selftests/bpf: Reject graph kptr use after RCU unlockgrafted

Add a sleepable verifier test that loads a graph-node local kptr in an
explicit RCU read-side critical section, then passes its node to
bpf_rbtree_remove() after the section ends.

Before the verifier fix, the stale NON_OWN_REF flag makes the node look like
a live borrowed reference and the program is accepted. After the fix, the
pointer is demoted without NON_OWN_REF and the graph kfunc argument is
rejected.

Also exercise a graph kptr loaded while a spin lock provides implicit RCU
protection. The pointer must be invalidated when the lock is released, which
guards the required ordering between non-owning-reference invalidation and
RCU demotion.

Update the existing fault-protected load test state description. The
post-unlock pointer no longer carries NON_OWN_REF, but remains readable
because the load is rewritten to use BPF_PROBE_MEM.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20260904084325.52250-7-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 1358 insertions, 0 deletions

diff --git a/rust/zerocopy/src/ref.rs b/rust/zerocopy/src/ref.rs
new file mode 100644
index 000000000..e49f2a887
--- /dev/null
+++ b/rust/zerocopy/src/ref.rs
@@ -0,0 +1,1358 @@
+// SPDX-License-Identifier: (BSD-2-Clause OR Apache-2.0) OR MIT
+//
+// Copyright 2024 The Fuchsia Authors
+//
+// Licensed under the 2-Clause BSD License <LICENSE-BSD or
+// https://opensource.org/license/bsd-2-clause>, Apache License, Version 2.0
+// <LICENSE-APACHE or https://www.apache.org/licenses/LICENSE-2.0>, or the MIT
+// license <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your option.
+// This file may not be copied, modified, or distributed except according to
+// those terms.
+use super::*;
+use crate::pointer::{
+    BecauseInvariantsEq, BecauseMutationCompatible, MutationCompatible, TransmuteFromPtr,
+};
+
+mod def {
+    use core::marker::PhantomData;
+
+    use crate::{
+        ByteSlice, ByteSliceMut, CloneableByteSlice, CopyableByteSlice, IntoByteSlice,
+        IntoByteSliceMut,
+    };
+
+    /// A typed reference derived from a byte slice.
+    ///
+    /// A `Ref<B, T>` is a reference to a `T` which is stored in a byte slice, `B`.
+    /// Unlike a native reference (`&T` or `&mut T`), `Ref<B, T>` has the same
+    /// mutability as the byte slice it was constructed from (`B`).
+    ///
+    /// # Examples
+    ///
+    /// `Ref` can be used to treat a sequence of bytes as a structured type, and
+    /// to read and write the fields of that type as if the byte slice reference
+    /// were simply a reference to that type.
+    ///
+    /// ```rust
+    /// use zerocopy::*;
+    /// # use zerocopy_derive::*;
+    ///
+    /// #[derive(FromBytes, IntoBytes, KnownLayout, Immutable, Unaligned)]
+    /// #[repr(C)]
+    /// struct UdpHeader {
+    ///     src_port: [u8; 2],
+    ///     dst_port: [u8; 2],
+    ///     length: [u8; 2],
+    ///     checksum: [u8; 2],
+    /// }
+    ///
+    /// #[derive(FromBytes, IntoBytes, KnownLayout, Immutable, Unaligned)]
+    /// #[repr(C, packed)]
+    /// struct UdpPacket {
+    ///     header: UdpHeader,
+    ///     body: [u8],
+    /// }
+    ///
+    /// impl UdpPacket {
+    ///     pub fn parse<B: ByteSlice>(bytes: B) -> Option<Ref<B, UdpPacket>> {
+    ///         Ref::from_bytes(bytes).ok()
+    ///     }
+    /// }
+    /// ```
+    pub struct Ref<B, T: ?Sized>(
+        // INVARIANTS: The referent (via `.deref`, `.deref_mut`, `.into`) byte
+        // slice is aligned to `T`'s alignment and its size corresponds to a
+        // valid size for `T`.
+        B,
+        PhantomData<T>,
+    );
+
+    impl<B, T: ?Sized> Ref<B, T> {
+        /// Constructs a new `Ref`.
+        ///
+        /// # Safety
+        ///
+        /// `bytes` dereferences (via [`deref`], [`deref_mut`], and [`into`]) to
+        /// a byte slice which is aligned to `T`'s alignment and whose size is a
+        /// valid size for `T`.
+        ///
+        /// [`deref`]: core::ops::Deref::deref
+        /// [`deref_mut`]: core::ops::DerefMut::deref_mut
+        /// [`into`]: core::convert::Into::into
+        pub(crate) unsafe fn new_unchecked(bytes: B) -> Ref<B, T> {
+            // INVARIANTS: The caller has promised that `bytes`'s referent is
+            // validly-aligned and has a valid size.
+            Ref(bytes, PhantomData)
+        }
+    }
+
+    impl<B: ByteSlice, T: ?Sized> Ref<B, T> {
+        /// Access the byte slice as a [`ByteSlice`].
+        ///
+        /// # Safety
+        ///
+        /// The caller promises not to call methods on the returned
+        /// [`ByteSlice`] other than `ByteSlice` methods (for example, via
+        /// `Any::downcast_ref`).
+        ///
+        /// `as_byte_slice` promises to return a `ByteSlice` whose referent is
+        /// validly-aligned for `T` and has a valid size for `T`.
+        pub(crate) unsafe fn as_byte_slice(&self) -> &impl ByteSlice {
+            // INVARIANTS: The caller promises not to call methods other than
+            // those on `ByteSlice`. Since `B: ByteSlice`, dereference stability
+            // guarantees that calling `ByteSlice` methods will not change the
+            // address or length of `self.0`'s referent.
+            //
+            // SAFETY: By invariant on `self.0`, the alignment and size
+            // post-conditions are upheld.
+            &self.0
+        }
+    }
+
+    impl<B: ByteSliceMut, T: ?Sized> Ref<B, T> {
+        /// Access the byte slice as a [`ByteSliceMut`].
+        ///
+        /// # Safety
+        ///
+        /// The caller promises not to call methods on the returned
+        /// [`ByteSliceMut`] other than `ByteSliceMut` methods (for example, via
+        /// `Any::downcast_mut`).
+        ///
+        /// `as_byte_slice` promises to return a `ByteSlice` whose referent is
+        /// validly-aligned for `T` and has a valid size for `T`.
+        pub(crate) unsafe fn as_byte_slice_mut(&mut self) -> &mut impl ByteSliceMut {
+            // INVARIANTS: The caller promises not to call methods other than
+            // those on `ByteSliceMut`. Since `B: ByteSlice`, dereference
+            // stability guarantees that calling `ByteSlice` methods will not
+            // change the address or length of `self.0`'s referent.
+            //
+            // SAFETY: By invariant on `self.0`, the alignment and size
+            // post-conditions are upheld.
+            &mut self.0
+        }
+    }
+
+    impl<'a, B: IntoByteSlice<'a>, T: ?Sized> Ref<B, T> {
+        /// Access the byte slice as an [`IntoByteSlice`].
+        ///
+        /// # Safety
+        ///
+        /// The caller promises not to call methods on the returned
+        /// [`IntoByteSlice`] other than `IntoByteSlice` methods (for example,
+        /// via `Any::downcast_ref`).
+        ///
+        /// `as_byte_slice` promises to return a `ByteSlice` whose referent is
+        /// validly-aligned for `T` and has a valid size for `T`.
+        pub(crate) unsafe fn into_byte_slice(self) -> impl IntoByteSlice<'a> {
+            // INVARIANTS: The caller promises not to call methods other than
+            // those on `IntoByteSlice`. Since `B: ByteSlice`, dereference
+            // stability guarantees that calling `ByteSlice` methods will not
+            // change the address or length of `self.0`'s referent.
+            //
+            // SAFETY: By invariant on `self.0`, the alignment and size
+            // post-conditions are upheld.
+            self.0
+        }
+    }
+
+    impl<'a, B: IntoByteSliceMut<'a>, T: ?Sized> Ref<B, T> {
+        /// Access the byte slice as an [`IntoByteSliceMut`].
+        ///
+        /// # Safety
+        ///
+        /// The caller promises not to call methods on the returned
+        /// [`IntoByteSliceMut`] other than `IntoByteSliceMut` methods (for
+        /// example, via `Any::downcast_mut`).
+        ///
+        /// `as_byte_slice` promises to return a `ByteSlice` whose referent is
+        /// validly-aligned for `T` and has a valid size for `T`.
+        pub(crate) unsafe fn into_byte_slice_mut(self) -> impl IntoByteSliceMut<'a> {
+            // INVARIANTS: The caller promises not to call methods other than
+            // those on `IntoByteSliceMut`. Since `B: ByteSlice`, dereference
+            // stability guarantees that calling `ByteSlice` methods will not
+            // change the address or length of `self.0`'s referent.
+            //
+            // SAFETY: By invariant on `self.0`, the alignment and size
+            // post-conditions are upheld.
+            self.0
+        }
+    }
+
+    impl<B: CloneableByteSlice + Clone, T: ?Sized> Clone for Ref<B, T> {
+        #[inline]
+        fn clone(&self) -> Ref<B, T> {
+            // INVARIANTS: Since `B: CloneableByteSlice`, `self.0.clone()` has
+            // the same address and length as `self.0`. Since `self.0` upholds
+            // the field invariants, so does `self.0.clone()`.
+            Ref(self.0.clone(), PhantomData)
+        }
+    }
+
+    // INVARIANTS: Since `B: CopyableByteSlice`, the copied `Ref`'s `.0` has the
+    // same address and length as the original `Ref`'s `.0`. Since the original
+    // upholds the field invariants, so does the copy.
+    impl<B: CopyableByteSlice + Copy, T: ?Sized> Copy for Ref<B, T> {}
+}
+
+#[allow(unreachable_pub)] // This is a false positive on our MSRV toolchain.
+pub use def::Ref;
+
+use crate::pointer::{
+    invariant::{Aligned, BecauseExclusive, Initialized, Unaligned, Valid},
+    BecauseRead, PtrInner,
+};
+
+impl<B, T> Ref<B, T>
+where
+    B: ByteSlice,
+{
+    #[must_use = "has no side effects"]
+    pub(crate) fn sized_from(bytes: B) -> Result<Ref<B, T>, CastError<B, T>> {
+        if bytes.len() != mem::size_of::<T>() {
+            return Err(SizeError::new(bytes).into());
+        }
+        if let Err(err) = util::validate_aligned_to::<_, T>(bytes.deref()) {
+            return Err(err.with_src(bytes).into());
+        }
+
+        // SAFETY: We just validated size and alignment.
+        Ok(unsafe { Ref::new_unchecked(bytes) })
+    }
+}
+
+impl<B, T> Ref<B, T>
+where
+    B: SplitByteSlice,
+{
+    #[must_use = "has no side effects"]
+    pub(crate) fn sized_from_prefix(bytes: B) -> Result<(Ref<B, T>, B), CastError<B, T>> {
+        if bytes.len() < mem::size_of::<T>() {
+            return Err(SizeError::new(bytes).into());
+        }
+        if let Err(err) = util::validate_aligned_to::<_, T>(bytes.deref()) {
+            return Err(err.with_src(bytes).into());
+        }
+        let (bytes, suffix) = bytes.split_at(mem::size_of::<T>()).map_err(
+            #[inline(always)]
+            |b| SizeError::new(b).into(),
+        )?;
+        // SAFETY: We just validated alignment and that `bytes` is at least as
+        // large as `T`. `bytes.split_at(mem::size_of::<T>())?` ensures that the
+        // new `bytes` is exactly the size of `T`. By safety postcondition on
+        // `SplitByteSlice::split_at` we can rely on `split_at` to produce the
+        // correct `bytes` and `suffix`.
+        let r = unsafe { Ref::new_unchecked(bytes) };
+        Ok((r, suffix))
+    }
+
+    #[must_use = "has no side effects"]
+    pub(crate) fn sized_from_suffix(bytes: B) -> Result<(B, Ref<B, T>), CastError<B, T>> {
+        let bytes_len = bytes.len();
+        let split_at = if let Some(split_at) = bytes_len.checked_sub(mem::size_of::<T>()) {
+            split_at
+        } else {
+            return Err(SizeError::new(bytes).into());
+        };
+        let (prefix, bytes) = bytes.split_at(split_at).map_err(|b| SizeError::new(b).into())?;
+        if let Err(err) = util::validate_aligned_to::<_, T>(bytes.deref()) {
+            return Err(err.with_src(bytes).into());
+        }
+        // SAFETY: Since `split_at` is defined as `bytes_len - size_of::<T>()`,
+        // the `bytes` which results from `let (prefix, bytes) =
+        // bytes.split_at(split_at)?` has length `size_of::<T>()`. After
+        // constructing `bytes`, we validate that it has the proper alignment.
+        // By safety postcondition on `SplitByteSlice::split_at` we can rely on
+        // `split_at` to produce the correct `prefix` and `bytes`.
+        let r = unsafe { Ref::new_unchecked(bytes) };
+        Ok((prefix, r))
+    }
+}
+
+impl<B, T> Ref<B, T>
+where
+    B: ByteSlice,
+    T: KnownLayout + Immutable + ?Sized,
+{
+    /// Constructs a `Ref` from a byte slice.
+    ///
+    /// If the length of `source` is not a [valid size of `T`][valid-size], or
+    /// if `source` is not appropriately aligned for `T`, this returns `Err`. If
+    /// [`T: Unaligned`][t-unaligned], you can [infallibly discard the alignment
+    /// error][size-error-from].
+    ///
+    /// `T` may be a sized type, a slice, or a [slice DST][slice-dst].
+    ///
+    /// [valid-size]: crate::KnownLayout#what-is-a-valid-size
+    /// [t-unaligned]: crate::Unaligned
+    /// [size-error-from]: error/struct.SizeError.html#method.from-1
+    /// [slice-dst]: KnownLayout#dynamically-sized-types
+    ///
+    /// # Compile-Time Assertions
+    ///
+    /// This method cannot yet be used on unsized types whose dynamically-sized
+    /// component is zero-sized. Attempting to use this method on such types
+    /// results in a compile-time assertion error; e.g.:
+    ///
+    /// ```compile_fail,E0080
+    /// use zerocopy::*;
+    /// # use zerocopy_derive::*;
+    ///
+    /// #[derive(Immutable, KnownLayout)]
+    /// #[repr(C)]
+    /// struct ZSTy {
+    ///     leading_sized: u16,
+    ///     trailing_dst: [()],
+    /// }
+    ///
+    /// let _ = Ref::<_, ZSTy>::from_bytes(&b"UU"[..]); // ⚠ Compile Error!
+    /// ```
+    #[must_use = "has no side effects"]
+    #[inline]
+    pub fn from_bytes(source: B) -> Result<Ref<B, T>, CastError<B, T>> {
+        static_assert_dst_is_not_zst!(T);
+        if let Err(e) =
+            Ptr::from_ref(source.deref()).try_cast_into_no_leftover::<T, BecauseImmutable>(None)
+        {
+            return Err(e.with_src(()).with_src(source));
+        }
+        // SAFETY: `try_cast_into_no_leftover` validates size and alignment.
+        Ok(unsafe { Ref::new_unchecked(source) })
+    }
+}
+
+impl<B, T> Ref<B, T>
+where
+    B: SplitByteSlice,
+    T: KnownLayout + Immutable + ?Sized,
+{
+    /// Constructs a `Ref` from the prefix of a byte slice.
+    ///
+    /// This method computes the [largest possible size of `T`][valid-size] that
+    /// can fit in the leading bytes of `source`, then attempts to return both a
+    /// `Ref` to those bytes, and a reference to the remaining bytes. If there
+    /// are insufficient bytes, or if `source` is not appropriately aligned,
+    /// this returns `Err`. If [`T: Unaligned`][t-unaligned], you can
+    /// [infallibly discard the alignment error][size-error-from].
+    ///
+    /// `T` may be a sized type, a slice, or a [slice DST][slice-dst].
+    ///
+    /// [valid-size]: crate::KnownLayout#what-is-a-valid-size
+    /// [t-unaligned]: crate::Unaligned
+    /// [size-error-from]: error/struct.SizeError.html#method.from-1
+    /// [slice-dst]: KnownLayout#dynamically-sized-types
+    ///
+    /// # Compile-Time Assertions
+    ///
+    /// This method cannot yet be used on unsized types whose dynamically-sized
+    /// component is zero-sized. Attempting to use this method on such types
+    /// results in a compile-time assertion error; e.g.:
+    ///
+    /// ```compile_fail,E0080
+    /// use zerocopy::*;
+    /// # use zerocopy_derive::*;
+    ///
+    /// #[derive(Immutable, KnownLayout)]
+    /// #[repr(C)]
+    /// struct ZSTy {
+    ///     leading_sized: u16,
+    ///     trailing_dst: [()],
+    /// }
+    ///
+    /// let _ = Ref::<_, ZSTy>::from_prefix(&b"UU"[..]); // ⚠ Compile Error!
+    /// ```
+    #[must_use = "has no side effects"]
+    #[inline]
+    pub fn from_prefix(source: B) -> Result<(Ref<B, T>, B), CastError<B, T>> {
+        static_assert_dst_is_not_zst!(T);
+        let remainder = match Ptr::from_ref(source.deref())
+            .try_cast_into::<T, BecauseImmutable>(CastType::Prefix, None)
+        {
+            Ok((_, remainder)) => remainder,
+            Err(e) => {
+                return Err(e.with_src(()).with_src(source));
+            }
+        };
+
+        // SAFETY: `remainder` is constructed as a subset of `source`, and so it
+        // cannot have a larger size than `source`. Both of their `len` methods
+        // measure bytes (`source` deref's to `[u8]`, and `remainder` is a
+        // `Ptr<[u8]>`), so `source.len() >= remainder.len()`. Thus, this cannot
+        // underflow.
+        #[allow(unstable_name_collisions)]
+        let split_at = unsafe { source.len().unchecked_sub(remainder.len()) };
+        let (bytes, suffix) = source.split_at(split_at).map_err(|b| SizeError::new(b).into())?;
+        // SAFETY: `try_cast_into` validates size and alignment, and returns a
+        // `split_at` that indicates how many bytes of `source` correspond to a
+        // valid `T`. By safety postcondition on `SplitByteSlice::split_at` we
+        // can rely on `split_at` to produce the correct `source` and `suffix`.
+        let r = unsafe { Ref::new_unchecked(bytes) };
+        Ok((r, suffix))
+    }
+
+    /// Constructs a `Ref` from the suffix of a byte slice.
+    ///
+    /// This method computes the [largest possible size of `T`][valid-size] that
+    /// can fit in the trailing bytes of `source`, then attempts to return both
+    /// a `Ref` to those bytes, and a reference to the preceding bytes. If there
+    /// are insufficient bytes, or if that suffix of `source` is not
+    /// appropriately aligned, this returns `Err`. If [`T:
+    /// Unaligned`][t-unaligned], you can [infallibly discard the alignment
+    /// error][size-error-from].
+    ///
+    /// `T` may be a sized type, a slice, or a [slice DST][slice-dst].
+    ///
+    /// [valid-size]: crate::KnownLayout#what-is-a-valid-size
+    /// [t-unaligned]: crate::Unaligned
+    /// [size-error-from]: error/struct.SizeError.html#method.from-1
+    /// [slice-dst]: KnownLayout#dynamically-sized-types
+    ///
+    /// # Compile-Time Assertions
+    ///
+    /// This method cannot yet be used on unsized types whose dynamically-sized
+    /// component is zero-sized. Attempting to use this method on such types
+    /// results in a compile-time assertion error; e.g.:
+    ///
+    /// ```compile_fail,E0080
+    /// use zerocopy::*;
+    /// # use zerocopy_derive::*;
+    ///
+    /// #[derive(Immutable, KnownLayout)]
+    /// #[repr(C)]
+    /// struct ZSTy {
+    ///     leading_sized: u16,
+    ///     trailing_dst: [()],
+    /// }
+    ///
+    /// let _ = Ref::<_, ZSTy>::from_suffix(&b"UU"[..]); // ⚠ Compile Error!
+    /// ```
+    #[must_use = "has no side effects"]
+    #[inline]
+    pub fn from_suffix(source: B) -> Result<(B, Ref<B, T>), CastError<B, T>> {
+        static_assert_dst_is_not_zst!(T);
+        let remainder = match Ptr::from_ref(source.deref())
+            .try_cast_into::<T, BecauseImmutable>(CastType::Suffix, None)
+        {
+            Ok((_, remainder)) => remainder,
+            Err(e) => {
+                let e = e.with_src(());
+                return Err(e.with_src(source));
+            }
+        };
+
+        let split_at = remainder.len();
+        let (prefix, bytes) = source.split_at(split_at).map_err(|b| SizeError::new(b).into())?;
+        // SAFETY: `try_cast_into` validates size and alignment, and returns a
+        // `split_at` that indicates how many bytes of `source` correspond to a
+        // valid `T`. By safety postcondition on `SplitByteSlice::split_at` we
+        // can rely on `split_at` to produce the correct `prefix` and `bytes`.
+        let r = unsafe { Ref::new_unchecked(bytes) };
+        Ok((prefix, r))
+    }
+}
+
+impl<B, T> Ref<B, T>
+where
+    B: ByteSlice,
+    T: KnownLayout<PointerMetadata = usize> + Immutable + ?Sized,
+{
+    /// Constructs a `Ref` from the given bytes with DST length equal to `count`
+    /// without copying.
+    ///
+    /// This method attempts to return a `Ref` to the prefix of `source`
+    /// interpreted as a `T` with `count` trailing elements, and a reference to
+    /// the remaining bytes. If the length of `source` is not equal to the size
+    /// of `Self` with `count` elements, or if `source` is not appropriately
+    /// aligned, this returns `Err`. If [`T: Unaligned`][t-unaligned], you can
+    /// [infallibly discard the alignment error][size-error-from].
+    ///
+    /// [t-unaligned]: crate::Unaligned
+    /// [size-error-from]: error/struct.SizeError.html#method.from-1
+    ///
+    /// # Compile-Time Assertions
+    ///
+    /// This method cannot yet be used on unsized types whose dynamically-sized
+    /// component is zero-sized. Attempting to use this method on such types
+    /// results in a compile-time assertion error; e.g.:
+    ///
+    /// ```compile_fail,E0080
+    /// use zerocopy::*;
+    /// # use zerocopy_derive::*;
+    ///
+    /// #[derive(Immutable, KnownLayout)]
+    /// #[repr(C)]
+    /// struct ZSTy {
+    ///     leading_sized: u16,
+    ///     trailing_dst: [()],
+    /// }
+    ///
+    /// let _ = Ref::<_, ZSTy>::from_bytes_with_elems(&b"UU"[..], 42); // ⚠ Compile Error!
+    /// ```
+    #[inline]
+    pub fn from_bytes_with_elems(source: B, count: usize) -> Result<Ref<B, T>, CastError<B, T>> {
+        static_assert_dst_is_not_zst!(T);
+        let expected_len = match T::size_for_metadata(count) {
+            Some(len) => len,
+            None => return Err(SizeError::new(source).into()),
+        };
+        if source.len() != expected_len {
+            return Err(SizeError::new(source).into());
+        }
+        Self::from_bytes(source)
+    }
+}
+
+impl<B, T> Ref<B, T>
+where
+    B: SplitByteSlice,
+    T: KnownLayout<PointerMetadata = usize> + Immutable + ?Sized,
+{
+    /// Constructs a `Ref` from the prefix of the given bytes with DST
+    /// length equal to `count` without copying.
+    ///
+    /// This method attempts to return a `Ref` to the prefix of `source`
+    /// interpreted as a `T` with `count` trailing elements, and a reference to
+    /// the remaining bytes. If there are insufficient bytes, or if `source` is
+    /// not appropriately aligned, this returns `Err`. If [`T:
+    /// Unaligned`][t-unaligned], you can [infallibly discard the alignment
+    /// error][size-error-from].
+    ///
+    /// [t-unaligned]: crate::Unaligned
+    /// [size-error-from]: error/struct.SizeError.html#method.from-1
+    ///
+    /// # Compile-Time Assertions
+    ///
+    /// This method cannot yet be used on unsized types whose dynamically-sized
+    /// component is zero-sized. Attempting to use this method on such types
+    /// results in a compile-time assertion error; e.g.:
+    ///
+    /// ```compile_fail,E0080
+    /// use zerocopy::*;
+    /// # use zerocopy_derive::*;
+    ///
+    /// #[derive(Immutable, KnownLayout)]
+    /// #[repr(C)]
+    /// struct ZSTy {
+    ///     leading_sized: u16,
+    ///     trailing_dst: [()],
+    /// }
+    ///
+    /// let _ = Ref::<_, ZSTy>::from_prefix_with_elems(&b"UU"[..], 42); // ⚠ Compile Error!
+    /// ```
+    #[inline]
+    pub fn from_prefix_with_elems(
+        source: B,
+        count: usize,
+    ) -> Result<(Ref<B, T>, B), CastError<B, T>> {
+        static_assert_dst_is_not_zst!(T);
+        let expected_len = match T::size_for_metadata(count) {
+            Some(len) => len,
+            None => return Err(SizeError::new(source).into()),
+        };
+        let (prefix, bytes) = source.split_at(expected_len).map_err(SizeError::new)?;
+        Self::from_bytes(prefix).map(move |l| (l, bytes))
+    }
+
+    /// Constructs a `Ref` from the suffix of the given bytes with DST length
+    /// equal to `count` without copying.
+    ///
+    /// This method attempts to return a `Ref` to the suffix of `source`
+    /// interpreted as a `T` with `count` trailing elements, and a reference to
+    /// the preceding bytes. If there are insufficient bytes, or if that suffix
+    /// of `source` is not appropriately aligned, this returns `Err`. If [`T:
+    /// Unaligned`][t-unaligned], you can [infallibly discard the alignment
+    /// error][size-error-from].
+    ///
+    /// [t-unaligned]: crate::Unaligned
+    /// [size-error-from]: error/struct.SizeError.html#method.from-1
+    ///
+    /// # Compile-Time Assertions
+    ///
+    /// This method cannot yet be used on unsized types whose dynamically-sized
+    /// component is zero-sized. Attempting to use this method on such types
+    /// results in a compile-time assertion error; e.g.:
+    ///
+    /// ```compile_fail,E0080
+    /// use zerocopy::*;
+    /// # use zerocopy_derive::*;
+    ///
+    /// #[derive(Immutable, KnownLayout)]
+    /// #[repr(C)]
+    /// struct ZSTy {
+    ///     leading_sized: u16,
+    ///     trailing_dst: [()],
+    /// }
+    ///
+    /// let _ = Ref::<_, ZSTy>::from_suffix_with_elems(&b"UU"[..], 42); // ⚠ Compile Error!
+    /// ```
+    #[inline]
+    pub fn from_suffix_with_elems(
+        source: B,
+        count: usize,
+    ) -> Result<(B, Ref<B, T>), CastError<B, T>> {
+        static_assert_dst_is_not_zst!(T);
+        let expected_len = match T::size_for_metadata(count) {
+            Some(len) => len,
+            None => return Err(SizeError::new(source).into()),
+        };
+        let split_at = if let Some(split_at) = source.len().checked_sub(expected_len) {
+            split_at
+        } else {
+            return Err(SizeError::new(source).into());
+        };
+        // SAFETY: The preceding `source.len().checked_sub(expected_len)`
+        // guarantees that `split_at` is in-bounds.
+        let (bytes, suffix) = unsafe { source.split_at_unchecked(split_at) };
+        Self::from_bytes(suffix).map(move |l| (bytes, l))
+    }
+}
+
+impl<'a, B, T> Ref<B, T>
+where
+    B: 'a + IntoByteSlice<'a>,
+    T: FromBytes + KnownLayout + Immutable + ?Sized,
+{
+    /// Converts this `Ref` into a reference.
+    ///
+    /// `into_ref` consumes the `Ref`, and returns a reference to `T`.
+    ///
+    /// Note: this is an associated function, which means that you have to call
+    /// it as `Ref::into_ref(r)` instead of `r.into_ref()`. This is so that
+    /// there is no conflict with a method on the inner type.
+    #[must_use = "has no side effects"]
+    #[inline(always)]
+    pub fn into_ref(r: Self) -> &'a T {
+        // Presumably unreachable, since we've guarded each constructor of `Ref`.
+        static_assert_dst_is_not_zst!(T);
+
+        // SAFETY: We don't call any methods on `b` other than those provided by
+        // `IntoByteSlice`.
+        let b = unsafe { r.into_byte_slice() };
+        let b = b.into_byte_slice();
+
+        if let crate::layout::SizeInfo::Sized { .. } = T::LAYOUT.size_info {
+            let ptr = Ptr::from_ref(b);
+            // SAFETY: We just checked that `T: Sized`. By invariant on `r`,
+            // `b`'s size is equal to `size_of::<T>()`.
+            let ptr = unsafe { cast_for_sized::<T, _, _, _>(ptr) };
+
+            // SAFETY: None of the preceding transformations modifies the
+            // address of the pointer, and by invariant on `r`, we know that it
+            // is validly-aligned.
+            let ptr = unsafe { ptr.assume_alignment::<Aligned>() };
+            return ptr.as_ref();
+        }
+
+        // PANICS: By post-condition on `into_byte_slice`, `b`'s size and
+        // alignment are valid for `T`. By post-condition, `b.into_byte_slice()`
+        // produces a byte slice with identical address and length to that
+        // produced by `b.deref()`.
+        let ptr = Ptr::from_ref(b.into_byte_slice())
+            .try_cast_into_no_leftover::<T, BecauseImmutable>(None)
+            .expect("zerocopy internal error: into_ref should be infallible");
+        let ptr = ptr.recall_validity();
+        ptr.as_ref()
+    }
+}
+
+impl<'a, B, T> Ref<B, T>
+where
+    B: 'a + IntoByteSliceMut<'a>,
+    T: FromBytes + IntoBytes + KnownLayout + ?Sized,
+{
+    /// Converts this `Ref` into a mutable reference.
+    ///
+    /// `into_mut` consumes the `Ref`, and returns a mutable reference to `T`.
+    ///
+    /// Note: this is an associated function, which means that you have to call
+    /// it as `Ref::into_mut(r)` instead of `r.into_mut()`. This is so that
+    /// there is no conflict with a method on the inner type.
+    #[must_use = "has no side effects"]
+    #[inline(always)]
+    pub fn into_mut(r: Self) -> &'a mut T {
+        // Presumably unreachable, since we've guarded each constructor of `Ref`.
+        static_assert_dst_is_not_zst!(T);
+
+        // SAFETY: We don't call any methods on `b` other than those provided by
+        // `IntoByteSliceMut`.
+        let b = unsafe { r.into_byte_slice_mut() };
+        let b = b.into_byte_slice_mut();
+
+        if let crate::layout::SizeInfo::Sized { .. } = T::LAYOUT.size_info {
+            let ptr = Ptr::from_mut(b);
+            // SAFETY: We just checked that `T: Sized`. By invariant on `r`,
+            // `b`'s size is equal to `size_of::<T>()`.
+            let ptr = unsafe {
+                cast_for_sized::<
+                    T,
+                    _,
+                    (BecauseRead, BecauseExclusive),
+                    (BecauseMutationCompatible, BecauseInvariantsEq),
+                >(ptr)
+            };
+
+            // SAFETY: None of the preceding transformations modifies the
+            // address of the pointer, and by invariant on `r`, we know that it
+            // is validly-aligned.
+            let ptr = unsafe { ptr.assume_alignment::<Aligned>() };
+            return ptr.as_mut();
+        }
+
+        // PANICS: By post-condition on `into_byte_slice_mut`, `b`'s size and
+        // alignment are valid for `T`. By post-condition,
+        // `b.into_byte_slice_mut()` produces a byte slice with identical
+        // address and length to that produced by `b.deref_mut()`.
+        let ptr = Ptr::from_mut(b.into_byte_slice_mut())
+            .try_cast_into_no_leftover::<T, BecauseExclusive>(None)
+            .expect("zerocopy internal error: into_ref should be infallible");
+        let ptr = ptr.recall_validity::<_, (_, (_, _))>();
+        ptr.as_mut()
+    }
+}
+
+impl<B, T> Ref<B, T>
+where
+    B: ByteSlice,
+    T: ?Sized,
+{
+    /// Gets the underlying bytes.
+    ///
+    /// Note: this is an associated function, which means that you have to call
+    /// it as `Ref::bytes(r)` instead of `r.bytes()`. This is so that there is
+    /// no conflict with a method on the inner type.
+    #[inline]
+    pub fn bytes(r: &Self) -> &[u8] {
+        // SAFETY: We don't call any methods on `b` other than those provided by
+        // `ByteSlice`.
+        unsafe { r.as_byte_slice().deref() }
+    }
+}
+
+impl<B, T> Ref<B, T>
+where
+    B: ByteSliceMut,
+    T: ?Sized,
+{
+    /// Gets the underlying bytes mutably.
+    ///
+    /// Note: this is an associated function, which means that you have to call
+    /// it as `Ref::bytes_mut(r)` instead of `r.bytes_mut()`. This is so that
+    /// there is no conflict with a method on the inner type.
+    #[inline]
+    pub fn bytes_mut(r: &mut Self) -> &mut [u8] {
+        // SAFETY: We don't call any methods on `b` other than those provided by
+        // `ByteSliceMut`.
+        unsafe { r.as_byte_slice_mut().deref_mut() }
+    }
+}
+
+impl<B, T> Ref<B, T>
+where
+    B: ByteSlice,
+    T: FromBytes,
+{
+    /// Reads a copy of `T`.
+    ///
+    /// Note: this is an associated function, which means that you have to call
+    /// it as `Ref::read(r)` instead of `r.read()`. This is so that there is no
+    /// conflict with a method on the inner type.
+    #[must_use = "has no side effects"]
+    #[inline]
+    pub fn read(r: &Self) -> T {
+        // SAFETY: We don't call any methods on `b` other than those provided by
+        // `ByteSlice`.
+        let b = unsafe { r.as_byte_slice() };
+
+        // SAFETY: By postcondition on `as_byte_slice`, we know that `b` is a
+        // valid size and alignment for `T`. By safety invariant on `ByteSlice`,
+        // we know that this is preserved via `.deref()`. Because `T:
+        // FromBytes`, it is sound to interpret these bytes as a `T`.
+        unsafe { ptr::read(b.deref().as_ptr().cast::<T>()) }
+    }
+}
+
+impl<B, T> Ref<B, T>
+where
+    B: ByteSliceMut,
+    T: IntoBytes,
+{
+    /// Writes the bytes of `t` and then forgets `t`.
+    ///
+    /// Note: this is an associated function, which means that you have to call
+    /// it as `Ref::write(r, t)` instead of `r.write(t)`. This is so that there
+    /// is no conflict with a method on the inner type.
+    #[inline]
+    pub fn write(r: &mut Self, t: T) {
+        // SAFETY: We don't call any methods on `b` other than those provided by
+        // `ByteSliceMut`.
+        let b = unsafe { r.as_byte_slice_mut() };
+
+        // SAFETY: By postcondition on `as_byte_slice_mut`, we know that `b` is
+        // a valid size and alignment for `T`. By safety invariant on
+        // `ByteSlice`, we know that this is preserved via `.deref()`. Writing
+        // `t` to the buffer will allow all of the bytes of `t` to be accessed
+        // as a `[u8]`, but because `T: IntoBytes`, we know that this is sound.
+        unsafe { ptr::write(b.deref_mut().as_mut_ptr().cast::<T>(), t) }
+    }
+}
+
+impl<B, T> Deref for Ref<B, T>
+where
+    B: ByteSlice,
+    T: FromBytes + KnownLayout + Immutable + ?Sized,
+{
+    type Target = T;
+    #[inline]
+    fn deref(&self) -> &T {
+        // Presumably unreachable, since we've guarded each constructor of `Ref`.
+        static_assert_dst_is_not_zst!(T);
+
+        // SAFETY: We don't call any methods on `b` other than those provided by
+        // `ByteSlice`.
+        let b = unsafe { self.as_byte_slice() };
+        let b = b.deref();
+
+        if let crate::layout::SizeInfo::Sized { .. } = T::LAYOUT.size_info {
+            let ptr = Ptr::from_ref(b);
+            // SAFETY: We just checked that `T: Sized`. By invariant on `r`,
+            // `b`'s size is equal to `size_of::<T>()`.
+            let ptr = unsafe { cast_for_sized::<T, _, _, _>(ptr) };
+
+            // SAFETY: None of the preceding transformations modifies the
+            // address of the pointer, and by invariant on `r`, we know that it
+            // is validly-aligned.
+            let ptr = unsafe { ptr.assume_alignment::<Aligned>() };
+            return ptr.as_ref();
+        }
+
+        // PANICS: By postcondition on `as_byte_slice`, `b`'s size and alignment
+        // are valid for `T`, and by invariant on `ByteSlice`, these are
+        // preserved through `.deref()`, so this `unwrap` will not panic.
+        let ptr = Ptr::from_ref(b)
+            .try_cast_into_no_leftover::<T, BecauseImmutable>(None)
+            .expect("zerocopy internal error: Deref::deref should be infallible");
+        let ptr = ptr.recall_validity();
+        ptr.as_ref()
+    }
+}
+
+impl<B, T> DerefMut for Ref<B, T>
+where
+    B: ByteSliceMut,
+    // FIXME(#251): We can't remove `Immutable` here because it's required by
+    // the impl of `Deref`, which is a super-trait of `DerefMut`. Maybe we can
+    // add a separate inherent method for this?
+    T: FromBytes + IntoBytes + KnownLayout + Immutable + ?Sized,
+{
+    #[inline]
+    fn deref_mut(&mut self) -> &mut T {
+        // Presumably unreachable, since we've guarded each constructor of `Ref`.
+        static_assert_dst_is_not_zst!(T);
+
+        // SAFETY: We don't call any methods on `b` other than those provided by
+        // `ByteSliceMut`.
+        let b = unsafe { self.as_byte_slice_mut() };
+        let b = b.deref_mut();
+
+        if let crate::layout::SizeInfo::Sized { .. } = T::LAYOUT.size_info {
+            let ptr = Ptr::from_mut(b);
+            // SAFETY: We just checked that `T: Sized`. By invariant on `r`,
+            // `b`'s size is equal to `size_of::<T>()`.
+            let ptr = unsafe {
+                cast_for_sized::<
+                    T,
+                    _,
+                    (BecauseRead, BecauseExclusive),
+                    (BecauseMutationCompatible, BecauseInvariantsEq),
+                >(ptr)
+            };
+
+            // SAFETY: None of the preceding transformations modifies the
+            // address of the pointer, and by invariant on `r`, we know that it
+            // is validly-aligned.
+            let ptr = unsafe { ptr.assume_alignment::<Aligned>() };
+            return ptr.as_mut();
+        }
+
+        // PANICS: By postcondition on `as_byte_slice_mut`, `b`'s size and
+        // alignment are valid for `T`, and by invariant on `ByteSlice`, these
+        // are preserved through `.deref_mut()`, so this `unwrap` will not
+        // panic.
+        let ptr = Ptr::from_mut(b)
+            .try_cast_into_no_leftover::<T, BecauseExclusive>(None)
+            .expect("zerocopy internal error: DerefMut::deref_mut should be infallible");
+        let ptr = ptr.recall_validity::<_, (_, (_, BecauseExclusive))>();
+        ptr.as_mut()
+    }
+}
+
+impl<T, B> Display for Ref<B, T>
+where
+    B: ByteSlice,
+    T: FromBytes + Display + KnownLayout + Immutable + ?Sized,
+{
+    #[inline]
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> fmt::Result {
+        let inner: &T = self;
+        inner.fmt(fmt)
+    }
+}
+
+impl<T, B> Debug for Ref<B, T>
+where
+    B: ByteSlice,
+    T: FromBytes + Debug + KnownLayout + Immutable + ?Sized,
+{
+    #[inline]
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> fmt::Result {
+        let inner: &T = self;
+        fmt.debug_tuple("Ref").field(&inner).finish()
+    }
+}
+
+impl<T, B> Eq for Ref<B, T>
+where
+    B: ByteSlice,
+    T: FromBytes + Eq + KnownLayout + Immutable + ?Sized,
+{
+}
+
+impl<T, B> PartialEq for Ref<B, T>
+where
+    B: ByteSlice,
+    T: FromBytes + PartialEq + KnownLayout + Immutable + ?Sized,
+{
+    #[inline]
+    fn eq(&self, other: &Self) -> bool {
+        self.deref().eq(other.deref())
+    }
+}
+
+impl<T, B> Ord for Ref<B, T>
+where
+    B: ByteSlice,
+    T: FromBytes + Ord + KnownLayout + Immutable + ?Sized,
+{
+    #[inline]
+    fn cmp(&self, other: &Self) -> Ordering {
+        let inner: &T = self;
+        let other_inner: &T = other;
+        inner.cmp(other_inner)
+    }
+}
+
+impl<T, B> PartialOrd for Ref<B, T>
+where
+    B: ByteSlice,
+    T: FromBytes + PartialOrd + KnownLayout + Immutable + ?Sized,
+{
+    #[inline]
+    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+        let inner: &T = self;
+        let other_inner: &T = other;
+        inner.partial_cmp(other_inner)
+    }
+}
+
+/// # Safety
+///
+/// `T: Sized` and `ptr`'s referent must have size `size_of::<T>()`.
+#[inline(always)]
+unsafe fn cast_for_sized<'a, T, A, R, S>(
+    ptr: Ptr<'a, [u8], (A, Aligned, Valid)>,
+) -> Ptr<'a, T, (A, Unaligned, Valid)>
+where
+    T: FromBytes + KnownLayout + ?Sized,
+    A: crate::invariant::Aliasing,
+    [u8]: MutationCompatible<T, A, Initialized, Initialized, R>,
+    T: TransmuteFromPtr<T, A, Initialized, Valid, crate::pointer::cast::IdCast, S>,
+{
+    use crate::pointer::cast::{Cast, Project};
+
+    enum CastForSized {}
+
+    // SAFETY: `CastForSized` is only used below with the input `ptr`, which the
+    // caller promises has size `size_of::<T>()`. Thus, the referent produced in
+    // this cast has the same size as `ptr`'s referent. All operations preserve
+    // provenance.
+    unsafe impl<T: ?Sized + KnownLayout> Project<[u8], T> for CastForSized {
+        #[inline(always)]
+        fn project(src: PtrInner<'_, [u8]>) -> *mut T {
+            T::raw_from_ptr_len(
+                src.as_non_null().cast(),
+                <T::PointerMetadata as crate::PointerMetadata>::from_elem_count(0),
+            )
+            .as_ptr()
+        }
+    }
+
+    // SAFETY: The `Project::project` impl preserves referent address.
+    unsafe impl<T: ?Sized + KnownLayout> Cast<[u8], T> for CastForSized {}
+
+    ptr.recall_validity::<Initialized, (_, (_, _))>()
+        .cast::<_, CastForSized, _>()
+        .recall_validity::<Valid, _>()
+}
+
+#[cfg(test)]
+#[allow(clippy::assertions_on_result_states)]
+mod tests {
+    use core::convert::TryInto as _;
+
+    use super::*;
+    use crate::util::testutil::*;
+
+    #[test]
+    fn test_mut_slice_into_ref() {
+        // Prior to #1260/#1299, calling `into_ref` on a `&mut [u8]`-backed
+        // `Ref` was not supported.
+        let mut buf = [0u8];
+        let r = Ref::<&mut [u8], u8>::from_bytes(&mut buf).unwrap();
+        assert_eq!(Ref::into_ref(r), &0);
+    }
+
+    #[test]
+    fn test_address() {
+        // Test that the `Deref` and `DerefMut` implementations return a
+        // reference which points to the right region of memory.
+
+        let buf = [0];
+        let r = Ref::<_, u8>::from_bytes(&buf[..]).unwrap();
+        let buf_ptr = buf.as_ptr();
+        let deref_ptr: *const u8 = r.deref();
+        assert_eq!(buf_ptr, deref_ptr);
+
+        let buf = [0];
+        let r = Ref::<_, [u8]>::from_bytes(&buf[..]).unwrap();
+        let buf_ptr = buf.as_ptr();
+        let deref_ptr = r.deref().as_ptr();
+        assert_eq!(buf_ptr, deref_ptr);
+    }
+
+    // Verify that values written to a `Ref` are properly shared between the
+    // typed and untyped representations, that reads via `deref` and `read`
+    // behave the same, and that writes via `deref_mut` and `write` behave the
+    // same.
+    fn test_new_helper(mut r: Ref<&mut [u8], AU64>) {
+        // assert that the value starts at 0
+        assert_eq!(*r, AU64(0));
+        assert_eq!(Ref::read(&r), AU64(0));
+
+        // Assert that values written to the typed value are reflected in the
+        // byte slice.
+        const VAL1: AU64 = AU64(0xFF00FF00FF00FF00);
+        *r = VAL1;
+        assert_eq!(Ref::bytes(&r), &VAL1.to_bytes());
+        *r = AU64(0);
+        Ref::write(&mut r, VAL1);
+        assert_eq!(Ref::bytes(&r), &VAL1.to_bytes());
+
+        // Assert that values written to the byte slice are reflected in the
+        // typed value.
+        const VAL2: AU64 = AU64(!VAL1.0); // different from `VAL1`
+        Ref::bytes_mut(&mut r).copy_from_slice(&VAL2.to_bytes()[..]);
+        assert_eq!(*r, VAL2);
+        assert_eq!(Ref::read(&r), VAL2);
+    }
+
+    // Verify that values written to a `Ref` are properly shared between the
+    // typed and untyped representations; pass a value with `typed_len` `AU64`s
+    // backed by an array of `typed_len * 8` bytes.
+    fn test_new_helper_slice(mut r: Ref<&mut [u8], [AU64]>, typed_len: usize) {
+        // Assert that the value starts out zeroed.
+        assert_eq!(&*r, vec![AU64(0); typed_len].as_slice());
+
+        // Check the backing storage is the exact same slice.
+        let untyped_len = typed_len * 8;
+        assert_eq!(Ref::bytes(&r).len(), untyped_len);
+        assert_eq!(Ref::bytes(&r).as_ptr(), r.as_ptr().cast::<u8>());
+
+        // Assert that values written to the typed value are reflected in the
+        // byte slice.
+        const VAL1: AU64 = AU64(0xFF00FF00FF00FF00);
+        for typed in &mut *r {
+            *typed = VAL1;
+        }
+        assert_eq!(Ref::bytes(&r), VAL1.0.to_ne_bytes().repeat(typed_len).as_slice());
+
+        // Assert that values written to the byte slice are reflected in the
+        // typed value.
+        const VAL2: AU64 = AU64(!VAL1.0); // different from VAL1
+        Ref::bytes_mut(&mut r).copy_from_slice(&VAL2.0.to_ne_bytes().repeat(typed_len));
+        assert!(r.iter().copied().all(|x| x == VAL2));
+    }
+
+    #[test]
+    fn test_new_aligned_sized() {
+        // Test that a properly-aligned, properly-sized buffer works for new,
+        // new_from_prefix, and new_from_suffix, and that new_from_prefix and
+        // new_from_suffix return empty slices. Test that a properly-aligned
+        // buffer whose length is a multiple of the element size works for
+        // new_slice.
+
+        // A buffer with an alignment of 8.
+        let mut buf = Align::<[u8; 8], AU64>::default();
+        // `buf.t` should be aligned to 8, so this should always succeed.
+        test_new_helper(Ref::<_, AU64>::from_bytes(&mut buf.t[..]).unwrap());
+        {
+            // In a block so that `r` and `suffix` don't live too long.
+            buf.set_default();
+            let (r, suffix) = Ref::<_, AU64>::from_prefix(&mut buf.t[..]).unwrap();
+            assert!(suffix.is_empty());
+            test_new_helper(r);
+        }
+        {
+            buf.set_default();
+            let (prefix, r) = Ref::<_, AU64>::from_suffix(&mut buf.t[..]).unwrap();
+            assert!(prefix.is_empty());
+            test_new_helper(r);
+        }
+
+        // A buffer with alignment 8 and length 24. We choose this length very
+        // intentionally: if we instead used length 16, then the prefix and
+        // suffix lengths would be identical. In the past, we used length 16,
+        // which resulted in this test failing to discover the bug uncovered in
+        // #506.
+        let mut buf = Align::<[u8; 24], AU64>::default();
+        // `buf.t` should be aligned to 8 and have a length which is a multiple
+        // of `size_of::<AU64>()`, so this should always succeed.
+        test_new_helper_slice(Ref::<_, [AU64]>::from_bytes(&mut buf.t[..]).unwrap(), 3);
+        buf.set_default();
+        let r = Ref::<_, [AU64]>::from_bytes_with_elems(&mut buf.t[..], 3).unwrap();
+        test_new_helper_slice(r, 3);
+
+        let ascending: [u8; 24] = (0..24).collect::<Vec<_>>().try_into().unwrap();
+        // 16 ascending bytes followed by 8 zeros.
+        let mut ascending_prefix = ascending;
+        ascending_prefix[16..].copy_from_slice(&[0, 0, 0, 0, 0, 0, 0, 0]);
+        // 8 zeros followed by 16 ascending bytes.
+        let mut ascending_suffix = ascending;
+        ascending_suffix[..8].copy_from_slice(&[0, 0, 0, 0, 0, 0, 0, 0]);
+        {
+            buf.t = ascending_suffix;
+            let (r, suffix) = Ref::<_, [AU64]>::from_prefix_with_elems(&mut buf.t[..], 1).unwrap();
+            assert_eq!(suffix, &ascending[8..]);
+            test_new_helper_slice(r, 1);
+        }
+        {
+            buf.t = ascending_prefix;
+            let (prefix, r) = Ref::<_, [AU64]>::from_suffix_with_elems(&mut buf.t[..], 1).unwrap();
+            assert_eq!(prefix, &ascending[..16]);
+            test_new_helper_slice(r, 1);
+        }
+    }
+
+    #[test]
+    fn test_new_oversized() {
+        // Test that a properly-aligned, overly-sized buffer works for
+        // `new_from_prefix` and `new_from_suffix`, and that they return the
+        // remainder and prefix of the slice respectively.
+
+        let mut buf = Align::<[u8; 16], AU64>::default();
+        {
+            // In a block so that `r` and `suffix` don't live too long. `buf.t`
+            // should be aligned to 8, so this should always succeed.
+            let (r, suffix) = Ref::<_, AU64>::from_prefix(&mut buf.t[..]).unwrap();
+            assert_eq!(suffix.len(), 8);
+            test_new_helper(r);
+        }
+        {
+            buf.set_default();
+            // `buf.t` should be aligned to 8, so this should always succeed.
+            let (prefix, r) = Ref::<_, AU64>::from_suffix(&mut buf.t[..]).unwrap();
+            assert_eq!(prefix.len(), 8);
+            test_new_helper(r);
+        }
+    }
+
+    #[test]
+    #[allow(clippy::cognitive_complexity)]
+    fn test_new_error() {
+        // Fail because the buffer is too large.
+
+        // A buffer with an alignment of 8.
+        let buf = Align::<[u8; 16], AU64>::default();
+        // `buf.t` should be aligned to 8, so only the length check should fail.
+        assert!(Ref::<_, AU64>::from_bytes(&buf.t[..]).is_err());
+
+        // Fail because the buffer is too small.
+
+        // A buffer with an alignment of 8.
+        let buf = Align::<[u8; 4], AU64>::default();
+        // `buf.t` should be aligned to 8, so only the length check should fail.
+        assert!(Ref::<_, AU64>::from_bytes(&buf.t[..]).is_err());
+        assert!(Ref::<_, AU64>::from_prefix(&buf.t[..]).is_err());
+        assert!(Ref::<_, AU64>::from_suffix(&buf.t[..]).is_err());
+
+        // Fail because the length is not a multiple of the element size.
+
+        let buf = Align::<[u8; 12], AU64>::default();
+        // `buf.t` has length 12, but element size is 8.
+        assert!(Ref::<_, [AU64]>::from_bytes(&buf.t[..]).is_err());
+
+        // Fail because the buffer is too short.
+        let buf = Align::<[u8; 12], AU64>::default();
+        // `buf.t` has length 12, but the element size is 8 (and we're expecting
+        // two of them). For each function, we test with a length that would
+        // cause the size to overflow `usize`, and with a normal length that
+        // will fail thanks to the buffer being too short; these are different
+        // error paths, and while the error types are the same, the distinction
+        // shows up in code coverage metrics.
+        let n = (usize::MAX / mem::size_of::<AU64>()) + 1;
+        assert!(Ref::<_, [AU64]>::from_bytes_with_elems(&buf.t[..], n).is_err());
+        assert!(Ref::<_, [AU64]>::from_bytes_with_elems(&buf.t[..], 2).is_err());
+        assert!(Ref::<_, [AU64]>::from_prefix_with_elems(&buf.t[..], n).is_err());
+        assert!(Ref::<_, [AU64]>::from_prefix_with_elems(&buf.t[..], 2).is_err());
+        assert!(Ref::<_, [AU64]>::from_suffix_with_elems(&buf.t[..], n).is_err());
+        assert!(Ref::<_, [AU64]>::from_suffix_with_elems(&buf.t[..], 2).is_err());
+
+        // Fail because the alignment is insufficient.
+
+        // A buffer with an alignment of 8. An odd buffer size is chosen so that
+        // the last byte of the buffer has odd alignment.
+        let buf = Align::<[u8; 13], AU64>::default();
+        // Slicing from 1, we get a buffer with size 12 (so the length check
+        // should succeed) but an alignment of only 1, which is insufficient.
+        assert!(Ref::<_, AU64>::from_bytes(&buf.t[1..]).is_err());
+        assert!(Ref::<_, AU64>::from_prefix(&buf.t[1..]).is_err());
+        assert!(Ref::<_, [AU64]>::from_bytes(&buf.t[1..]).is_err());
+        assert!(Ref::<_, [AU64]>::from_bytes_with_elems(&buf.t[1..], 1).is_err());
+        assert!(Ref::<_, [AU64]>::from_prefix_with_elems(&buf.t[1..], 1).is_err());
+        assert!(Ref::<_, [AU64]>::from_suffix_with_elems(&buf.t[1..], 1).is_err());
+        // Slicing is unnecessary here because `new_from_suffix` uses the suffix
+        // of the slice, which has odd alignment.
+        assert!(Ref::<_, AU64>::from_suffix(&buf.t[..]).is_err());
+
+        // Fail due to arithmetic overflow.
+
+        let buf = Align::<[u8; 16], AU64>::default();
+        let unreasonable_len = usize::MAX / mem::size_of::<AU64>() + 1;
+        assert!(Ref::<_, [AU64]>::from_prefix_with_elems(&buf.t[..], unreasonable_len).is_err());
+        assert!(Ref::<_, [AU64]>::from_suffix_with_elems(&buf.t[..], unreasonable_len).is_err());
+    }
+
+    #[test]
+    #[allow(unstable_name_collisions)]
+    #[allow(clippy::as_conversions)]
+    fn test_into_ref_mut() {
+        #[allow(unused)]
+        use crate::util::AsAddress as _;
+
+        let mut buf = Align::<[u8; 8], u64>::default();
+        let r = Ref::<_, u64>::from_bytes(&buf.t[..]).unwrap();
+        let rf = Ref::into_ref(r);
+        assert_eq!(rf, &0u64);
+        let buf_addr = (&buf.t as *const [u8; 8]).addr();
+        assert_eq!((rf as *const u64).addr(), buf_addr);
+
+        let r = Ref::<_, u64>::from_bytes(&mut buf.t[..]).unwrap();
+        let rf = Ref::into_mut(r);
+        assert_eq!(rf, &mut 0u64);
+        assert_eq!((rf as *mut u64).addr(), buf_addr);
+
+        *rf = u64::MAX;
+        assert_eq!(buf.t, [0xFF; 8]);
+    }
+
+    #[test]
+    fn test_display_debug() {
+        let buf = Align::<[u8; 8], u64>::default();
+        let r = Ref::<_, u64>::from_bytes(&buf.t[..]).unwrap();
+        assert_eq!(format!("{}", r), "0");
+        assert_eq!(format!("{:?}", r), "Ref(0)");
+
+        let buf = Align::<[u8; 8], u64>::default();
+        let r = Ref::<_, [u64]>::from_bytes(&buf.t[..]).unwrap();
+        assert_eq!(format!("{:?}", r), "Ref([0])");
+    }
+
+    #[test]
+    fn test_eq() {
+        let buf1 = 0_u64;
+        let r1 = Ref::<_, u64>::from_bytes(buf1.as_bytes()).unwrap();
+        let buf2 = 0_u64;
+        let r2 = Ref::<_, u64>::from_bytes(buf2.as_bytes()).unwrap();
+        assert_eq!(r1, r2);
+    }
+
+    #[test]
+    fn test_ne() {
+        let buf1 = 0_u64;
+        let r1 = Ref::<_, u64>::from_bytes(buf1.as_bytes()).unwrap();
+        let buf2 = 1_u64;
+        let r2 = Ref::<_, u64>::from_bytes(buf2.as_bytes()).unwrap();
+        assert_ne!(r1, r2);
+    }
+
+    #[test]
+    fn test_ord() {
+        let buf1 = 0_u64;
+        let r1 = Ref::<_, u64>::from_bytes(buf1.as_bytes()).unwrap();
+        let buf2 = 1_u64;
+        let r2 = Ref::<_, u64>::from_bytes(buf2.as_bytes()).unwrap();
+        assert!(r1 < r2);
+        assert_eq!(PartialOrd::partial_cmp(&r1, &r2), Some(Ordering::Less));
+        assert_eq!(Ord::cmp(&r1, &r2), Ordering::Less);
+    }
+}
+
+#[cfg(all(test, __ZEROCOPY_INTERNAL_USE_ONLY_NIGHTLY_FEATURES_IN_TESTS))]
+mod benches {
+    use test::{self, Bencher};
+
+    use super::*;
+    use crate::util::testutil::*;
+
+    #[bench]
+    fn bench_from_bytes_sized(b: &mut Bencher) {
+        let buf = Align::<[u8; 8], AU64>::default();
+        // `buf.t` should be aligned to 8, so this should always succeed.
+        let bytes = &buf.t[..];
+        b.iter(|| test::black_box(Ref::<_, AU64>::from_bytes(test::black_box(bytes)).unwrap()));
+    }
+
+    #[bench]
+    fn bench_into_ref_sized(b: &mut Bencher) {
+        let buf = Align::<[u8; 8], AU64>::default();
+        let bytes = &buf.t[..];
+        let r = Ref::<_, AU64>::from_bytes(bytes).unwrap();
+        b.iter(|| test::black_box(Ref::into_ref(test::black_box(r))));
+    }
+
+    #[bench]
+    fn bench_into_mut_sized(b: &mut Bencher) {
+        let mut buf = Align::<[u8; 8], AU64>::default();
+        let buf = &mut buf.t[..];
+        let _ = Ref::<_, AU64>::from_bytes(&mut *buf).unwrap();
+        b.iter(move || {
+            // SAFETY: The preceding `from_bytes` succeeded, and so we know that
+            // `buf` is validly-aligned and has the correct length.
+            let r = unsafe { Ref::<&mut [u8], AU64>::new_unchecked(&mut *buf) };
+            test::black_box(Ref::into_mut(test::black_box(r)));
+        });
+    }
+
+    #[bench]
+    fn bench_deref_sized(b: &mut Bencher) {
+        let buf = Align::<[u8; 8], AU64>::default();
+        let bytes = &buf.t[..];
+        let r = Ref::<_, AU64>::from_bytes(bytes).unwrap();
+        b.iter(|| {
+            let temp = test::black_box(r);
+            test::black_box(temp.deref());
+        });
+    }
+
+    #[bench]
+    fn bench_deref_mut_sized(b: &mut Bencher) {
+        let mut buf = Align::<[u8; 8], AU64>::default();
+        let buf = &mut buf.t[..];
+        let _ = Ref::<_, AU64>::from_bytes(&mut *buf).unwrap();
+        b.iter(|| {
+            // SAFETY: The preceding `from_bytes` succeeded, and so we know that
+            // `buf` is validly-aligned and has the correct length.
+            let r = unsafe { Ref::<&mut [u8], AU64>::new_unchecked(&mut *buf) };
+            let mut temp = test::black_box(r);
+            test::black_box(temp.deref_mut());
+        });
+    }
+}