rust/zerocopy/src/impls.rs - pub/scm/linux/kernel/git/rafael/linux-pm - Git at Google

 // 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 core::{
     cell::{Cell, UnsafeCell},
     mem::MaybeUninit as CoreMaybeUninit,
     ptr::NonNull,
 };

 use super::*;
 use crate::pointer::cast::{CastSizedExact, CastUnsized};

 // SAFETY: Per the reference [1], "the unit tuple (`()`) ... is guaranteed as a
 // zero-sized type to have a size of 0 and an alignment of 1."
 // - `Immutable`: `()` self-evidently does not contain any `UnsafeCell`s.
 // - `TryFromBytes` (with no validator), `FromZeros`, `FromBytes`: There is only
 //   one possible sequence of 0 bytes, and `()` is inhabited.
 // - `IntoBytes`: Since `()` has size 0, it contains no padding bytes.
 // - `Unaligned`: `()` has alignment 1.
 //
 // [1] https://doc.rust-lang.org/1.81.0/reference/type-layout.html#tuple-layout
 #[allow(clippy::multiple_unsafe_ops_per_block)]
 const _: () = unsafe {
     unsafe_impl!((): Immutable, TryFromBytes, FromZeros, FromBytes, IntoBytes, Unaligned);
     assert_unaligned!(());
 };

 // SAFETY:
 // - `Immutable`: These types self-evidently do not contain any `UnsafeCell`s.
 // - `TryFromBytes` (with no validator), `FromZeros`, `FromBytes`: all bit
 //   patterns are valid for numeric types [1]
 // - `IntoBytes`: numeric types have no padding bytes [1]
 // - `Unaligned` (`u8` and `i8` only): The reference [2] specifies the size of
 //   `u8` and `i8` as 1 byte. We also know that:
 //   - Alignment is >= 1 [3]
 //   - Size is an integer multiple of alignment [4]
 //   - The only value >= 1 for which 1 is an integer multiple is 1 Therefore,
 //   the only possible alignment for `u8` and `i8` is 1.
 //
 // [1] Per https://doc.rust-lang.org/1.81.0/reference/types/numeric.html#bit-validity:
 //
 //     For every numeric type, `T`, the bit validity of `T` is equivalent to
 //     the bit validity of `[u8; size_of::<T>()]`. An uninitialized byte is
 //     not a valid `u8`.
 //
 // [2] https://doc.rust-lang.org/1.81.0/reference/type-layout.html#primitive-data-layout
 //
 // [3] Per https://doc.rust-lang.org/1.81.0/reference/type-layout.html#size-and-alignment:
 //
 //     Alignment is measured in bytes, and must be at least 1.
 //
 // [4] Per https://doc.rust-lang.org/1.81.0/reference/type-layout.html#size-and-alignment:
 //
 //     The size of a value is always a multiple of its alignment.
 //
 // FIXME(#278): Once we've updated the trait docs to refer to `u8`s rather than
 // bits or bytes, update this comment, especially the reference to [1].
 #[allow(clippy::multiple_unsafe_ops_per_block)]
 const _: () = unsafe {
     unsafe_impl!(u8: Immutable, TryFromBytes, FromZeros, FromBytes, IntoBytes, Unaligned);
     unsafe_impl!(i8: Immutable, TryFromBytes, FromZeros, FromBytes, IntoBytes, Unaligned);
     assert_unaligned!(u8, i8);
     unsafe_impl!(u16: Immutable, TryFromBytes, FromZeros, FromBytes, IntoBytes);
     unsafe_impl!(i16: Immutable, TryFromBytes, FromZeros, FromBytes, IntoBytes);
     unsafe_impl!(u32: Immutable, TryFromBytes, FromZeros, FromBytes, IntoBytes);
     unsafe_impl!(i32: Immutable, TryFromBytes, FromZeros, FromBytes, IntoBytes);
     unsafe_impl!(u64: Immutable, TryFromBytes, FromZeros, FromBytes, IntoBytes);
     unsafe_impl!(i64: Immutable, TryFromBytes, FromZeros, FromBytes, IntoBytes);
     unsafe_impl!(u128: Immutable, TryFromBytes, FromZeros, FromBytes, IntoBytes);
     unsafe_impl!(i128: Immutable, TryFromBytes, FromZeros, FromBytes, IntoBytes);
     unsafe_impl!(usize: Immutable, TryFromBytes, FromZeros, FromBytes, IntoBytes);
     unsafe_impl!(isize: Immutable, TryFromBytes, FromZeros, FromBytes, IntoBytes);
     unsafe_impl!(f32: Immutable, TryFromBytes, FromZeros, FromBytes, IntoBytes);
     unsafe_impl!(f64: Immutable, TryFromBytes, FromZeros, FromBytes, IntoBytes);
     #[cfg(feature = "float-nightly")]
     unsafe_impl!(#[cfg_attr(doc_cfg, doc(cfg(feature = "float-nightly")))] f16: Immutable, TryFromBytes, FromZeros, FromBytes, IntoBytes);
     #[cfg(feature = "float-nightly")]
     unsafe_impl!(#[cfg_attr(doc_cfg, doc(cfg(feature = "float-nightly")))] f128: Immutable, TryFromBytes, FromZeros, FromBytes, IntoBytes);
 };

 // SAFETY:
 // - `Immutable`: `bool` self-evidently does not contain any `UnsafeCell`s.
 // - `FromZeros`: Valid since "[t]he value false has the bit pattern 0x00" [1].
 // - `IntoBytes`: Since "the boolean type has a size and alignment of 1 each"
 //   and "The value false has the bit pattern 0x00 and the value true has the
 //   bit pattern 0x01" [1]. Thus, the only byte of the bool is always
 //   initialized.
 // - `Unaligned`: Per the reference [1], "[a]n object with the boolean type has
 //   a size and alignment of 1 each."
 //
 // [1] https://doc.rust-lang.org/1.81.0/reference/types/boolean.html
 #[allow(clippy::multiple_unsafe_ops_per_block)]
 const _: () = unsafe { unsafe_impl!(bool: Immutable, FromZeros, IntoBytes, Unaligned) };
 assert_unaligned!(bool);

 // SAFETY: The impl must only return `true` for its argument if the original
 // `Maybe<bool>` refers to a valid `bool`. We only return true if the `u8` value
 // is 0 or 1, and both of these are valid values for `bool` [1].
 //
 // [1] Per https://doc.rust-lang.org/1.81.0/reference/types/boolean.html:
 //
 //   The value false has the bit pattern 0x00 and the value true has the bit
 //   pattern 0x01.
 const _: () = unsafe {
     unsafe_impl!(=> TryFromBytes for bool; |byte| {
         let byte = byte.transmute_with::<u8, invariant::Valid, CastSizedExact, BecauseImmutable>();
         *byte.unaligned_as_ref() < 2
     })
 };

 // SAFETY:
 // - `Immutable`: `char` self-evidently does not contain any `UnsafeCell`s.
 // - `FromZeros`: Per reference [1], "[a] value of type char is a Unicode scalar
 //   value (i.e. a code point that is not a surrogate), represented as a 32-bit
 //   unsigned word in the 0x0000 to 0xD7FF or 0xE000 to 0x10FFFF range" which
 //   contains 0x0000.
 // - `IntoBytes`: `char` is per reference [1] "represented as a 32-bit unsigned
 //   word" (`u32`) which is `IntoBytes`. Note that unlike `u32`, not all bit
 //   patterns are valid for `char`.
 //
 // [1] https://doc.rust-lang.org/1.81.0/reference/types/textual.html
 #[allow(clippy::multiple_unsafe_ops_per_block)]
 const _: () = unsafe { unsafe_impl!(char: Immutable, FromZeros, IntoBytes) };

 // SAFETY: The impl must only return `true` for its argument if the original
 // `Maybe<char>` refers to a valid `char`. `char::from_u32` guarantees that it
 // returns `None` if its input is not a valid `char` [1].
 //
 // [1] Per https://doc.rust-lang.org/core/primitive.char.html#method.from_u32:
 //
 //   `from_u32()` will return `None` if the input is not a valid value for a
 //   `char`.
 const _: () = unsafe {
     unsafe_impl!(=> TryFromBytes for char; |c| {
         let c = c.transmute_with::<Unalign<u32>, invariant::Valid, CastSizedExact, BecauseImmutable>();
         let c = c.read().into_inner();
         char::from_u32(c).is_some()
     });
 };

 // SAFETY: Per the Reference [1], `str` has the same layout as `[u8]`.
 // - `Immutable`: `[u8]` does not contain any `UnsafeCell`s.
 // - `FromZeros`, `IntoBytes`, `Unaligned`: `[u8]` is `FromZeros`, `IntoBytes`,
 //   and `Unaligned`.
 //
 // Note that we don't `assert_unaligned!(str)` because `assert_unaligned!` uses
 // `align_of`, which only works for `Sized` types.
 //
 // FIXME(#429): Improve safety proof for `FromZeros` and `IntoBytes`; having the same
 // layout as `[u8]` isn't sufficient.
 //
 // [1] Per https://doc.rust-lang.org/1.81.0/reference/type-layout.html#str-layout:
 //
 //   String slices are a UTF-8 representation of characters that have the same
 //   layout as slices of type `[u8]`.
 #[allow(clippy::multiple_unsafe_ops_per_block)]
 const _: () = unsafe { unsafe_impl!(str: Immutable, FromZeros, IntoBytes, Unaligned) };

 // SAFETY: The impl must only return `true` for its argument if the original
 // `Maybe<str>` refers to a valid `str`. `str::from_utf8` guarantees that it
 // returns `Err` if its input is not a valid `str` [1].
 //
 // [1] Per https://doc.rust-lang.org/core/str/fn.from_utf8.html#errors:
 //
 //   Returns `Err` if the slice is not UTF-8.
 const _: () = unsafe {
     unsafe_impl!(=> TryFromBytes for str; |c| {
         let c = c.transmute_with::<[u8], invariant::Valid, CastUnsized, BecauseImmutable>();
         let c = c.unaligned_as_ref();
         core::str::from_utf8(c).is_ok()
     })
 };

 macro_rules! unsafe_impl_try_from_bytes_for_nonzero {
     ($($nonzero:ident[$prim:ty]),*) => {
         $(
             unsafe_impl!(=> TryFromBytes for $nonzero; |n| {
                 let n = n.transmute_with::<Unalign<$prim>, invariant::Valid, CastSizedExact, BecauseImmutable>();
                 $nonzero::new(n.read().into_inner()).is_some()
             });
         )*
     }
 }

 // `NonZeroXxx` is `IntoBytes`, but not `FromZeros` or `FromBytes`.
 //
 // SAFETY:
 // - `IntoBytes`: `NonZeroXxx` has the same layout as its associated primitive.
 //    Since it is the same size, this guarantees it has no padding - integers
 //    have no padding, and there's no room for padding if it can represent all
 //    of the same values except 0.
 // - `Unaligned`: `NonZeroU8` and `NonZeroI8` document that `Option<NonZeroU8>`
 //   and `Option<NonZeroI8>` both have size 1. [1] [2] This is worded in a way
 //   that makes it unclear whether it's meant as a guarantee, but given the
 //   purpose of those types, it's virtually unthinkable that that would ever
 //   change. `Option` cannot be smaller than its contained type, which implies
 //   that, and `NonZeroX8` are of size 1 or 0. `NonZeroX8` can represent
 //   multiple states, so they cannot be 0 bytes, which means that they must be 1
 //   byte. The only valid alignment for a 1-byte type is 1.
 //
 // FIXME(#429):
 // - Add quotes from documentation.
 // - Add safety comment for `Immutable`. How can we prove that `NonZeroXxx`
 //   doesn't contain any `UnsafeCell`s? It's obviously true, but it's not clear
 //   how we'd prove it short of adding text to the stdlib docs that says so
 //   explicitly, which likely wouldn't be accepted.
 //
 // [1] Per https://doc.rust-lang.org/1.81.0/std/num/type.NonZeroU8.html:
 //
 //     `NonZeroU8` is guaranteed to have the same layout and bit validity as `u8` with
 //     the exception that 0 is not a valid instance.
 //
 // [2] Per https://doc.rust-lang.org/1.81.0/std/num/type.NonZeroI8.html:
 //
 //     `NonZeroI8` is guaranteed to have the same layout and bit validity as `i8` with
 //     the exception that 0 is not a valid instance.
 #[allow(clippy::multiple_unsafe_ops_per_block)]
 const _: () = unsafe {
     unsafe_impl!(NonZeroU8: Immutable, IntoBytes, Unaligned);
     unsafe_impl!(NonZeroI8: Immutable, IntoBytes, Unaligned);
     assert_unaligned!(NonZeroU8, NonZeroI8);
     unsafe_impl!(NonZeroU16: Immutable, IntoBytes);
     unsafe_impl!(NonZeroI16: Immutable, IntoBytes);
     unsafe_impl!(NonZeroU32: Immutable, IntoBytes);
     unsafe_impl!(NonZeroI32: Immutable, IntoBytes);
     unsafe_impl!(NonZeroU64: Immutable, IntoBytes);
     unsafe_impl!(NonZeroI64: Immutable, IntoBytes);
     unsafe_impl!(NonZeroU128: Immutable, IntoBytes);
     unsafe_impl!(NonZeroI128: Immutable, IntoBytes);
     unsafe_impl!(NonZeroUsize: Immutable, IntoBytes);
     unsafe_impl!(NonZeroIsize: Immutable, IntoBytes);
     unsafe_impl_try_from_bytes_for_nonzero!(
         NonZeroU8[u8],
         NonZeroI8[i8],
         NonZeroU16[u16],
         NonZeroI16[i16],
         NonZeroU32[u32],
         NonZeroI32[i32],
         NonZeroU64[u64],
         NonZeroI64[i64],
         NonZeroU128[u128],
         NonZeroI128[i128],
         NonZeroUsize[usize],
         NonZeroIsize[isize]
     );
 };

 // SAFETY:
 // - `TryFromBytes` (with no validator), `FromZeros`, `FromBytes`, `IntoBytes`:
 //   The Rust compiler reuses `0` value to represent `None`, so
 //   `size_of::<Option<NonZeroXxx>>() == size_of::<xxx>()`; see `NonZeroXxx`
 //   documentation.
 // - `Unaligned`: `NonZeroU8` and `NonZeroI8` document that `Option<NonZeroU8>`
 //   and `Option<NonZeroI8>` both have size 1. [1] [2] This is worded in a way
 //   that makes it unclear whether it's meant as a guarantee, but given the
 //   purpose of those types, it's virtually unthinkable that that would ever
 //   change. The only valid alignment for a 1-byte type is 1.
 //
 // [1] Per https://doc.rust-lang.org/1.81.0/std/num/type.NonZeroU8.html:
 //
 //     `Option<NonZeroU8>` is guaranteed to be compatible with `u8`, including in FFI.
 //
 //     Thanks to the null pointer optimization, `NonZeroU8` and `Option<NonZeroU8>`
 //     are guaranteed to have the same size and alignment:
 //
 // [2] Per https://doc.rust-lang.org/1.81.0/std/num/type.NonZeroI8.html:
 //
 //     `Option<NonZeroI8>` is guaranteed to be compatible with `i8`, including in FFI.
 //
 //     Thanks to the null pointer optimization, `NonZeroI8` and `Option<NonZeroI8>`
 //     are guaranteed to have the same size and alignment:
 #[allow(clippy::multiple_unsafe_ops_per_block)]
 const _: () = unsafe {
     unsafe_impl!(Option<NonZeroU8>: TryFromBytes, FromZeros, FromBytes, IntoBytes, Unaligned);
     unsafe_impl!(Option<NonZeroI8>: TryFromBytes, FromZeros, FromBytes, IntoBytes, Unaligned);
     assert_unaligned!(Option<NonZeroU8>, Option<NonZeroI8>);
     unsafe_impl!(Option<NonZeroU16>: TryFromBytes, FromZeros, FromBytes, IntoBytes);
     unsafe_impl!(Option<NonZeroI16>: TryFromBytes, FromZeros, FromBytes, IntoBytes);
     unsafe_impl!(Option<NonZeroU32>: TryFromBytes, FromZeros, FromBytes, IntoBytes);
     unsafe_impl!(Option<NonZeroI32>: TryFromBytes, FromZeros, FromBytes, IntoBytes);
     unsafe_impl!(Option<NonZeroU64>: TryFromBytes, FromZeros, FromBytes, IntoBytes);
     unsafe_impl!(Option<NonZeroI64>: TryFromBytes, FromZeros, FromBytes, IntoBytes);
     unsafe_impl!(Option<NonZeroU128>: TryFromBytes, FromZeros, FromBytes, IntoBytes);
     unsafe_impl!(Option<NonZeroI128>: TryFromBytes, FromZeros, FromBytes, IntoBytes);
     unsafe_impl!(Option<NonZeroUsize>: TryFromBytes, FromZeros, FromBytes, IntoBytes);
     unsafe_impl!(Option<NonZeroIsize>: TryFromBytes, FromZeros, FromBytes, IntoBytes);
 };

 // SAFETY: While it's not fully documented, the consensus is that `Box<T>` does
 // not contain any `UnsafeCell`s for `T: Sized` [1]. This is not a complete
 // proof, but we are accepting this as a known risk per #1358.
 //
 // [1] https://github.com/rust-lang/unsafe-code-guidelines/issues/492
 #[cfg(feature = "alloc")]
 const _: () = unsafe {
     unsafe_impl!(
         #[cfg_attr(doc_cfg, doc(cfg(feature = "alloc")))]
         T: Sized => Immutable for Box<T>
     )
 };

 // SAFETY: The following types can be transmuted from `[0u8; size_of::<T>()]`. [1]
 //
 // [1] Per https://doc.rust-lang.org/1.89.0/core/option/index.html#representation:
 //
 //   Rust guarantees to optimize the following types `T` such that [`Option<T>`]
 //   has the same size and alignment as `T`. In some of these cases, Rust
 //   further guarantees that `transmute::<_, Option<T>>([0u8; size_of::<T>()])`
 //   is sound and produces `Option::<T>::None`. These cases are identified by
 //   the second column:
 //
 //   | `T`                               | `transmute::<_, Option<T>>([0u8; size_of::<T>()])` sound? |
 //   |-----------------------------------|-----------------------------------------------------------|
 //   | [`Box<U>`]                        | when `U: Sized`                                           |
 //   | `&U`                              | when `U: Sized`                                           |
 //   | `&mut U`                          | when `U: Sized`                                           |
 //   | [`ptr::NonNull<U>`]               | when `U: Sized`                                           |
 //   | `fn`, `extern "C" fn`[^extern_fn] | always                                                    |
 //
 //   [^extern_fn]: this remains true for `unsafe` variants, any argument/return
 //     types, and any other ABI: `[unsafe] extern "abi" fn` (_e.g._, `extern
 //     "system" fn`)
 #[allow(clippy::multiple_unsafe_ops_per_block)]
 const _: () = unsafe {
     #[cfg(feature = "alloc")]
     unsafe_impl!(
         #[cfg_attr(doc_cfg, doc(cfg(feature = "alloc")))]
         T => TryFromBytes for Option<Box<T>>; |c| pointer::is_zeroed(c)
     );
     #[cfg(feature = "alloc")]
     unsafe_impl!(
         #[cfg_attr(doc_cfg, doc(cfg(feature = "alloc")))]
         T => FromZeros for Option<Box<T>>
     );
     unsafe_impl!(
         T => TryFromBytes for Option<&'_ T>; |c| pointer::is_zeroed(c)
     );
     unsafe_impl!(T => FromZeros for Option<&'_ T>);
     unsafe_impl!(
             T => TryFromBytes for Option<&'_ mut T>; |c| pointer::is_zeroed(c)
     );
     unsafe_impl!(T => FromZeros for Option<&'_ mut T>);
     unsafe_impl!(
         T => TryFromBytes for Option<NonNull<T>>; |c| pointer::is_zeroed(c)
     );
     unsafe_impl!(T => FromZeros for Option<NonNull<T>>);
     unsafe_impl_for_power_set!(A, B, C, D, E, F, G, H, I, J, K, L -> M => FromZeros for opt_fn!(...));
     unsafe_impl_for_power_set!(
         A, B, C, D, E, F, G, H, I, J, K, L -> M => TryFromBytes for opt_fn!(...);
         |c| pointer::is_zeroed(c)
     );
     unsafe_impl_for_power_set!(A, B, C, D, E, F, G, H, I, J, K, L -> M => FromZeros for opt_unsafe_fn!(...));
     unsafe_impl_for_power_set!(
         A, B, C, D, E, F, G, H, I, J, K, L -> M => TryFromBytes for opt_unsafe_fn!(...);
         |c| pointer::is_zeroed(c)
     );
     unsafe_impl_for_power_set!(A, B, C, D, E, F, G, H, I, J, K, L -> M => FromZeros for opt_extern_c_fn!(...));
     unsafe_impl_for_power_set!(
         A, B, C, D, E, F, G, H, I, J, K, L -> M => TryFromBytes for opt_extern_c_fn!(...);
         |c| pointer::is_zeroed(c)
     );
     unsafe_impl_for_power_set!(A, B, C, D, E, F, G, H, I, J, K, L -> M => FromZeros for opt_unsafe_extern_c_fn!(...));
     unsafe_impl_for_power_set!(
         A, B, C, D, E, F, G, H, I, J, K, L -> M => TryFromBytes for opt_unsafe_extern_c_fn!(...);
         |c| pointer::is_zeroed(c)
     );
 };

 // SAFETY: `[unsafe] [extern "C"] fn()` self-evidently do not contain
 // `UnsafeCell`s. This is not a proof, but we are accepting this as a known risk
 // per #1358.
 #[allow(clippy::multiple_unsafe_ops_per_block)]
 const _: () = unsafe {
     unsafe_impl_for_power_set!(A, B, C, D, E, F, G, H, I, J, K, L -> M => Immutable for opt_fn!(...));
     unsafe_impl_for_power_set!(A, B, C, D, E, F, G, H, I, J, K, L -> M => Immutable for opt_unsafe_fn!(...));
     unsafe_impl_for_power_set!(A, B, C, D, E, F, G, H, I, J, K, L -> M => Immutable for opt_extern_c_fn!(...));
     unsafe_impl_for_power_set!(A, B, C, D, E, F, G, H, I, J, K, L -> M => Immutable for opt_unsafe_extern_c_fn!(...));
 };

 #[cfg(all(
     not(no_zerocopy_target_has_atomics_1_60_0),
     any(
         target_has_atomic = "8",
         target_has_atomic = "16",
         target_has_atomic = "32",
         target_has_atomic = "64",
         target_has_atomic = "ptr"
     )
 ))]
 #[cfg_attr(doc_cfg, doc(cfg(rust = "1.60.0")))]
 mod atomics {
     use super::*;

     macro_rules! impl_traits_for_atomics {
         ($($atomics:tt [$primitives:ty]),* $(,)?) => {
             $(
                 impl_known_layout!($atomics);
                 impl_for_transmute_from!(=> FromZeros for $atomics [$primitives]);
                 impl_for_transmute_from!(=> FromBytes for $atomics [$primitives]);
                 impl_for_transmute_from!(=> TryFromBytes for $atomics [$primitives]);
                 impl_for_transmute_from!(=> IntoBytes for $atomics [$primitives]);
             )*
         };
     }

     /// Implements `TransmuteFrom` for `$atomic`, `$prim`, and
     /// `UnsafeCell<$prim>`.
     ///
     /// # Safety
     ///
     /// `$atomic` must have the same size and bit validity as `$prim`.
     macro_rules! unsafe_impl_transmute_from_for_atomic {
         ($($($tyvar:ident)? => $atomic:ty [$prim:ty]),*) => {{
             crate::util::macros::__unsafe();

             use crate::pointer::{SizeEq, TransmuteFrom, invariant::Valid};

             $(
                 // SAFETY: The caller promised that `$atomic` and `$prim` have
                 // the same size and bit validity.
                 unsafe impl<$($tyvar)?> TransmuteFrom<$atomic, Valid, Valid> for $prim {}
                 // SAFETY: The caller promised that `$atomic` and `$prim` have
                 // the same size and bit validity.
                 unsafe impl<$($tyvar)?> TransmuteFrom<$prim, Valid, Valid> for $atomic {}

                 impl<$($tyvar)?> SizeEq<ReadOnly<$atomic>> for ReadOnly<$prim> {
                     type CastFrom = $crate::pointer::cast::CastSizedExact;
                 }

                 // SAFETY: The caller promised that `$atomic` and `$prim` have
                 // the same bit validity. `UnsafeCell<T>` has the same bit
                 // validity as `T` [1].
                 //
                 // [1] Per https://doc.rust-lang.org/1.85.0/std/cell/struct.UnsafeCell.html#memory-layout:
                 //
                 //   `UnsafeCell<T>` has the same in-memory representation as
                 //   its inner type `T`. A consequence of this guarantee is that
                 //   it is possible to convert between `T` and `UnsafeCell<T>`.
                 unsafe impl<$($tyvar)?> TransmuteFrom<$atomic, Valid, Valid> for core::cell::UnsafeCell<$prim> {}
                 // SAFETY: See previous safety comment.
                 unsafe impl<$($tyvar)?> TransmuteFrom<core::cell::UnsafeCell<$prim>, Valid, Valid> for $atomic {}
             )*
         }};
     }

     #[cfg(target_has_atomic = "8")]
     #[cfg_attr(doc_cfg, doc(cfg(target_has_atomic = "8")))]
     mod atomic_8 {
         use core::sync::atomic::{AtomicBool, AtomicI8, AtomicU8};

         use super::*;

         impl_traits_for_atomics!(AtomicU8[u8], AtomicI8[i8]);

         impl_known_layout!(AtomicBool);
         impl_for_transmute_from!(=> FromZeros for AtomicBool [bool]);
         impl_for_transmute_from!(=> TryFromBytes for AtomicBool [bool]);
         impl_for_transmute_from!(=> IntoBytes for AtomicBool [bool]);

         // SAFETY: Per [1], `AtomicBool`, `AtomicU8`, and `AtomicI8` have the
         // same size as `bool`, `u8`, and `i8` respectively. Since a type's
         // alignment cannot be smaller than 1 [2], and since its alignment
         // cannot be greater than its size [3], the only possible value for the
         // alignment is 1. Thus, it is sound to implement `Unaligned`.
         //
         // [1] Per (for example) https://doc.rust-lang.org/1.81.0/std/sync/atomic/struct.AtomicU8.html:
         //
         //   This type has the same size, alignment, and bit validity as the
         //   underlying integer type
         //
         // [2] Per https://doc.rust-lang.org/1.81.0/reference/type-layout.html#size-and-alignment:
         //
         //     Alignment is measured in bytes, and must be at least 1.
         //
         // [3] Per https://doc.rust-lang.org/1.81.0/reference/type-layout.html#size-and-alignment:
         //
         //     The size of a value is always a multiple of its alignment.
         #[allow(clippy::multiple_unsafe_ops_per_block)]
         const _: () = unsafe {
             unsafe_impl!(AtomicBool: Unaligned);
             unsafe_impl!(AtomicU8: Unaligned);
             unsafe_impl!(AtomicI8: Unaligned);
             assert_unaligned!(AtomicBool, AtomicU8, AtomicI8);
         };

         // SAFETY: `AtomicU8`, `AtomicI8`, and `AtomicBool` have the same size
         // and bit validity as `u8`, `i8`, and `bool` respectively [1][2][3].
         //
         // [1] Per https://doc.rust-lang.org/1.85.0/std/sync/atomic/struct.AtomicU8.html:
         //
         //   This type has the same size, alignment, and bit validity as the
         //   underlying integer type, `u8`.
         //
         // [2] Per https://doc.rust-lang.org/1.85.0/std/sync/atomic/struct.AtomicI8.html:
         //
         //   This type has the same size, alignment, and bit validity as the
         //   underlying integer type, `i8`.
         //
         // [3] Per https://doc.rust-lang.org/1.85.0/std/sync/atomic/struct.AtomicBool.html:
         //
         //   This type has the same size, alignment, and bit validity a `bool`.
         #[allow(clippy::multiple_unsafe_ops_per_block)]
         const _: () = unsafe {
             unsafe_impl_transmute_from_for_atomic!(
                 => AtomicU8 [u8],
                 => AtomicI8 [i8],
                 => AtomicBool [bool]
             )
         };
     }

     #[cfg(target_has_atomic = "16")]
     #[cfg_attr(doc_cfg, doc(cfg(target_has_atomic = "16")))]
     mod atomic_16 {
         use core::sync::atomic::{AtomicI16, AtomicU16};

         use super::*;

         impl_traits_for_atomics!(AtomicU16[u16], AtomicI16[i16]);

         // SAFETY: `AtomicU16` and `AtomicI16` have the same size and bit
         // validity as `u16` and `i16` respectively [1][2].
         //
         // [1] Per https://doc.rust-lang.org/1.85.0/std/sync/atomic/struct.AtomicU16.html:
         //
         //   This type has the same size and bit validity as the underlying
         //   integer type, `u16`.
         //
         // [2] Per https://doc.rust-lang.org/1.85.0/std/sync/atomic/struct.AtomicI16.html:
         //
         //   This type has the same size and bit validity as the underlying
         //   integer type, `i16`.
         #[allow(clippy::multiple_unsafe_ops_per_block)]
         const _: () = unsafe {
             unsafe_impl_transmute_from_for_atomic!(=> AtomicU16 [u16], => AtomicI16 [i16])
         };
     }

     #[cfg(target_has_atomic = "32")]
     #[cfg_attr(doc_cfg, doc(cfg(target_has_atomic = "32")))]
     mod atomic_32 {
         use core::sync::atomic::{AtomicI32, AtomicU32};

         use super::*;

         impl_traits_for_atomics!(AtomicU32[u32], AtomicI32[i32]);

         // SAFETY: `AtomicU32` and `AtomicI32` have the same size and bit
         // validity as `u32` and `i32` respectively [1][2].
         //
         // [1] Per https://doc.rust-lang.org/1.85.0/std/sync/atomic/struct.AtomicU32.html:
         //
         //   This type has the same size and bit validity as the underlying
         //   integer type, `u32`.
         //
         // [2] Per https://doc.rust-lang.org/1.85.0/std/sync/atomic/struct.AtomicI32.html:
         //
         //   This type has the same size and bit validity as the underlying
         //   integer type, `i32`.
         #[allow(clippy::multiple_unsafe_ops_per_block)]
         const _: () = unsafe {
             unsafe_impl_transmute_from_for_atomic!(=> AtomicU32 [u32], => AtomicI32 [i32])
         };
     }

     #[cfg(target_has_atomic = "64")]
     #[cfg_attr(doc_cfg, doc(cfg(target_has_atomic = "64")))]
     mod atomic_64 {
         use core::sync::atomic::{AtomicI64, AtomicU64};

         use super::*;

         impl_traits_for_atomics!(AtomicU64[u64], AtomicI64[i64]);

         // SAFETY: `AtomicU64` and `AtomicI64` have the same size and bit
         // validity as `u64` and `i64` respectively [1][2].
         //
         // [1] Per https://doc.rust-lang.org/1.85.0/std/sync/atomic/struct.AtomicU64.html:
         //
         //   This type has the same size and bit validity as the underlying
         //   integer type, `u64`.
         //
         // [2] Per https://doc.rust-lang.org/1.85.0/std/sync/atomic/struct.AtomicI64.html:
         //
         //   This type has the same size and bit validity as the underlying
         //   integer type, `i64`.
         #[allow(clippy::multiple_unsafe_ops_per_block)]
         const _: () = unsafe {
             unsafe_impl_transmute_from_for_atomic!(=> AtomicU64 [u64], => AtomicI64 [i64])
         };
     }

     #[cfg(target_has_atomic = "ptr")]
     #[cfg_attr(doc_cfg, doc(cfg(target_has_atomic = "ptr")))]
     mod atomic_ptr {
         use core::sync::atomic::{AtomicIsize, AtomicPtr, AtomicUsize};

         use super::*;

         impl_traits_for_atomics!(AtomicUsize[usize], AtomicIsize[isize]);

         // FIXME(#170): Implement `FromBytes` and `IntoBytes` once we implement
         // those traits for `*mut T`.
         impl_known_layout!(T => AtomicPtr<T>);
         impl_for_transmute_from!(T => TryFromBytes for AtomicPtr<T> [*mut T]);
         impl_for_transmute_from!(T => FromZeros for AtomicPtr<T> [*mut T]);

         // SAFETY: `AtomicUsize` and `AtomicIsize` have the same size and bit
         // validity as `usize` and `isize` respectively [1][2].
         //
         // [1] Per https://doc.rust-lang.org/1.85.0/std/sync/atomic/struct.AtomicUsize.html:
         //
         //   This type has the same size and bit validity as the underlying
         //   integer type, `usize`.
         //
         // [2] Per https://doc.rust-lang.org/1.85.0/std/sync/atomic/struct.AtomicIsize.html:
         //
         //   This type has the same size and bit validity as the underlying
         //   integer type, `isize`.
         #[allow(clippy::multiple_unsafe_ops_per_block)]
         const _: () = unsafe {
             unsafe_impl_transmute_from_for_atomic!(=> AtomicUsize [usize], => AtomicIsize [isize])
         };

         // SAFETY: Per
         // https://doc.rust-lang.org/1.85.0/std/sync/atomic/struct.AtomicPtr.html:
         //
         //   This type has the same size and bit validity as a `*mut T`.
         #[allow(clippy::multiple_unsafe_ops_per_block)]
         const _: () = unsafe { unsafe_impl_transmute_from_for_atomic!(T => AtomicPtr<T> [*mut T]) };
     }
 }

 // SAFETY: Per reference [1]: "For all T, the following are guaranteed:
 // size_of::<PhantomData<T>>() == 0 align_of::<PhantomData<T>>() == 1". This
 // gives:
 // - `Immutable`: `PhantomData` has no fields.
 // - `TryFromBytes` (with no validator), `FromZeros`, `FromBytes`: There is only
 //   one possible sequence of 0 bytes, and `PhantomData` is inhabited.
 // - `IntoBytes`: Since `PhantomData` has size 0, it contains no padding bytes.
 // - `Unaligned`: Per the preceding reference, `PhantomData` has alignment 1.
 //
 // [1] https://doc.rust-lang.org/1.81.0/std/marker/struct.PhantomData.html#layout-1
 #[allow(clippy::multiple_unsafe_ops_per_block)]
 const _: () = unsafe {
     unsafe_impl!(T: ?Sized => Immutable for PhantomData<T>);
     unsafe_impl!(T: ?Sized => TryFromBytes for PhantomData<T>);
     unsafe_impl!(T: ?Sized => FromZeros for PhantomData<T>);
     unsafe_impl!(T: ?Sized => FromBytes for PhantomData<T>);
     unsafe_impl!(T: ?Sized => IntoBytes for PhantomData<T>);
     unsafe_impl!(T: ?Sized => Unaligned for PhantomData<T>);
     assert_unaligned!(PhantomData<()>, PhantomData<u8>, PhantomData<u64>);
 };

 impl_for_transmute_from!(T: TryFromBytes => TryFromBytes for Wrapping<T>[T]);
 impl_for_transmute_from!(T: FromZeros => FromZeros for Wrapping<T>[T]);
 impl_for_transmute_from!(T: FromBytes => FromBytes for Wrapping<T>[T]);
 impl_for_transmute_from!(T: IntoBytes => IntoBytes for Wrapping<T>[T]);
 assert_unaligned!(Wrapping<()>, Wrapping<u8>);

 // SAFETY: Per [1], `Wrapping<T>` has the same layout as `T`. Since its single
 // field (of type `T`) is public, it would be a breaking change to add or remove
 // fields. Thus, we know that `Wrapping<T>` contains a `T` (as opposed to just
 // having the same size and alignment as `T`) with no pre- or post-padding.
 // Thus, `Wrapping<T>` must have `UnsafeCell`s covering the same byte ranges as
 // `Inner = T`.
 //
 // [1] Per https://doc.rust-lang.org/1.81.0/std/num/struct.Wrapping.html#layout-1:
 //
 //   `Wrapping<T>` is guaranteed to have the same layout and ABI as `T`
 const _: () = unsafe { unsafe_impl!(T: Immutable => Immutable for Wrapping<T>) };

 // SAFETY: Per [1] in the preceding safety comment, `Wrapping<T>` has the same
 // alignment as `T`.
 const _: () = unsafe { unsafe_impl!(T: Unaligned => Unaligned for Wrapping<T>) };

 // SAFETY: `TryFromBytes` (with no validator), `FromZeros`, `FromBytes`:
 // `MaybeUninit<T>` has no restrictions on its contents.
 #[allow(clippy::multiple_unsafe_ops_per_block)]
 const _: () = unsafe {
     unsafe_impl!(T => TryFromBytes for CoreMaybeUninit<T>);
     unsafe_impl!(T => FromZeros for CoreMaybeUninit<T>);
     unsafe_impl!(T => FromBytes for CoreMaybeUninit<T>);
 };

 // SAFETY: `MaybeUninit<T>` has `UnsafeCell`s covering the same byte ranges as
 // `Inner = T`. This is not explicitly documented, but it can be inferred. Per
 // [1], `MaybeUninit<T>` has the same size as `T`. Further, note the signature
 // of `MaybeUninit::assume_init_ref` [2]:
 //
 //   pub unsafe fn assume_init_ref(&self) -> &T
 //
 // If the argument `&MaybeUninit<T>` and the returned `&T` had `UnsafeCell`s at
 // different offsets, this would be unsound. Its existence is proof that this is
 // not the case.
 //
 // [1] Per https://doc.rust-lang.org/1.81.0/std/mem/union.MaybeUninit.html#layout-1:
 //
 // `MaybeUninit<T>` is guaranteed to have the same size, alignment, and ABI as
 // `T`.
 //
 // [2] https://doc.rust-lang.org/1.81.0/std/mem/union.MaybeUninit.html#method.assume_init_ref
 const _: () = unsafe { unsafe_impl!(T: Immutable => Immutable for CoreMaybeUninit<T>) };

 // SAFETY: Per [1] in the preceding safety comment, `MaybeUninit<T>` has the
 // same alignment as `T`.
 const _: () = unsafe { unsafe_impl!(T: Unaligned => Unaligned for CoreMaybeUninit<T>) };
 assert_unaligned!(CoreMaybeUninit<()>, CoreMaybeUninit<u8>);

 // SAFETY: `ManuallyDrop<T>` has the same layout as `T` [1]. This strongly
 // implies, but does not guarantee, that it contains `UnsafeCell`s covering the
 // same byte ranges as in `T`. However, it also implements `Defer<Target = T>`
 // [2], which provides the ability to convert `&ManuallyDrop<T> -> &T`. This,
 // combined with having the same size as `T`, implies that `ManuallyDrop<T>`
 // exactly contains a `T` with the same fields and `UnsafeCell`s covering the
 // same byte ranges, or else the `Deref` impl would permit safe code to obtain
 // different shared references to the same region of memory with different
 // `UnsafeCell` coverage, which would in turn permit interior mutation that
 // would violate the invariants of a shared reference.
 //
 // [1] Per https://doc.rust-lang.org/1.85.0/std/mem/struct.ManuallyDrop.html:
 //
 //   `ManuallyDrop<T>` is guaranteed to have the same layout and bit validity as
 //   `T`
 //
 // [2] https://doc.rust-lang.org/1.85.0/std/mem/struct.ManuallyDrop.html#impl-Deref-for-ManuallyDrop%3CT%3E
 const _: () = unsafe { unsafe_impl!(T: ?Sized + Immutable => Immutable for ManuallyDrop<T>) };

 impl_for_transmute_from!(T: ?Sized + TryFromBytes => TryFromBytes for ManuallyDrop<T>[T]);
 impl_for_transmute_from!(T: ?Sized + FromZeros => FromZeros for ManuallyDrop<T>[T]);
 impl_for_transmute_from!(T: ?Sized + FromBytes => FromBytes for ManuallyDrop<T>[T]);
 impl_for_transmute_from!(T: ?Sized + IntoBytes => IntoBytes for ManuallyDrop<T>[T]);
 // SAFETY: `ManuallyDrop<T>` has the same layout as `T` [1], and thus has the
 // same alignment as `T`.
 //
 // [1] Per https://doc.rust-lang.org/1.81.0/std/mem/struct.ManuallyDrop.html:
 //
 //   `ManuallyDrop<T>` is guaranteed to have the same layout and bit validity as
 //   `T`
 const _: () = unsafe { unsafe_impl!(T: ?Sized + Unaligned => Unaligned for ManuallyDrop<T>) };
 assert_unaligned!(ManuallyDrop<()>, ManuallyDrop<u8>);

 const _: () = {
     #[allow(
         non_camel_case_types,
         missing_copy_implementations,
         missing_debug_implementations,
         missing_docs
     )]
     pub enum value {}

     // SAFETY: See safety comment on `ProjectToTag`.
     unsafe impl<T: ?Sized> HasTag for ManuallyDrop<T> {
         #[inline]
         fn only_derive_is_allowed_to_implement_this_trait()
         where
             Self: Sized,
         {
         }

         type Tag = ();

         // SAFETY: It is trivially sound to project any pointer to a pointer to
         // a type of size zero and alignment 1 (which `()` is [1]). Such a
         // pointer will trivially satisfy its aliasing and validity requirements
         // (since it has a zero-sized referent), and its alignment requirement
         // (since it is aligned to 1).
         //
         // [1] Per https://doc.rust-lang.org/1.92.0/reference/type-layout.html#r-layout.tuple.unit:
         //
         //     [T]he unit tuple (`()`)... is guaranteed as a zero-sized type to
         //     have a size of 0 and an alignment of 1.
         type ProjectToTag = crate::pointer::cast::CastToUnit;
     }

     // SAFETY: `ManuallyDrop<T>` has a field of type `T` at offset `0` without
     // any safety invariants beyond those of `T`.  Its existence is not
     // explicitly documented, but it can be inferred; per [1] `ManuallyDrop<T>`
     // has the same size and bit validity as `T`. This field is not literally
     // public, but is effectively so; the field can be transparently:
     //
     //  - initialized via `ManuallyDrop::new`
     //  - moved via `ManuallyDrop::into_inner`
     //  - referenced via `ManuallyDrop::deref`
     //  - exclusively referenced via `ManuallyDrop::deref_mut`
     //
     // We call this field `value`, both because that is both the name of this
     // private field, and because it is the name it is referred to in the public
     // documentation of `ManuallyDrop::new`, `ManuallyDrop::into_inner`,
     // `ManuallyDrop::take` and `ManuallyDrop::drop`.
     unsafe impl<T: ?Sized>
         HasField<value, { crate::STRUCT_VARIANT_ID }, { crate::ident_id!(value) }>
         for ManuallyDrop<T>
     {
         #[inline]
         fn only_derive_is_allowed_to_implement_this_trait()
         where
             Self: Sized,
         {
         }

         type Type = T;

         #[inline(always)]
         fn project(slf: PtrInner<'_, Self>) -> *mut T {
             // SAFETY: `ManuallyDrop<T>` has the same layout and bit validity as
             // `T` [1].
             //
             // [1] Per https://doc.rust-lang.org/1.85.0/std/mem/struct.ManuallyDrop.html:
             //
             //   `ManuallyDrop<T>` is guaranteed to have the same layout and bit
             //   validity as `T`
             #[allow(clippy::as_conversions)]
             return slf.as_ptr() as *mut T;
         }
     }
 };

 impl_for_transmute_from!(T: ?Sized + TryFromBytes => TryFromBytes for Cell<T>[T]);
 impl_for_transmute_from!(T: ?Sized + FromZeros => FromZeros for Cell<T>[T]);
 impl_for_transmute_from!(T: ?Sized + FromBytes => FromBytes for Cell<T>[T]);
 impl_for_transmute_from!(T: ?Sized + IntoBytes => IntoBytes for Cell<T>[T]);
 // SAFETY: `Cell<T>` has the same in-memory representation as `T` [1], and thus
 // has the same alignment as `T`.
 //
 // [1] Per https://doc.rust-lang.org/1.81.0/core/cell/struct.Cell.html#memory-layout:
 //
 //   `Cell<T>` has the same in-memory representation as its inner type `T`.
 const _: () = unsafe { unsafe_impl!(T: ?Sized + Unaligned => Unaligned for Cell<T>) };

 impl_for_transmute_from!(T: ?Sized + FromZeros => FromZeros for UnsafeCell<T>[T]);
 impl_for_transmute_from!(T: ?Sized + FromBytes => FromBytes for UnsafeCell<T>[T]);
 impl_for_transmute_from!(T: ?Sized + IntoBytes => IntoBytes for UnsafeCell<T>[T]);
 // SAFETY: `UnsafeCell<T>` has the same in-memory representation as `T` [1], and
 // thus has the same alignment as `T`.
 //
 // [1] Per https://doc.rust-lang.org/1.81.0/core/cell/struct.UnsafeCell.html#memory-layout:
 //
 //   `UnsafeCell<T>` has the same in-memory representation as its inner type
 //   `T`.
 const _: () = unsafe { unsafe_impl!(T: ?Sized + Unaligned => Unaligned for UnsafeCell<T>) };
 assert_unaligned!(UnsafeCell<()>, UnsafeCell<u8>);

 // SAFETY: See safety comment in `is_bit_valid` impl.
 unsafe impl<T: TryFromBytes + ?Sized> TryFromBytes for UnsafeCell<T> {
     #[allow(clippy::missing_inline_in_public_items)]
     fn only_derive_is_allowed_to_implement_this_trait()
     where
         Self: Sized,
     {
     }

     #[inline(always)]
     fn is_bit_valid<A>(candidate: Maybe<'_, Self, A>) -> bool
     where
         A: invariant::Alignment,
     {
         T::is_bit_valid(candidate.transmute::<_, _, BecauseImmutable>())
     }
 }

 // SAFETY: Per the reference [1]:
 //
 //   An array of `[T; N]` has a size of `size_of::<T>() * N` and the same
 //   alignment of `T`. Arrays are laid out so that the zero-based `nth` element
 //   of the array is offset from the start of the array by `n * size_of::<T>()`
 //   bytes.
 //
 //   ...
 //
 //   Slices have the same layout as the section of the array they slice.
 //
 // In other words, the layout of a `[T]` or `[T; N]` is a sequence of `T`s laid
 // out back-to-back with no bytes in between. Therefore, `[T]` or `[T; N]` are
 // `Immutable`, `TryFromBytes`, `FromZeros`, `FromBytes`, and `IntoBytes` if `T`
 // is (respectively). Furthermore, since an array/slice has "the same alignment
 // of `T`", `[T]` and `[T; N]` are `Unaligned` if `T` is.
 //
 // Note that we don't `assert_unaligned!` for slice types because
 // `assert_unaligned!` uses `align_of`, which only works for `Sized` types.
 //
 // [1] https://doc.rust-lang.org/1.81.0/reference/type-layout.html#array-layout
 #[allow(clippy::multiple_unsafe_ops_per_block)]
 const _: () = unsafe {
     unsafe_impl!(const N: usize, T: Immutable => Immutable for [T; N]);
     unsafe_impl!(const N: usize, T: TryFromBytes => TryFromBytes for [T; N]; |c| {
         let c: Ptr<'_, [ReadOnly<T>; N], _> = c.cast::<_, crate::pointer::cast::CastSized, _>();
         let c: Ptr<'_, [ReadOnly<T>], _> = c.as_slice();
         let c: Ptr<'_, ReadOnly<[T]>, _> = c.cast::<_, crate::pointer::cast::CastUnsized, _>();

         // Note that this call may panic, but it would still be sound even if it
         // did. `is_bit_valid` does not promise that it will not panic (in fact,
         // it explicitly warns that it's a possibility), and we have not
         // violated any safety invariants that we must fix before returning.
         <[T] as TryFromBytes>::is_bit_valid(c)
     });
     unsafe_impl!(const N: usize, T: FromZeros => FromZeros for [T; N]);
     unsafe_impl!(const N: usize, T: FromBytes => FromBytes for [T; N]);
     unsafe_impl!(const N: usize, T: IntoBytes => IntoBytes for [T; N]);
     unsafe_impl!(const N: usize, T: Unaligned => Unaligned for [T; N]);
     assert_unaligned!([(); 0], [(); 1], [u8; 0], [u8; 1]);
     unsafe_impl!(T: Immutable => Immutable for [T]);
     unsafe_impl!(T: TryFromBytes => TryFromBytes for [T]; |c| {
         let c: Ptr<'_, [ReadOnly<T>], _> = c.cast::<_, crate::pointer::cast::CastUnsized, _>();

         // SAFETY: Per the reference [1]:
         //
         //   An array of `[T; N]` has a size of `size_of::<T>() * N` and the
         //   same alignment of `T`. Arrays are laid out so that the zero-based
         //   `nth` element of the array is offset from the start of the array by
         //   `n * size_of::<T>()` bytes.
         //
         //   ...
         //
         //   Slices have the same layout as the section of the array they slice.
         //
         // In other words, the layout of a `[T] is a sequence of `T`s laid out
         // back-to-back with no bytes in between. If all elements in `candidate`
         // are `is_bit_valid`, so too is `candidate`.
         //
         // Note that any of the below calls may panic, but it would still be
         // sound even if it did. `is_bit_valid` does not promise that it will
         // not panic (in fact, it explicitly warns that it's a possibility), and
         // we have not violated any safety invariants that we must fix before
         // returning.
         c.iter().all(<T as TryFromBytes>::is_bit_valid)
     });
     unsafe_impl!(T: FromZeros => FromZeros for [T]);
     unsafe_impl!(T: FromBytes => FromBytes for [T]);
     unsafe_impl!(T: IntoBytes => IntoBytes for [T]);
     unsafe_impl!(T: Unaligned => Unaligned for [T]);
 };

 // SAFETY:
 // - `Immutable`: Raw pointers do not contain any `UnsafeCell`s.
 // - `FromZeros`: For thin pointers (note that `T: Sized`), the zero pointer is
 //   considered "null". [1] No operations which require provenance are legal on
 //   null pointers, so this is not a footgun.
 // - `TryFromBytes`: By the same reasoning as for `FromZeroes`, we can implement
 //   `TryFromBytes` for thin pointers provided that
 //   [`TryFromByte::is_bit_valid`] only produces `true` for zeroed bytes.
 //
 // NOTE(#170): Implementing `FromBytes` and `IntoBytes` for raw pointers would
 // be sound, but carries provenance footguns. We want to support `FromBytes` and
 // `IntoBytes` for raw pointers eventually, but we are holding off until we can
 // figure out how to address those footguns.
 //
 // [1] Per https://doc.rust-lang.org/1.81.0/std/ptr/fn.null.html:
 //
 //   Creates a null raw pointer.
 //
 //   This function is equivalent to zero-initializing the pointer:
 //   `MaybeUninit::<*const T>::zeroed().assume_init()`.
 //
 //   The resulting pointer has the address 0.
 #[allow(clippy::multiple_unsafe_ops_per_block)]
 const _: () = unsafe {
     unsafe_impl!(T: ?Sized => Immutable for *const T);
     unsafe_impl!(T: ?Sized => Immutable for *mut T);
     unsafe_impl!(T => TryFromBytes for *const T; |c| pointer::is_zeroed(c));
     unsafe_impl!(T => FromZeros for *const T);
     unsafe_impl!(T => TryFromBytes for *mut T; |c| pointer::is_zeroed(c));
     unsafe_impl!(T => FromZeros for *mut T);
 };

 // SAFETY: `NonNull<T>` self-evidently does not contain `UnsafeCell`s. This is
 // not a proof, but we are accepting this as a known risk per #1358.
 const _: () = unsafe { unsafe_impl!(T: ?Sized => Immutable for NonNull<T>) };

 // SAFETY: Reference types do not contain any `UnsafeCell`s.
 #[allow(clippy::multiple_unsafe_ops_per_block)]
 const _: () = unsafe {
     unsafe_impl!(T: ?Sized => Immutable for &'_ T);
     unsafe_impl!(T: ?Sized => Immutable for &'_ mut T);
 };

 // SAFETY: `Option` is not `#[non_exhaustive]` [1], which means that the types
 // in its variants cannot change, and no new variants can be added. `Option<T>`
 // does not contain any `UnsafeCell`s outside of `T`. [1]
 //
 // [1] https://doc.rust-lang.org/core/option/enum.Option.html
 const _: () = unsafe { unsafe_impl!(T: Immutable => Immutable for Option<T>) };

 mod tuples {
     use super::*;

     /// Generates various trait implementations for tuples.
     ///
     /// # Safety
     ///
     /// `impl_tuple!` should be provided name-number pairs, where each number is
     /// the ordinal of the preceding type name.
     macro_rules! impl_tuple {
         // Entry point.
         ($($T:ident $I:tt),+ $(,)?) => {
             crate::util::macros::__unsafe();
             impl_tuple!(@all [] [$($T $I)+]);
         };

         // Build up the set of tuple types (i.e., `(A,)`, `(A, B)`, `(A, B, C)`,
         // etc.) Trait implementations that do not depend on field index may be
         // added to this branch.
         (@all [$($head_T:ident $head_I:tt)*] [$next_T:ident $next_I:tt $($tail:tt)*]) => {
             // SAFETY: If all fields of the tuple `Self` are `Immutable`, so too is `Self`.
             unsafe_impl!($($head_T: Immutable,)* $next_T: Immutable => Immutable for ($($head_T,)* $next_T,));

             // SAFETY: If all fields in `c` are `is_bit_valid`, so too is `c`.
             unsafe_impl!($($head_T: TryFromBytes,)* $next_T: TryFromBytes => TryFromBytes for ($($head_T,)* $next_T,); |c| {
                 let mut c = c;
                 $(TryFromBytes::is_bit_valid(into_inner!(c.reborrow().project::<_, { crate::STRUCT_VARIANT_ID }, { crate::ident_id!($head_I) }>())) &&)*
                     TryFromBytes::is_bit_valid(into_inner!(c.reborrow().project::<_, { crate::STRUCT_VARIANT_ID }, { crate::ident_id!($next_I) }>()))
             });

             // SAFETY: If all fields in `Self` are `FromZeros`, so too is `Self`.
             unsafe_impl!($($head_T: FromZeros,)* $next_T: FromZeros => FromZeros for ($($head_T,)* $next_T,));

             // SAFETY: If all fields in `Self` are `FromBytes`, so too is `Self`.
             unsafe_impl!($($head_T: FromBytes,)* $next_T: FromBytes => FromBytes for ($($head_T,)* $next_T,));

             // SAFETY: See safety comment on `ProjectToTag`.
             unsafe impl<$($head_T,)* $next_T> crate::HasTag for ($($head_T,)* $next_T,) {
                 #[inline]
                 fn only_derive_is_allowed_to_implement_this_trait()
                 where
                     Self: Sized
                 {}

                 type Tag = ();

                 // SAFETY: It is trivially sound to project any pointer to a
                 // pointer to a type of size zero and alignment 1 (which `()` is
                 // [1]). Such a pointer will trivially satisfy its aliasing and
                 // validity requirements (since it has a zero-sized referent),
                 // and its alignment requirement (since it is aligned to 1).
                 //
                 // [1] Per https://doc.rust-lang.org/1.92.0/reference/type-layout.html#r-layout.tuple.unit:
                 //
                 //     [T]he unit tuple (`()`)... is guaranteed as a zero-sized
                 //     type to have a size of 0 and an alignment of 1.
                 type ProjectToTag = crate::pointer::cast::CastToUnit;
             }

             // Generate impls that depend on tuple index.
             impl_tuple!(@variants
                 [$($head_T $head_I)* $next_T $next_I]
                 []
                 [$($head_T $head_I)* $next_T $next_I]
             );

             // Recurse to next tuple size
             impl_tuple!(@all [$($head_T $head_I)* $next_T $next_I] [$($tail)*]);
         };
         (@all [$($head_T:ident $head_I:tt)*] []) => {};

         // Emit trait implementations that depend on field index.
         (@variants
             // The full tuple definition in type–index pairs.
             [$($AllT:ident $AllI:tt)+]
             // Types before the current index.
             [$($BeforeT:ident)*]
             // The types and indices at and after the current index.
             [$CurrT:ident $CurrI:tt $($AfterT:ident $AfterI:tt)*]
         ) => {
             // SAFETY:
             // - `Self` is a struct (albeit anonymous), so `VARIANT_ID` is
             //   `STRUCT_VARIANT_ID`.
             // - `$CurrI` is the field at index `$CurrI`, so `FIELD_ID` is
             //   `zerocopy::ident_id!($CurrI)`
             // - `()` has the same visibility as the `.$CurrI` field (ie, `.0`,
             //   `.1`, etc)
             // - `Type` has the same type as `$CurrI`; i.e., `$CurrT`.
             unsafe impl<$($AllT),+> crate::HasField<
                 (),
                 { crate::STRUCT_VARIANT_ID },
                 { crate::ident_id!($CurrI)}
             > for ($($AllT,)+) {
                 #[inline]
                 fn only_derive_is_allowed_to_implement_this_trait()
                 where
                     Self: Sized
                 {}

                 type Type = $CurrT;

                 #[inline(always)]
                 fn project(slf: crate::PtrInner<'_, Self>) -> *mut Self::Type {
                     let slf = slf.as_non_null().as_ptr();
                     // SAFETY: `PtrInner` promises it references either a zero-sized
                     // byte range, or else will reference a byte range that is
                     // entirely contained within an allocated object. In either
                     // case, this guarantees that `(*slf).$CurrI` is in-bounds of
                     // `slf`.
                     unsafe { core::ptr::addr_of_mut!((*slf).$CurrI) }
                 }
             }

             // SAFETY: See comments on items.
             unsafe impl<Aliasing, Alignment, $($AllT),+> crate::ProjectField<
                 (),
                 (Aliasing, Alignment, crate::invariant::Uninit),
                 { crate::STRUCT_VARIANT_ID },
                 { crate::ident_id!($CurrI)}
             > for ($($AllT,)+)
             where
                 Aliasing: crate::invariant::Aliasing,
                 Alignment: crate::invariant::Alignment,
             {
                 #[inline]
                 fn only_derive_is_allowed_to_implement_this_trait()
                 where
                     Self: Sized
                 {}

                 // SAFETY: Tuples are product types whose fields are
                 // well-aligned, so projection preserves both the alignment and
                 // validity invariants of the outer pointer.
                 type Invariants = (Aliasing, Alignment, crate::invariant::Uninit);

                 // SAFETY: Tuples are product types and so projection is infallible;
                 type Error = core::convert::Infallible;
             }

             // SAFETY: See comments on items.
             unsafe impl<Aliasing, Alignment, $($AllT),+> crate::ProjectField<
                 (),
                 (Aliasing, Alignment, crate::invariant::Initialized),
                 { crate::STRUCT_VARIANT_ID },
                 { crate::ident_id!($CurrI)}
             > for ($($AllT,)+)
             where
                 Aliasing: crate::invariant::Aliasing,
                 Alignment: crate::invariant::Alignment,
             {
                 #[inline]
                 fn only_derive_is_allowed_to_implement_this_trait()
                 where
                     Self: Sized
                 {}

                 // SAFETY: Tuples are product types whose fields are
                 // well-aligned, so projection preserves both the alignment and
                 // validity invariants of the outer pointer.
                 type Invariants = (Aliasing, Alignment, crate::invariant::Initialized);

                 // SAFETY: Tuples are product types and so projection is infallible;
                 type Error = core::convert::Infallible;
             }

             // SAFETY: See comments on items.
             unsafe impl<Aliasing, Alignment, $($AllT),+> crate::ProjectField<
                 (),
                 (Aliasing, Alignment, crate::invariant::Valid),
                 { crate::STRUCT_VARIANT_ID },
                 { crate::ident_id!($CurrI)}
             > for ($($AllT,)+)
             where
                 Aliasing: crate::invariant::Aliasing,
                 Alignment: crate::invariant::Alignment,
             {
                 #[inline]
                 fn only_derive_is_allowed_to_implement_this_trait()
                 where
                     Self: Sized
                 {}

                 // SAFETY: Tuples are product types whose fields are
                 // well-aligned, so projection preserves both the alignment and
                 // validity invariants of the outer pointer.
                 type Invariants = (Aliasing, Alignment, crate::invariant::Valid);

                 // SAFETY: Tuples are product types and so projection is infallible;
                 type Error = core::convert::Infallible;
             }

             // Recurse to the next index.
             impl_tuple!(@variants [$($AllT $AllI)+] [$($BeforeT)* $CurrT] [$($AfterT $AfterI)*]);
         };
         (@variants [$($AllT:ident $AllI:tt)+] [$($BeforeT:ident)*] []) => {};
     }

     // SAFETY: `impl_tuple` is provided name-number pairs, where number is the
     // ordinal of the name.
     #[allow(clippy::multiple_unsafe_ops_per_block)]
     const _: () = unsafe {
         impl_tuple! {
             A 0,
             B 1,
             C 2,
             D 3,
             E 4,
             F 5,
             G 6,
             H 7,
             I 8,
             J 9,
             K 10,
             L 11,
             M 12,
             N 13,
             O 14,
             P 15,
             Q 16,
             R 17,
             S 18,
             T 19,
             U 20,
             V 21,
             W 22,
             X 23,
             Y 24,
             Z 25,
         };
     };
 }

 // SIMD support
 //
 // Per the Unsafe Code Guidelines Reference [1]:
 //
 //   Packed SIMD vector types are `repr(simd)` homogeneous tuple-structs
 //   containing `N` elements of type `T` where `N` is a power-of-two and the
 //   size and alignment requirements of `T` are equal:
 //
 //   ```rust
 //   #[repr(simd)]
 //   struct Vector<T, N>(T_0, ..., T_(N - 1));
 //   ```
 //
 //   ...
 //
 //   The size of `Vector` is `N * size_of::<T>()` and its alignment is an
 //   implementation-defined function of `T` and `N` greater than or equal to
 //   `align_of::<T>()`.
 //
 //   ...
 //
 //   Vector elements are laid out in source field order, enabling random access
 //   to vector elements by reinterpreting the vector as an array:
 //
 //   ```rust
 //   union U {
 //      vec: Vector<T, N>,
 //      arr: [T; N]
 //   }
 //
 //   assert_eq!(size_of::<Vector<T, N>>(), size_of::<[T; N]>());
 //   assert!(align_of::<Vector<T, N>>() >= align_of::<[T; N]>());
 //
 //   unsafe {
 //     let u = U { vec: Vector<T, N>(t_0, ..., t_(N - 1)) };
 //
 //     assert_eq!(u.vec.0, u.arr[0]);
 //     // ...
 //     assert_eq!(u.vec.(N - 1), u.arr[N - 1]);
 //   }
 //   ```
 //
 // Given this background, we can observe that:
 // - The size and bit pattern requirements of a SIMD type are equivalent to the
 //   equivalent array type. Thus, for any SIMD type whose primitive `T` is
 //   `Immutable`, `TryFromBytes`, `FromZeros`, `FromBytes`, or `IntoBytes`, that
 //   SIMD type is also `Immutable`, `TryFromBytes`, `FromZeros`, `FromBytes`, or
 //   `IntoBytes` respectively.
 // - Since no upper bound is placed on the alignment, no SIMD type can be
 //   guaranteed to be `Unaligned`.
 //
 // Also per [1]:
 //
 //   This chapter represents the consensus from issue #38. The statements in
 //   here are not (yet) "guaranteed" not to change until an RFC ratifies them.
 //
 // See issue #38 [2]. While this behavior is not technically guaranteed, the
 // likelihood that the behavior will change such that SIMD types are no longer
 // `TryFromBytes`, `FromZeros`, `FromBytes`, or `IntoBytes` is next to zero, as
 // that would defeat the entire purpose of SIMD types. Nonetheless, we put this
 // behavior behind the `simd` Cargo feature, which requires consumers to opt
 // into this stability hazard.
 //
 // [1] https://rust-lang.github.io/unsafe-code-guidelines/layout/packed-simd-vectors.html
 // [2] https://github.com/rust-lang/unsafe-code-guidelines/issues/38
 #[cfg(feature = "simd")]
 #[cfg_attr(doc_cfg, doc(cfg(feature = "simd")))]
 mod simd {
     /// Defines a module which implements `TryFromBytes`, `FromZeros`,
     /// `FromBytes`, and `IntoBytes` for a set of types from a module in
     /// `core::arch`.
     ///
     /// `$arch` is both the name of the defined module and the name of the
     /// module in `core::arch`, and `$typ` is the list of items from that module
     /// to implement `FromZeros`, `FromBytes`, and `IntoBytes` for.
     #[allow(unused_macros)] // `allow(unused_macros)` is needed because some
                             // target/feature combinations don't emit any impls
                             // and thus don't use this macro.
     macro_rules! simd_arch_mod {
         ($(#[cfg $cfg:tt])* $(#[cfg_attr $cfg_attr:tt])? $arch:ident, $mod:ident, $($typ:ident),*) => {
             $(#[cfg $cfg])*
             #[cfg_attr(doc_cfg, doc(cfg $($cfg)*))]
             $(#[cfg_attr $cfg_attr])?
             mod $mod {
                 use core::arch::$arch::{$($typ),*};

                 use crate::*;
                 impl_known_layout!($($typ),*);
                 // SAFETY: See comment on module definition for justification.
                 #[allow(clippy::multiple_unsafe_ops_per_block)]
                 const _: () = unsafe {
                     $( unsafe_impl!($typ: Immutable, TryFromBytes, FromZeros, FromBytes, IntoBytes); )*
                 };
             }
         };
     }

     #[rustfmt::skip]
     const _: () = {
         simd_arch_mod!(
             #[cfg(target_arch = "x86")]
             x86, x86, __m128, __m128d, __m128i, __m256, __m256d, __m256i
         );
         #[cfg(not(no_zerocopy_simd_x86_avx12_1_89_0))]
         simd_arch_mod!(
             #[cfg(target_arch = "x86")]
             #[cfg_attr(doc_cfg, doc(cfg(rust = "1.89.0")))]
             x86, x86_nightly, __m512bh, __m512, __m512d, __m512i
         );
         simd_arch_mod!(
             #[cfg(target_arch = "x86_64")]
             x86_64, x86_64, __m128, __m128d, __m128i, __m256, __m256d, __m256i
         );
         #[cfg(not(no_zerocopy_simd_x86_avx12_1_89_0))]
         simd_arch_mod!(
             #[cfg(target_arch = "x86_64")]
             #[cfg_attr(doc_cfg, doc(cfg(rust = "1.89.0")))]
             x86_64, x86_64_nightly, __m512bh, __m512, __m512d, __m512i
         );
         simd_arch_mod!(
             #[cfg(target_arch = "wasm32")]
             wasm32, wasm32, v128
         );
         simd_arch_mod!(
             #[cfg(all(feature = "simd-nightly", target_arch = "powerpc"))]
             powerpc, powerpc, vector_bool_long, vector_double, vector_signed_long, vector_unsigned_long
         );
         simd_arch_mod!(
             #[cfg(all(feature = "simd-nightly", target_arch = "powerpc64"))]
             powerpc64, powerpc64, vector_bool_long, vector_double, vector_signed_long, vector_unsigned_long
         );
         // NOTE: NEON intrinsics were broken on big-endian platforms from their stabilization up to
         // Rust 1.87. (Context in https://github.com/rust-lang/stdarch/issues/1484). Support is
         // split in two different version ranges on top of the base configuration, requiring either
         // little endian or the more recent version to be detected as well.
         #[cfg(not(no_zerocopy_aarch64_simd_1_59_0))]
         simd_arch_mod!(
             #[cfg(all(
                 target_arch = "aarch64",
                 any(
                     target_endian = "little",
                     not(no_zerocopy_aarch64_simd_be_1_87_0)
                 )
             ))]
             #[cfg_attr(
                 doc_cfg,
                 doc(cfg(all(target_arch = "aarch64", any(
                     all(rust = "1.59.0", target_endian = "little"),
                     rust = "1.87.0",
                 ))))
             )]
             aarch64, aarch64, float32x2_t, float32x4_t, float64x1_t, float64x2_t, int8x8_t, int8x8x2_t,
             int8x8x3_t, int8x8x4_t, int8x16_t, int8x16x2_t, int8x16x3_t, int8x16x4_t, int16x4_t,
             int16x8_t, int32x2_t, int32x4_t, int64x1_t, int64x2_t, poly8x8_t, poly8x8x2_t, poly8x8x3_t,
             poly8x8x4_t, poly8x16_t, poly8x16x2_t, poly8x16x3_t, poly8x16x4_t, poly16x4_t, poly16x8_t,
             poly64x1_t, poly64x2_t, uint8x8_t, uint8x8x2_t, uint8x8x3_t, uint8x8x4_t, uint8x16_t,
             uint8x16x2_t, uint8x16x3_t, uint8x16x4_t, uint16x4_t, uint16x4x2_t, uint16x4x3_t,
             uint16x4x4_t, uint16x8_t, uint32x2_t, uint32x4_t, uint64x1_t, uint64x2_t
         );
     };
 }

 #[cfg(test)]
 mod tests {
     use super::*;

     #[test]
     fn test_impls() {
         // A type that can supply test cases for testing
         // `TryFromBytes::is_bit_valid`. All types passed to `assert_impls!`
         // must implement this trait; that macro uses it to generate runtime
         // tests for `TryFromBytes` impls.
         //
         // All `T: FromBytes` types are provided with a blanket impl. Other
         // types must implement `TryFromBytesTestable` directly (ie using
         // `impl_try_from_bytes_testable!`).
         trait TryFromBytesTestable {
             fn with_passing_test_cases<F: Fn(Box<ReadOnly<Self>>)>(f: F);
             fn with_failing_test_cases<F: Fn(&mut [u8])>(f: F);
         }

         impl<T: FromBytes> TryFromBytesTestable for T {
             fn with_passing_test_cases<F: Fn(Box<ReadOnly<Self>>)>(f: F) {
                 // Test with a zeroed value.
                 f(ReadOnly::<Self>::new_box_zeroed().unwrap());

                 let ffs = {
                     let mut t = ReadOnly::new(Self::new_zeroed());
                     let ptr: *mut T = ReadOnly::as_mut(&mut t);
                     // SAFETY: `T: FromBytes`
                     unsafe { ptr::write_bytes(ptr.cast::<u8>(), 0xFF, mem::size_of::<T>()) };
                     t
                 };

                 // Test with a value initialized with 0xFF.
                 f(Box::new(ffs));
             }

             fn with_failing_test_cases<F: Fn(&mut [u8])>(_f: F) {}
         }

         macro_rules! impl_try_from_bytes_testable_for_null_pointer_optimization {
             ($($tys:ty),*) => {
                 $(
                     impl TryFromBytesTestable for Option<$tys> {
                         fn with_passing_test_cases<F: Fn(Box<ReadOnly<Self>>)>(f: F) {
                             // Test with a zeroed value.
                             f(Box::new(ReadOnly::new(None)));
                         }

                         fn with_failing_test_cases<F: Fn(&mut [u8])>(f: F) {
                             for pos in 0..mem::size_of::<Self>() {
                                 let mut bytes = [0u8; mem::size_of::<Self>()];
                                 bytes[pos] = 0x01;
                                 f(&mut bytes[..]);
                             }
                         }
                     }
                 )*
             };
         }

         // Implements `TryFromBytesTestable`.
         macro_rules! impl_try_from_bytes_testable {
             // Base case for recursion (when the list of types has run out).
             (=> @success $($success_case:expr),* $(, @failure $($failure_case:expr),*)?) => {};
             // Implements for type(s) with no type parameters.
             ($ty:ty $(,$tys:ty)* => @success $($success_case:expr),* $(, @failure $($failure_case:expr),*)?) => {
                 impl TryFromBytesTestable for $ty {
                     impl_try_from_bytes_testable!(
                         @methods     @success $($success_case),*
                                  $(, @failure $($failure_case),*)?
                     );
                 }
                 impl_try_from_bytes_testable!($($tys),* => @success $($success_case),* $(, @failure $($failure_case),*)?);
             };
             // Implements for multiple types with no type parameters.
             ($($($ty:ty),* => @success $($success_case:expr), * $(, @failure $($failure_case:expr),*)?;)*) => {
                 $(
                     impl_try_from_bytes_testable!($($ty),* => @success $($success_case),* $(, @failure $($failure_case),*)*);
                 )*
             };
             // Implements only the methods; caller must invoke this from inside
             // an impl block.
             (@methods @success $($success_case:expr),* $(, @failure $($failure_case:expr),*)?) => {
                 fn with_passing_test_cases<F: Fn(Box<ReadOnly<Self>>)>(_f: F) {
                     $(
                         let bx = Box::<Self>::from($success_case);
                         let ro: Box<ReadOnly<_>> = {
                             let raw = Box::into_raw(bx);
                             // SAFETY: `ReadOnly<T>` has the same layout and bit
                             // validity as `T`.
                             #[allow(clippy::as_conversions)]
                             unsafe { Box::from_raw(raw as *mut _) }
                         };
                         _f(ro);
                     )*
                 }

                 fn with_failing_test_cases<F: Fn(&mut [u8])>(_f: F) {
                     $($(
                         let mut case = $failure_case;
                         _f(case.as_mut_bytes());
                     )*)?
                 }
             };
         }

         impl_try_from_bytes_testable_for_null_pointer_optimization!(
             Box<UnsafeCell<NotZerocopy>>,
             &'static UnsafeCell<NotZerocopy>,
             &'static mut UnsafeCell<NotZerocopy>,
             NonNull<UnsafeCell<NotZerocopy>>,
             fn(),
             FnManyArgs,
             extern "C" fn(),
             ECFnManyArgs
         );

         macro_rules! bx {
             ($e:expr) => {
                 Box::new($e)
             };
         }

         // Note that these impls are only for types which are not `FromBytes`.
         // `FromBytes` types are covered by a preceding blanket impl.
         impl_try_from_bytes_testable!(
             bool => @success true, false,
                     @failure 2u8, 3u8, 0xFFu8;
             char => @success '\u{0}', '\u{D7FF}', '\u{E000}', '\u{10FFFF}',
                     @failure 0xD800u32, 0xDFFFu32, 0x110000u32;
             str  => @success "", "hello", "❤️🧡💛💚💙💜",
                     @failure [0, 159, 146, 150];
             [u8] => @success vec![].into_boxed_slice(), vec![0, 1, 2].into_boxed_slice();
             NonZeroU8, NonZeroI8, NonZeroU16, NonZeroI16, NonZeroU32,
             NonZeroI32, NonZeroU64, NonZeroI64, NonZeroU128, NonZeroI128,
             NonZeroUsize, NonZeroIsize
                 => @success Self::new(1).unwrap(),
                    // Doing this instead of `0` ensures that we always satisfy
                    // the size and alignment requirements of `Self` (whereas `0`
                    // may be any integer type with a different size or alignment
                    // than some `NonZeroXxx` types).
                    @failure Option::<Self>::None;
             [bool; 0] => @success [];
             [bool; 1]
                 => @success [true], [false],
                    @failure [2u8], [3u8], [0xFFu8];
             [bool]
                 => @success vec![true, false].into_boxed_slice(), vec![false, true].into_boxed_slice(),
                     @failure [2u8], [3u8], [0xFFu8], [0u8, 1u8, 2u8];
             Unalign<bool>
                 => @success Unalign::new(false), Unalign::new(true),
                    @failure 2u8, 0xFFu8;
             ManuallyDrop<bool>
                 => @success ManuallyDrop::new(false), ManuallyDrop::new(true),
                    @failure 2u8, 0xFFu8;
             ManuallyDrop<[u8]>
                 => @success bx!(ManuallyDrop::new([])), bx!(ManuallyDrop::new([0u8])), bx!(ManuallyDrop::new([0u8, 1u8]));
             ManuallyDrop<[bool]>
                 => @success bx!(ManuallyDrop::new([])), bx!(ManuallyDrop::new([false])), bx!(ManuallyDrop::new([false, true])),
                    @failure [2u8], [3u8], [0xFFu8], [0u8, 1u8, 2u8];
             ManuallyDrop<[UnsafeCell<u8>]>
                 => @success bx!(ManuallyDrop::new([UnsafeCell::new(0)])), bx!(ManuallyDrop::new([UnsafeCell::new(0), UnsafeCell::new(1)]));
             ManuallyDrop<[UnsafeCell<bool>]>
                 => @success bx!(ManuallyDrop::new([UnsafeCell::new(false)])), bx!(ManuallyDrop::new([UnsafeCell::new(false), UnsafeCell::new(true)])),
                 @failure [2u8], [3u8], [0xFFu8], [0u8, 1u8, 2u8];
             Wrapping<bool>
                 => @success Wrapping(false), Wrapping(true),
                     @failure 2u8, 0xFFu8;
             *const NotZerocopy
                 => @success ptr::null::<NotZerocopy>(),
                    @failure [0x01; mem::size_of::<*const NotZerocopy>()];
             *mut NotZerocopy
                 => @success ptr::null_mut::<NotZerocopy>(),
                    @failure [0x01; mem::size_of::<*mut NotZerocopy>()];
         );

         // Use the trick described in [1] to allow us to call methods
         // conditional on certain trait bounds.
         //
         // In all of these cases, methods return `Option<R>`, where `R` is the
         // return type of the method we're conditionally calling. The "real"
         // implementations (the ones defined in traits using `&self`) return
         // `Some`, and the default implementations (the ones defined as inherent
         // methods using `&mut self`) return `None`.
         //
         // [1] https://github.com/dtolnay/case-studies/blob/master/autoref-specialization/README.md
         mod autoref_trick {
             use super::*;

             pub(super) struct AutorefWrapper<T: ?Sized>(pub(super) PhantomData<T>);

             pub(super) trait TestIsBitValidShared<T: ?Sized> {
                 #[allow(clippy::needless_lifetimes)]
                 fn test_is_bit_valid_shared<'ptr>(&self, candidate: Maybe<'ptr, T>)
                     -> Option<bool>;
             }

             impl<T: TryFromBytes + Immutable + ?Sized> TestIsBitValidShared<T> for AutorefWrapper<T> {
                 #[allow(clippy::needless_lifetimes)]
                 fn test_is_bit_valid_shared<'ptr>(
                     &self,
                     candidate: Maybe<'ptr, T>,
                 ) -> Option<bool> {
                     Some(T::is_bit_valid(candidate))
                 }
             }

             pub(super) trait TestTryFromRef<T: ?Sized> {
                 #[allow(clippy::needless_lifetimes)]
                 fn test_try_from_ref<'bytes>(
                     &self,
                     bytes: &'bytes [u8],
                 ) -> Option<Option<&'bytes T>>;
             }

             impl<T: TryFromBytes + Immutable + KnownLayout + ?Sized> TestTryFromRef<T> for AutorefWrapper<T> {
                 #[allow(clippy::needless_lifetimes)]
                 fn test_try_from_ref<'bytes>(
                     &self,
                     bytes: &'bytes [u8],
                 ) -> Option<Option<&'bytes T>> {
                     Some(T::try_ref_from_bytes(bytes).ok())
                 }
             }

             pub(super) trait TestTryFromMut<T: ?Sized> {
                 #[allow(clippy::needless_lifetimes)]
                 fn test_try_from_mut<'bytes>(
                     &self,
                     bytes: &'bytes mut [u8],
                 ) -> Option<Option<&'bytes mut T>>;
             }

             impl<T: TryFromBytes + IntoBytes + KnownLayout + ?Sized> TestTryFromMut<T> for AutorefWrapper<T> {
                 #[allow(clippy::needless_lifetimes)]
                 fn test_try_from_mut<'bytes>(
                     &self,
                     bytes: &'bytes mut [u8],
                 ) -> Option<Option<&'bytes mut T>> {
                     Some(T::try_mut_from_bytes(bytes).ok())
                 }
             }

             pub(super) trait TestTryReadFrom<T> {
                 fn test_try_read_from(&self, bytes: &[u8]) -> Option<Option<T>>;
             }

             impl<T: TryFromBytes> TestTryReadFrom<T> for AutorefWrapper<T> {
                 fn test_try_read_from(&self, bytes: &[u8]) -> Option<Option<T>> {
                     Some(T::try_read_from_bytes(bytes).ok())
                 }
             }

             pub(super) trait TestAsBytes<T: ?Sized> {
                 #[allow(clippy::needless_lifetimes)]
                 fn test_as_bytes<'slf, 't>(&'slf self, t: &'t ReadOnly<T>) -> Option<&'t [u8]>;
             }

             impl<T: IntoBytes + Immutable + ?Sized> TestAsBytes<T> for AutorefWrapper<T> {
                 #[allow(clippy::needless_lifetimes)]
                 fn test_as_bytes<'slf, 't>(&'slf self, t: &'t ReadOnly<T>) -> Option<&'t [u8]> {
                     Some(t.as_bytes())
                 }
             }
         }

         use autoref_trick::*;

         // Asserts that `$ty` is one of a list of types which are allowed to not
         // provide a "real" implementation for `$fn_name`. Since the
         // `autoref_trick` machinery fails silently, this allows us to ensure
         // that the "default" impls are only being used for types which we
         // expect.
         //
         // Note that, since this is a runtime test, it is possible to have an
         // allowlist which is too restrictive if the function in question is
         // never called for a particular type. For example, if `as_bytes` is not
         // supported for a particular type, and so `test_as_bytes` returns
         // `None`, methods such as `test_try_from_ref` may never be called for
         // that type. As a result, it's possible that, for example, adding
         // `as_bytes` support for a type would cause other allowlist assertions
         // to fail. This means that allowlist assertion failures should not
         // automatically be taken as a sign of a bug.
         macro_rules! assert_on_allowlist {
             ($fn_name:ident($ty:ty) $(: $($tys:ty),*)?) => {{
                 use core::any::TypeId;

                 let allowlist: &[TypeId] = &[ $($(TypeId::of::<$tys>()),*)? ];
                 let allowlist_names: &[&str] = &[ $($(stringify!($tys)),*)? ];

                 let id = TypeId::of::<$ty>();
                 assert!(allowlist.contains(&id), "{} is not on allowlist for {}: {:?}", stringify!($ty), stringify!($fn_name), allowlist_names);
             }};
         }

         // Asserts that `$ty` implements any `$trait` and doesn't implement any
         // `!$trait`. Note that all `$trait`s must come before any `!$trait`s.
         //
         // For `T: TryFromBytes`, uses `TryFromBytesTestable` to test success
         // and failure cases.
         macro_rules! assert_impls {
             ($ty:ty: TryFromBytes) => {
                 // "Default" implementations that match the "real"
                 // implementations defined in the `autoref_trick` module above.
                 #[allow(unused, non_local_definitions)]
                 impl AutorefWrapper<$ty> {
                     #[allow(clippy::needless_lifetimes)]
                     fn test_is_bit_valid_shared<'ptr>(
                         &mut self,
                         candidate: Maybe<'ptr, $ty>,
                     ) -> Option<bool> {
                         assert_on_allowlist!(
                             test_is_bit_valid_shared($ty):
                             ManuallyDrop<UnsafeCell<()>>,
                             ManuallyDrop<[UnsafeCell<u8>]>,
                             ManuallyDrop<[UnsafeCell<bool>]>,
                             CoreMaybeUninit<NotZerocopy>,
                             CoreMaybeUninit<UnsafeCell<()>>,
                             Wrapping<UnsafeCell<()>>
                         );

                         None
                     }

                     #[allow(clippy::needless_lifetimes)]
                     fn test_try_from_ref<'bytes>(&mut self, _bytes: &'bytes [u8]) -> Option<Option<&'bytes $ty>> {
                         assert_on_allowlist!(
                             test_try_from_ref($ty):
                             ManuallyDrop<[UnsafeCell<bool>]>
                         );

                         None
                     }

                     #[allow(clippy::needless_lifetimes)]
                     fn test_try_from_mut<'bytes>(&mut self, _bytes: &'bytes mut [u8]) -> Option<Option<&'bytes mut $ty>> {
                         assert_on_allowlist!(
                             test_try_from_mut($ty):
                             Option<Box<UnsafeCell<NotZerocopy>>>,
                             Option<&'static UnsafeCell<NotZerocopy>>,
                             Option<&'static mut UnsafeCell<NotZerocopy>>,
                             Option<NonNull<UnsafeCell<NotZerocopy>>>,
                             Option<fn()>,
                             Option<FnManyArgs>,
                             Option<extern "C" fn()>,
                             Option<ECFnManyArgs>,
                             *const NotZerocopy,
                             *mut NotZerocopy
                         );

                         None
                     }

                     fn test_try_read_from(&mut self, _bytes: &[u8]) -> Option<Option<&$ty>> {
                         assert_on_allowlist!(
                             test_try_read_from($ty):
                             str,
                             ManuallyDrop<[u8]>,
                             ManuallyDrop<[bool]>,
                             ManuallyDrop<[UnsafeCell<bool>]>,
                             [u8],
                             [bool]
                         );

                         None
                     }

                     fn test_as_bytes(&mut self, _t: &ReadOnly<$ty>) -> Option<&[u8]> {
                         assert_on_allowlist!(
                             test_as_bytes($ty):
                             Option<&'static UnsafeCell<NotZerocopy>>,
                             Option<&'static mut UnsafeCell<NotZerocopy>>,
                             Option<NonNull<UnsafeCell<NotZerocopy>>>,
                             Option<Box<UnsafeCell<NotZerocopy>>>,
                             Option<fn()>,
                             Option<FnManyArgs>,
                             Option<extern "C" fn()>,
                             Option<ECFnManyArgs>,
                             CoreMaybeUninit<u8>,
                             CoreMaybeUninit<NotZerocopy>,
                             CoreMaybeUninit<UnsafeCell<()>>,
                             ManuallyDrop<UnsafeCell<()>>,
                             ManuallyDrop<[UnsafeCell<u8>]>,
                             ManuallyDrop<[UnsafeCell<bool>]>,
                             Wrapping<UnsafeCell<()>>,
                             *const NotZerocopy,
                             *mut NotZerocopy
                         );

                         None
                     }
                 }

                 <$ty as TryFromBytesTestable>::with_passing_test_cases(|mut val| {
                     // FIXME(#494): These tests only get exercised for types
                     // which are `IntoBytes`. Once we implement #494, we should
                     // be able to support non-`IntoBytes` types by zeroing
                     // padding.

                     // We define `w` and `ww` since, in the case of the inherent
                     // methods, Rust thinks they're both borrowed mutably at the
                     // same time (given how we use them below). If we just
                     // defined a single `w` and used it for multiple operations,
                     // this would conflict.
                     //
                     // We `#[allow(unused_mut]` for the cases where the "real"
                     // impls are used, which take `&self`.
                     #[allow(unused_mut)]
                     let (mut w, mut ww) = (AutorefWrapper::<$ty>(PhantomData), AutorefWrapper::<$ty>(PhantomData));

                     let c = Ptr::from_ref(&*val);
                     let c = c.forget_aligned();
                     // SAFETY: FIXME(#899): This is unsound. `$ty` is not
                     // necessarily `IntoBytes`, but that's the corner we've
                     // backed ourselves into by using `Ptr::from_ref`.
                     let c = unsafe { c.assume_initialized() };
                     let res = w.test_is_bit_valid_shared(c);
                     if let Some(res) = res {
                         assert!(res, "{}::is_bit_valid (shared `Ptr`): got false, expected true", stringify!($ty));
                     }

                     let c = Ptr::from_mut(&mut *val);
                     let c = c.forget_aligned();
                     // SAFETY: FIXME(#899): This is unsound. `$ty` is not
                     // necessarily `IntoBytes`, but that's the corner we've
                     // backed ourselves into by using `Ptr::from_ref`.
                     let mut c = unsafe { c.assume_initialized() };
                     let res = <$ty as TryFromBytes>::is_bit_valid(c.reborrow_shared());
                     assert!(res, "{}::is_bit_valid (exclusive `Ptr`): got false, expected true", stringify!($ty));

                     // `bytes` is `Some(val.as_bytes())` if `$ty: IntoBytes +
                     // Immutable` and `None` otherwise.
                     let bytes = w.test_as_bytes(&*val);

                     // The inner closure returns
                     // `Some($ty::try_ref_from_bytes(bytes))` if `$ty:
                     // Immutable` and `None` otherwise.
                     let res = bytes.and_then(|bytes| ww.test_try_from_ref(bytes));
                     if let Some(res) = res {
                         assert!(res.is_some(), "{}::try_ref_from_bytes: got `None`, expected `Some`", stringify!($ty));
                     }

                     if let Some(bytes) = bytes {
                         // We need to get a mutable byte slice, and so we clone
                         // into a `Vec`. However, we also need these bytes to
                         // satisfy `$ty`'s alignment requirement, which isn't
                         // guaranteed for `Vec<u8>`. In order to get around
                         // this, we create a `Vec` which is twice as long as we
                         // need. There is guaranteed to be an aligned byte range
                         // of size `size_of_val(val)` within that range.
                         let val = &*val;
                         let size = mem::size_of_val(val);
                         let align = mem::align_of_val(val);

                         let mut vec = bytes.to_vec();
                         vec.extend(bytes);
                         let slc = vec.as_slice();
                         let offset = slc.as_ptr().align_offset(align);
                         let bytes_mut = &mut vec.as_mut_slice()[offset..offset+size];
                         bytes_mut.copy_from_slice(bytes);

                         let res = ww.test_try_from_mut(bytes_mut);
                         if let Some(res) = res {
                             assert!(res.is_some(), "{}::try_mut_from_bytes: got `None`, expected `Some`", stringify!($ty));
                         }
                     }

                     let res = bytes.and_then(|bytes| ww.test_try_read_from(bytes));
                     if let Some(res) = res {
                         assert!(res.is_some(), "{}::try_read_from_bytes: got `None`, expected `Some`", stringify!($ty));
                     }
                 });
                 #[allow(clippy::as_conversions)]
                 <$ty as TryFromBytesTestable>::with_failing_test_cases(|c| {
                     #[allow(unused_mut)] // For cases where the "real" impls are used, which take `&self`.
                     let mut w = AutorefWrapper::<$ty>(PhantomData);

                     // This is `Some($ty::try_ref_from_bytes(c))` if `$ty:
                     // Immutable` and `None` otherwise.
                     let res = w.test_try_from_ref(c);
                     if let Some(res) = res {
                         assert!(res.is_none(), "{}::try_ref_from_bytes({:?}): got Some, expected None", stringify!($ty), c);
                     }

                     let res = w.test_try_from_mut(c);
                     if let Some(res) = res {
                         assert!(res.is_none(), "{}::try_mut_from_bytes({:?}): got Some, expected None", stringify!($ty), c);
                     }


                     let res = w.test_try_read_from(c);
                     if let Some(res) = res {
                         assert!(res.is_none(), "{}::try_read_from_bytes({:?}): got Some, expected None", stringify!($ty), c);
                     }
                 });

                 #[allow(dead_code)]
                 const _: () = { static_assertions::assert_impl_all!($ty: TryFromBytes); };
             };
             ($ty:ty: $trait:ident) => {
                 #[allow(dead_code)]
                 const _: () = { static_assertions::assert_impl_all!($ty: $trait); };
             };
             ($ty:ty: !$trait:ident) => {
                 #[allow(dead_code)]
                 const _: () = { static_assertions::assert_not_impl_any!($ty: $trait); };
             };
             ($ty:ty: $($trait:ident),* $(,)? $(!$negative_trait:ident),*) => {
                 $(
                     assert_impls!($ty: $trait);
                 )*

                 $(
                     assert_impls!($ty: !$negative_trait);
                 )*
             };
         }

         // NOTE: The negative impl assertions here are not necessarily
         // prescriptive. They merely serve as change detectors to make sure
         // we're aware of what trait impls are getting added with a given
         // change. Of course, some impls would be invalid (e.g., `bool:
         // FromBytes`), and so this change detection is very important.

         assert_impls!(
             (): KnownLayout,
             Immutable,
             TryFromBytes,
             FromZeros,
             FromBytes,
             IntoBytes,
             Unaligned
         );
         assert_impls!(
             u8: KnownLayout,
             Immutable,
             TryFromBytes,
             FromZeros,
             FromBytes,
             IntoBytes,
             Unaligned
         );
         assert_impls!(
             i8: KnownLayout,
             Immutable,
             TryFromBytes,
             FromZeros,
             FromBytes,
             IntoBytes,
             Unaligned
         );
         assert_impls!(
             u16: KnownLayout,
             Immutable,
             TryFromBytes,
             FromZeros,
             FromBytes,
             IntoBytes,
             !Unaligned
         );
         assert_impls!(
             i16: KnownLayout,
             Immutable,
             TryFromBytes,
             FromZeros,
             FromBytes,
             IntoBytes,
             !Unaligned
         );
         assert_impls!(
             u32: KnownLayout,
             Immutable,
             TryFromBytes,
             FromZeros,
             FromBytes,
             IntoBytes,
             !Unaligned
         );
         assert_impls!(
             i32: KnownLayout,
             Immutable,
             TryFromBytes,
             FromZeros,
             FromBytes,
             IntoBytes,
             !Unaligned
         );
         assert_impls!(
             u64: KnownLayout,
             Immutable,
             TryFromBytes,
             FromZeros,
             FromBytes,
             IntoBytes,
             !Unaligned
         );
         assert_impls!(
             i64: KnownLayout,
             Immutable,
             TryFromBytes,
             FromZeros,
             FromBytes,
             IntoBytes,
             !Unaligned
         );
         assert_impls!(
             u128: KnownLayout,
             Immutable,
             TryFromBytes,
             FromZeros,
             FromBytes,
             IntoBytes,
             !Unaligned
         );
         assert_impls!(
             i128: KnownLayout,
             Immutable,
             TryFromBytes,
             FromZeros,
             FromBytes,
             IntoBytes,
             !Unaligned
         );
         assert_impls!(
             usize: KnownLayout,
             Immutable,
             TryFromBytes,
             FromZeros,
             FromBytes,
             IntoBytes,
             !Unaligned
         );
         assert_impls!(
             isize: KnownLayout,
             Immutable,
             TryFromBytes,
             FromZeros,
             FromBytes,
             IntoBytes,
             !Unaligned
         );
         #[cfg(feature = "float-nightly")]
         assert_impls!(
             f16: KnownLayout,
             Immutable,
             TryFromBytes,
             FromZeros,
             FromBytes,
             IntoBytes,
             !Unaligned
         );
         assert_impls!(
             f32: KnownLayout,
             Immutable,
             TryFromBytes,
             FromZeros,
             FromBytes,
             IntoBytes,
             !Unaligned
         );
         assert_impls!(
             f64: KnownLayout,
             Immutable,
             TryFromBytes,
             FromZeros,
             FromBytes,
             IntoBytes,
             !Unaligned
         );
         #[cfg(feature = "float-nightly")]
         assert_impls!(
             f128: KnownLayout,
             Immutable,
             TryFromBytes,
             FromZeros,
             FromBytes,
             IntoBytes,
             !Unaligned
         );
         assert_impls!(
             bool: KnownLayout,
             Immutable,
             TryFromBytes,
             FromZeros,
             IntoBytes,
             Unaligned,
             !FromBytes
         );
         assert_impls!(
             char: KnownLayout,
             Immutable,
             TryFromBytes,
             FromZeros,
             IntoBytes,
             !FromBytes,
             !Unaligned
         );
         assert_impls!(
             str: KnownLayout,
             Immutable,
             TryFromBytes,
             FromZeros,
             IntoBytes,
             Unaligned,
             !FromBytes
         );

         assert_impls!(
             NonZeroU8: KnownLayout,
             Immutable,
             TryFromBytes,
             IntoBytes,
             Unaligned,
             !FromZeros,
             !FromBytes
         );
         assert_impls!(
             NonZeroI8: KnownLayout,
             Immutable,
             TryFromBytes,
             IntoBytes,
             Unaligned,
             !FromZeros,
             !FromBytes
         );
         assert_impls!(
             NonZeroU16: KnownLayout,
             Immutable,
             TryFromBytes,
             IntoBytes,
             !FromBytes,
             !Unaligned
         );
         assert_impls!(
             NonZeroI16: KnownLayout,
             Immutable,
             TryFromBytes,
             IntoBytes,
             !FromBytes,
             !Unaligned
         );
         assert_impls!(
             NonZeroU32: KnownLayout,
             Immutable,
             TryFromBytes,
             IntoBytes,
             !FromBytes,
             !Unaligned
         );
         assert_impls!(
             NonZeroI32: KnownLayout,
             Immutable,
             TryFromBytes,
             IntoBytes,
             !FromBytes,
             !Unaligned
         );
         assert_impls!(
             NonZeroU64: KnownLayout,
             Immutable,
             TryFromBytes,
             IntoBytes,
             !FromBytes,
             !Unaligned
         );
         assert_impls!(
             NonZeroI64: KnownLayout,
             Immutable,
             TryFromBytes,
             IntoBytes,
             !FromBytes,
             !Unaligned
         );
         assert_impls!(
             NonZeroU128: KnownLayout,
             Immutable,
             TryFromBytes,
             IntoBytes,
             !FromBytes,
             !Unaligned
         );
         assert_impls!(
             NonZeroI128: KnownLayout,
             Immutable,
             TryFromBytes,
             IntoBytes,
             !FromBytes,
             !Unaligned
         );
         assert_impls!(
             NonZeroUsize: KnownLayout,
             Immutable,
             TryFromBytes,
             IntoBytes,
             !FromBytes,
             !Unaligned
         );
         assert_impls!(
             NonZeroIsize: KnownLayout,
             Immutable,
             TryFromBytes,
             IntoBytes,
             !FromBytes,
             !Unaligned
         );

         assert_impls!(Option<NonZeroU8>: KnownLayout, Immutable, TryFromBytes, FromZeros, FromBytes, IntoBytes, Unaligned);
         assert_impls!(Option<NonZeroI8>: KnownLayout, Immutable, TryFromBytes, FromZeros, FromBytes, IntoBytes, Unaligned);
         assert_impls!(Option<NonZeroU16>: KnownLayout, Immutable, TryFromBytes, FromZeros, FromBytes, IntoBytes, !Unaligned);
         assert_impls!(Option<NonZeroI16>: KnownLayout, Immutable, TryFromBytes, FromZeros, FromBytes, IntoBytes, !Unaligned);
         assert_impls!(Option<NonZeroU32>: KnownLayout, Immutable, TryFromBytes, FromZeros, FromBytes, IntoBytes, !Unaligned);
         assert_impls!(Option<NonZeroI32>: KnownLayout, Immutable, TryFromBytes, FromZeros, FromBytes, IntoBytes, !Unaligned);
         assert_impls!(Option<NonZeroU64>: KnownLayout, Immutable, TryFromBytes, FromZeros, FromBytes, IntoBytes, !Unaligned);
         assert_impls!(Option<NonZeroI64>: KnownLayout, Immutable, TryFromBytes, FromZeros, FromBytes, IntoBytes, !Unaligned);
         assert_impls!(Option<NonZeroU128>: KnownLayout, Immutable, TryFromBytes, FromZeros, FromBytes, IntoBytes, !Unaligned);
         assert_impls!(Option<NonZeroI128>: KnownLayout, Immutable, TryFromBytes, FromZeros, FromBytes, IntoBytes, !Unaligned);
         assert_impls!(Option<NonZeroUsize>: KnownLayout, Immutable, TryFromBytes, FromZeros, FromBytes, IntoBytes, !Unaligned);
         assert_impls!(Option<NonZeroIsize>: KnownLayout, Immutable, TryFromBytes, FromZeros, FromBytes, IntoBytes, !Unaligned);

         // Implements none of the ZC traits.
         struct NotZerocopy;

         #[rustfmt::skip]
         type FnManyArgs = fn(
             NotZerocopy, u8, u8, u8, u8, u8, u8, u8, u8, u8, u8, u8,
         ) -> (NotZerocopy, NotZerocopy);

         // Allowed, because we're not actually using this type for FFI.
         #[allow(improper_ctypes_definitions)]
         #[rustfmt::skip]
         type ECFnManyArgs = extern "C" fn(
             NotZerocopy, u8, u8, u8, u8, u8, u8, u8, u8, u8, u8, u8,
         ) -> (NotZerocopy, NotZerocopy);

         #[cfg(feature = "alloc")]
         assert_impls!(Option<Box<UnsafeCell<NotZerocopy>>>: KnownLayout, Immutable, TryFromBytes, FromZeros, !FromBytes, !IntoBytes, !Unaligned);
         assert_impls!(Option<Box<[UnsafeCell<NotZerocopy>]>>: KnownLayout, !Immutable, !TryFromBytes, !FromZeros, !FromBytes, !IntoBytes, !Unaligned);
         assert_impls!(Option<&'static UnsafeCell<NotZerocopy>>: KnownLayout, Immutable, TryFromBytes, FromZeros, !FromBytes, !IntoBytes, !Unaligned);
         assert_impls!(Option<&'static [UnsafeCell<NotZerocopy>]>: KnownLayout, Immutable, !TryFromBytes, !FromZeros, !FromBytes, !IntoBytes, !Unaligned);
         assert_impls!(Option<&'static mut UnsafeCell<NotZerocopy>>: KnownLayout, Immutable, TryFromBytes, FromZeros, !FromBytes, !IntoBytes, !Unaligned);
         assert_impls!(Option<&'static mut [UnsafeCell<NotZerocopy>]>: KnownLayout, Immutable, !TryFromBytes, !FromZeros, !FromBytes, !IntoBytes, !Unaligned);
         assert_impls!(Option<NonNull<UnsafeCell<NotZerocopy>>>: KnownLayout, TryFromBytes, FromZeros, Immutable, !FromBytes, !IntoBytes, !Unaligned);
         assert_impls!(Option<NonNull<[UnsafeCell<NotZerocopy>]>>: KnownLayout, Immutable, !TryFromBytes, !FromZeros, !FromBytes, !IntoBytes, !Unaligned);
         assert_impls!(Option<fn()>: KnownLayout, Immutable, TryFromBytes, FromZeros, !FromBytes, !IntoBytes, !Unaligned);
         assert_impls!(Option<FnManyArgs>: KnownLayout, Immutable, TryFromBytes, FromZeros, !FromBytes, !IntoBytes, !Unaligned);
         assert_impls!(Option<extern "C" fn()>: KnownLayout, Immutable, TryFromBytes, FromZeros, !FromBytes, !IntoBytes, !Unaligned);
         assert_impls!(Option<ECFnManyArgs>: KnownLayout, Immutable, TryFromBytes, FromZeros, !FromBytes, !IntoBytes, !Unaligned);

         assert_impls!(PhantomData<NotZerocopy>: KnownLayout, Immutable, TryFromBytes, FromZeros, FromBytes, IntoBytes, Unaligned);
         assert_impls!(PhantomData<UnsafeCell<()>>: KnownLayout, Immutable, TryFromBytes, FromZeros, FromBytes, IntoBytes, Unaligned);
         assert_impls!(PhantomData<[u8]>: KnownLayout, Immutable, TryFromBytes, FromZeros, FromBytes, IntoBytes, Unaligned);

         assert_impls!(ManuallyDrop<u8>: KnownLayout, Immutable, TryFromBytes, FromZeros, FromBytes, IntoBytes, Unaligned);
         // This test is important because it allows us to test our hand-rolled
         // implementation of `<ManuallyDrop<T> as TryFromBytes>::is_bit_valid`.
         assert_impls!(ManuallyDrop<bool>: KnownLayout, Immutable, TryFromBytes, FromZeros, IntoBytes, Unaligned, !FromBytes);
         assert_impls!(ManuallyDrop<[u8]>: KnownLayout, Immutable, TryFromBytes, FromZeros, FromBytes, IntoBytes, Unaligned);
         // This test is important because it allows us to test our hand-rolled
         // implementation of `<ManuallyDrop<T> as TryFromBytes>::is_bit_valid`.
         assert_impls!(ManuallyDrop<[bool]>: KnownLayout, Immutable, TryFromBytes, FromZeros, IntoBytes, Unaligned, !FromBytes);
         assert_impls!(ManuallyDrop<NotZerocopy>: !Immutable, !TryFromBytes, !KnownLayout, !FromZeros, !FromBytes, !IntoBytes, !Unaligned);
         assert_impls!(ManuallyDrop<[NotZerocopy]>: KnownLayout, !Immutable, !TryFromBytes, !FromZeros, !FromBytes, !IntoBytes, !Unaligned);
         assert_impls!(ManuallyDrop<UnsafeCell<()>>: KnownLayout, TryFromBytes, FromZeros, FromBytes, IntoBytes, Unaligned, !Immutable);
         assert_impls!(ManuallyDrop<[UnsafeCell<u8>]>: KnownLayout, TryFromBytes, FromZeros, FromBytes, IntoBytes, Unaligned, !Immutable);
         assert_impls!(ManuallyDrop<[UnsafeCell<bool>]>: KnownLayout, TryFromBytes, FromZeros, IntoBytes, Unaligned, !Immutable, !FromBytes);

         assert_impls!(CoreMaybeUninit<u8>: KnownLayout, Immutable, TryFromBytes, FromZeros, FromBytes, Unaligned, !IntoBytes);
         assert_impls!(CoreMaybeUninit<NotZerocopy>: KnownLayout, TryFromBytes, FromZeros, FromBytes, !Immutable, !IntoBytes, !Unaligned);
         assert_impls!(CoreMaybeUninit<UnsafeCell<()>>: KnownLayout, TryFromBytes, FromZeros, FromBytes, Unaligned, !Immutable, !IntoBytes);

         assert_impls!(Wrapping<u8>: KnownLayout, Immutable, TryFromBytes, FromZeros, FromBytes, IntoBytes, Unaligned);
         // This test is important because it allows us to test our hand-rolled
         // implementation of `<Wrapping<T> as TryFromBytes>::is_bit_valid`.
         assert_impls!(Wrapping<bool>: KnownLayout, Immutable, TryFromBytes, FromZeros, IntoBytes, Unaligned, !FromBytes);
         assert_impls!(Wrapping<NotZerocopy>: KnownLayout, !Immutable, !TryFromBytes, !FromZeros, !FromBytes, !IntoBytes, !Unaligned);
         assert_impls!(Wrapping<UnsafeCell<()>>: KnownLayout, TryFromBytes, FromZeros, FromBytes, IntoBytes, Unaligned, !Immutable);

         assert_impls!(Unalign<u8>: KnownLayout, Immutable, TryFromBytes, FromZeros, FromBytes, IntoBytes, Unaligned);
         // This test is important because it allows us to test our hand-rolled
         // implementation of `<Unalign<T> as TryFromBytes>::is_bit_valid`.
         assert_impls!(Unalign<bool>: KnownLayout, Immutable, TryFromBytes, FromZeros, IntoBytes, Unaligned, !FromBytes);
         assert_impls!(Unalign<NotZerocopy>: KnownLayout, Unaligned, !Immutable, !TryFromBytes, !FromZeros, !FromBytes, !IntoBytes);

         assert_impls!(
             [u8]: KnownLayout,
             Immutable,
             TryFromBytes,
             FromZeros,
             FromBytes,
             IntoBytes,
             Unaligned
         );
         assert_impls!(
             [bool]: KnownLayout,
             Immutable,
             TryFromBytes,
             FromZeros,
             IntoBytes,
             Unaligned,
             !FromBytes
         );
         assert_impls!([NotZerocopy]: KnownLayout, !Immutable, !TryFromBytes, !FromZeros, !FromBytes, !IntoBytes, !Unaligned);
         assert_impls!(
             [u8; 0]: KnownLayout,
             Immutable,
             TryFromBytes,
             FromZeros,
             FromBytes,
             IntoBytes,
             Unaligned,
         );
         assert_impls!(
             [NotZerocopy; 0]: KnownLayout,
             !Immutable,
             !TryFromBytes,
             !FromZeros,
             !FromBytes,
             !IntoBytes,
             !Unaligned
         );
         assert_impls!(
             [u8; 1]: KnownLayout,
             Immutable,
             TryFromBytes,
             FromZeros,
             FromBytes,
             IntoBytes,
             Unaligned,
         );
         assert_impls!(
             [NotZerocopy; 1]: KnownLayout,
             !Immutable,
             !TryFromBytes,
             !FromZeros,
             !FromBytes,
             !IntoBytes,
             !Unaligned
         );

         assert_impls!(*const NotZerocopy: KnownLayout, Immutable, TryFromBytes, FromZeros, !FromBytes, !IntoBytes, !Unaligned);
         assert_impls!(*mut NotZerocopy: KnownLayout, Immutable, TryFromBytes, FromZeros, !FromBytes, !IntoBytes, !Unaligned);
         assert_impls!(*const [NotZerocopy]: KnownLayout, Immutable, !TryFromBytes, !FromZeros, !FromBytes, !IntoBytes, !Unaligned);
         assert_impls!(*mut [NotZerocopy]: KnownLayout, Immutable, !TryFromBytes, !FromZeros, !FromBytes, !IntoBytes, !Unaligned);
         assert_impls!(*const dyn Debug: KnownLayout, Immutable, !TryFromBytes, !FromZeros, !FromBytes, !IntoBytes, !Unaligned);
         assert_impls!(*mut dyn Debug: KnownLayout, Immutable, !TryFromBytes, !FromZeros, !FromBytes, !IntoBytes, !Unaligned);

         #[cfg(feature = "simd")]
         {
             #[allow(unused_macros)]
             macro_rules! test_simd_arch_mod {
                 ($arch:ident, $($typ:ident),*) => {
                     {
                         use core::arch::$arch::{$($typ),*};
                         use crate::*;
                         $( assert_impls!($typ: KnownLayout, Immutable, TryFromBytes, FromZeros, FromBytes, IntoBytes, !Unaligned); )*
                     }
                 };
             }
             #[cfg(target_arch = "x86")]
             test_simd_arch_mod!(x86, __m128, __m128d, __m128i, __m256, __m256d, __m256i);

             #[cfg(all(not(no_zerocopy_simd_x86_avx12_1_89_0), target_arch = "x86"))]
             test_simd_arch_mod!(x86, __m512bh, __m512, __m512d, __m512i);

             #[cfg(target_arch = "x86_64")]
             test_simd_arch_mod!(x86_64, __m128, __m128d, __m128i, __m256, __m256d, __m256i);

             #[cfg(all(not(no_zerocopy_simd_x86_avx12_1_89_0), target_arch = "x86_64"))]
             test_simd_arch_mod!(x86_64, __m512bh, __m512, __m512d, __m512i);

             #[cfg(target_arch = "wasm32")]
             test_simd_arch_mod!(wasm32, v128);

             #[cfg(all(feature = "simd-nightly", target_arch = "powerpc"))]
             test_simd_arch_mod!(
                 powerpc,
                 vector_bool_long,
                 vector_double,
                 vector_signed_long,
                 vector_unsigned_long
             );

             #[cfg(all(feature = "simd-nightly", target_arch = "powerpc64"))]
             test_simd_arch_mod!(
                 powerpc64,
                 vector_bool_long,
                 vector_double,
                 vector_signed_long,
                 vector_unsigned_long
             );
             #[cfg(all(target_arch = "aarch64", not(no_zerocopy_aarch64_simd_1_59_0)))]
             #[rustfmt::skip]
             test_simd_arch_mod!(
                 aarch64, float32x2_t, float32x4_t, float64x1_t, float64x2_t, int8x8_t, int8x8x2_t,
                 int8x8x3_t, int8x8x4_t, int8x16_t, int8x16x2_t, int8x16x3_t, int8x16x4_t, int16x4_t,
                 int16x8_t, int32x2_t, int32x4_t, int64x1_t, int64x2_t, poly8x8_t, poly8x8x2_t, poly8x8x3_t,
                 poly8x8x4_t, poly8x16_t, poly8x16x2_t, poly8x16x3_t, poly8x16x4_t, poly16x4_t, poly16x8_t,
                 poly64x1_t, poly64x2_t, uint8x8_t, uint8x8x2_t, uint8x8x3_t, uint8x8x4_t, uint8x16_t,
                 uint8x16x2_t, uint8x16x3_t, uint8x16x4_t, uint16x4_t, uint16x4x2_t, uint16x4x3_t,
                 uint16x4x4_t, uint16x8_t, uint32x2_t, uint32x4_t, uint64x1_t, uint64x2_t
             );
         }
     }
 }