set of referent bytes.
     unsafe impl<Src, Dst> Project<Src, Dst> for CastUnsized
     where
         Src: ?Sized + KnownLayout,
@@ -201,10 +196,11 @@ pub mod cast {
                 let dst = <Dst as KnownLayout>::LAYOUT;
                 match (src.size_info, dst.size_info) {
                     (SizeInfo::Sized { size: src_size }, SizeInfo::Sized { size: dst_size }) => src_size == dst_size,
-                    (
-                        SizeInfo::SliceDst(TrailingSliceLayout { offset: src_offset, elem_size: src_elem_size }),
-                        SizeInfo::SliceDst(TrailingSliceLayout { offset: dst_offset, elem_size: dst_elem_size })
-                    ) => src.align.get() == dst.align.get() && src_offset == dst_offset && src_elem_size == dst_elem_size,
+                    (SizeInfo::SliceDst(src_trailing), SizeInfo::SliceDst(dst_trailing)) => {
+                        src.align.get() == dst.align.get()
+                            && src_trailing.offset == dst_trailing.offset
+                            && src_trailing.has_same_size_sequence(dst_trailing)
+                    },
                     _ => false,
                 }
             });
@@ -225,10 +221,8 @@ pub mod cast {
     // SAFETY: By the `static_assert!` in `Project::project`, `Src` and `Dst`
     // are either:
     // - Both sized and equal in size
-    // - Both slice DSTs with the same alignment, trailing slice offset, and
-    //   element size. These ensure that any given pointer metadata encodes the
-    //   same size for both `Src` and `Dst` (note that the alignment is required
-    //   as it affects the amount of trailing padding).
+    // - Both slice DSTs with the same alignment and trailing-slice offset, and
+    //   with equal sizes for every metadata value.
     unsafe impl<Src, Dst> CastExact<Src, Dst> for CastUnsized
     where
         Src: ?Sized + KnownLayout,
@@ -240,18 +234,22 @@ pub mod cast {
     ///
     /// A `Projection` is a [`Project`] which implements projection by
     /// delegating to an implementation of [`HasField::project`].
+    ///
+    /// The `Client` parameter exists to select between implementations of
+    /// [`HasField`] that would otherwise conflict.
     #[allow(missing_debug_implementations, missing_copy_implementations)]
-    pub struct Projection<F: ?Sized, const VARIANT_ID: i128, const FIELD_ID: i128> {
+    pub struct Projection<Client, F: ?Sized, const VARIANT_ID: i128, const FIELD_ID: i128> {
         _never: core::convert::Infallible,
+        _client: PhantomData<Client>,
         _phantom: PhantomData<F>,
     }
 
     // SAFETY: `HasField::project` has the same safety post-conditions as
     // `Project::project`.
-    unsafe impl<T: ?Sized, F, const VARIANT_ID: i128, const FIELD_ID: i128> Project<T, T::Type>
-        for Projection<F, VARIANT_ID, FIELD_ID>
+    unsafe impl<T: ?Sized, Client, F, const VARIANT_ID: i128, const FIELD_ID: i128>
+        Project<T, T::Type> for Projection<Client, F, VARIANT_ID, FIELD_ID>
     where
-        T: HasField<F, VARIANT_ID, FIELD_ID>,
+        T: HasField<Client, F, VARIANT_ID, FIELD_ID>,
     {
         #[inline(always)]
         fn project(src: PtrInner<'_, T>) -> *mut T::Type {
@@ -292,10 +290,10 @@ pub mod cast {
     //
     // FIXME(https://github.com/rust-lang/unsafe-code-guidelines/issues/595):
     // Cite the documentation once it's updated.
-    unsafe impl<T: ?Sized, F, const FIELD_ID: i128> Cast<T, T::Type>
-        for Projection<F, { crate::REPR_C_UNION_VARIANT_ID }, FIELD_ID>
+    unsafe impl<T: ?Sized, Client, F, const FIELD_ID: i128> Cast<T, T::Type>
+        for Projection<Client, F, { crate::REPR_C_UNION_VARIANT_ID }, FIELD_ID>
     where
-        T: HasField<F, { crate::REPR_C_UNION_VARIANT_ID }, FIELD_ID>,
+        T: HasField<Client, F, { crate::REPR_C_UNION_VARIANT_ID }, FIELD_ID>,
     {
     }