rust/kernel/list/impl_list_item_mod.rs - linux - Git at Google

 // SPDX-License-Identifier: GPL-2.0

 // Copyright (C) 2024 Google LLC.

 //! Helpers for implementing list traits safely.

 use crate::list::ListLinks;

 /// Declares that this type has a `ListLinks<ID>` field at a fixed offset.
 ///
 /// This trait is only used to help implement `ListItem` safely. If `ListItem` is implemented
 /// manually, then this trait is not needed. Use the [`impl_has_list_links!`] macro to implement
 /// this trait.
 ///
 /// # Safety
 ///
 /// All values of this type must have a `ListLinks<ID>` field at the given offset.
 ///
 /// The behavior of `raw_get_list_links` must not be changed.
 pub unsafe trait HasListLinks<const ID: u64 = 0> {
     /// The offset of the `ListLinks` field.
     const OFFSET: usize;

     /// Returns a pointer to the [`ListLinks<T, ID>`] field.
     ///
     /// # Safety
     ///
     /// The provided pointer must point at a valid struct of type `Self`.
     ///
     /// [`ListLinks<T, ID>`]: ListLinks
     // We don't really need this method, but it's necessary for the implementation of
     // `impl_has_list_links!` to be correct.
     #[inline]
     unsafe fn raw_get_list_links(ptr: *mut Self) -> *mut ListLinks<ID> {
         // SAFETY: The caller promises that the pointer is valid. The implementer promises that the
         // `OFFSET` constant is correct.
         unsafe { (ptr as *mut u8).add(Self::OFFSET) as *mut ListLinks<ID> }
     }
 }

 /// Implements the [`HasListLinks`] trait for the given type.
 #[macro_export]
 macro_rules! impl_has_list_links {
     ($(impl$(<$($implarg:ident),*>)?
        HasListLinks$(<$id:tt>)?
        for $self:ident $(<$($selfarg:ty),*>)?
        { self$(.$field:ident)* }
     )*) => {$(
         // SAFETY: The implementation of `raw_get_list_links` only compiles if the field has the
         // right type.
         //
         // The behavior of `raw_get_list_links` is not changed since the `addr_of_mut!` macro is
         // equivalent to the pointer offset operation in the trait definition.
         unsafe impl$(<$($implarg),*>)? $crate::list::HasListLinks$(<$id>)? for
             $self $(<$($selfarg),*>)?
         {
             const OFFSET: usize = ::core::mem::offset_of!(Self, $($field).*) as usize;

             #[inline]
             unsafe fn raw_get_list_links(ptr: *mut Self) -> *mut $crate::list::ListLinks$(<$id>)? {
                 // SAFETY: The caller promises that the pointer is not dangling. We know that this
                 // expression doesn't follow any pointers, as the `offset_of!` invocation above
                 // would otherwise not compile.
                 unsafe { ::core::ptr::addr_of_mut!((*ptr)$(.$field)*) }
             }
         }
     )*};
 }
 pub use impl_has_list_links;

 /// Declares that the `ListLinks<ID>` field in this struct is inside a `ListLinksSelfPtr<T, ID>`.
 ///
 /// # Safety
 ///
 /// The `ListLinks<ID>` field of this struct at the offset `HasListLinks<ID>::OFFSET` must be
 /// inside a `ListLinksSelfPtr<T, ID>`.
 pub unsafe trait HasSelfPtr<T: ?Sized, const ID: u64 = 0>
 where
     Self: HasListLinks<ID>,
 {
 }

 /// Implements the [`HasListLinks`] and [`HasSelfPtr`] traits for the given type.
 #[macro_export]
 macro_rules! impl_has_list_links_self_ptr {
     ($(impl$({$($implarg:tt)*})?
        HasSelfPtr<$item_type:ty $(, $id:tt)?>
        for $self:ident $(<$($selfarg:ty),*>)?
        { self.$field:ident }
     )*) => {$(
         // SAFETY: The implementation of `raw_get_list_links` only compiles if the field has the
         // right type.
         unsafe impl$(<$($implarg)*>)? $crate::list::HasSelfPtr<$item_type $(, $id)?> for
             $self $(<$($selfarg),*>)?
         {}

         unsafe impl$(<$($implarg)*>)? $crate::list::HasListLinks$(<$id>)? for
             $self $(<$($selfarg),*>)?
         {
             const OFFSET: usize = ::core::mem::offset_of!(Self, $field) as usize;

             #[inline]
             unsafe fn raw_get_list_links(ptr: *mut Self) -> *mut $crate::list::ListLinks$(<$id>)? {
                 // SAFETY: The caller promises that the pointer is not dangling.
                 let ptr: *mut $crate::list::ListLinksSelfPtr<$item_type $(, $id)?> =
                     unsafe { ::core::ptr::addr_of_mut!((*ptr).$field) };
                 ptr.cast()
             }
         }
     )*};
 }
 pub use impl_has_list_links_self_ptr;

 /// Implements the [`ListItem`] trait for the given type.
 ///
 /// Requires that the type implements [`HasListLinks`]. Use the [`impl_has_list_links!`] macro to
 /// implement that trait.
 ///
 /// [`ListItem`]: crate::list::ListItem
 #[macro_export]
 macro_rules! impl_list_item {
     (
         $(impl$({$($generics:tt)*})? ListItem<$num:tt> for $t:ty {
             using ListLinks;
         })*
     ) => {$(
         // SAFETY: See GUARANTEES comment on each method.
         unsafe impl$(<$($generics)*>)? $crate::list::ListItem<$num> for $t {
             // GUARANTEES:
             // * This returns the same pointer as `prepare_to_insert` because `prepare_to_insert`
             //   is implemented in terms of `view_links`.
             // * By the type invariants of `ListLinks`, the `ListLinks` has two null pointers when
             //   this value is not in a list.
             unsafe fn view_links(me: *const Self) -> *mut $crate::list::ListLinks<$num> {
                 // SAFETY: The caller guarantees that `me` points at a valid value of type `Self`.
                 unsafe {
                     <Self as $crate::list::HasListLinks<$num>>::raw_get_list_links(me.cast_mut())
                 }
             }

             // GUARANTEES:
             // * `me` originates from the most recent call to `prepare_to_insert`, which just added
             //   `offset` to the pointer passed to `prepare_to_insert`. This method subtracts
             //   `offset` from `me` so it returns the pointer originally passed to
             //   `prepare_to_insert`.
             // * The pointer remains valid until the next call to `post_remove` because the caller
             //   of the most recent call to `prepare_to_insert` promised to retain ownership of the
             //   `ListArc` containing `Self` until the next call to `post_remove`. The value cannot
             //   be destroyed while a `ListArc` reference exists.
             unsafe fn view_value(me: *mut $crate::list::ListLinks<$num>) -> *const Self {
                 let offset = <Self as $crate::list::HasListLinks<$num>>::OFFSET;
                 // SAFETY: `me` originates from the most recent call to `prepare_to_insert`, so it
                 // points at the field at offset `offset` in a value of type `Self`. Thus,
                 // subtracting `offset` from `me` is still in-bounds of the allocation.
                 unsafe { (me as *const u8).sub(offset) as *const Self }
             }

             // GUARANTEES:
             // This implementation of `ListItem` will not give out exclusive access to the same
             // `ListLinks` several times because calls to `prepare_to_insert` and `post_remove`
             // must alternate and exclusive access is given up when `post_remove` is called.
             //
             // Other invocations of `impl_list_item!` also cannot give out exclusive access to the
             // same `ListLinks` because you can only implement `ListItem` once for each value of
             // `ID`, and the `ListLinks` fields only work with the specified `ID`.
             unsafe fn prepare_to_insert(me: *const Self) -> *mut $crate::list::ListLinks<$num> {
                 // SAFETY: The caller promises that `me` points at a valid value.
                 unsafe { <Self as $crate::list::ListItem<$num>>::view_links(me) }
             }

             // GUARANTEES:
             // * `me` originates from the most recent call to `prepare_to_insert`, which just added
             //   `offset` to the pointer passed to `prepare_to_insert`. This method subtracts
             //   `offset` from `me` so it returns the pointer originally passed to
             //   `prepare_to_insert`.
             unsafe fn post_remove(me: *mut $crate::list::ListLinks<$num>) -> *const Self {
                 let offset = <Self as $crate::list::HasListLinks<$num>>::OFFSET;
                 // SAFETY: `me` originates from the most recent call to `prepare_to_insert`, so it
                 // points at the field at offset `offset` in a value of type `Self`. Thus,
                 // subtracting `offset` from `me` is still in-bounds of the allocation.
                 unsafe { (me as *const u8).sub(offset) as *const Self }
             }
         }
     )*};

     (
         $(impl$({$($generics:tt)*})? ListItem<$num:tt> for $t:ty {
             using ListLinksSelfPtr;
         })*
     ) => {$(
         // SAFETY: See GUARANTEES comment on each method.
         unsafe impl$(<$($generics)*>)? $crate::list::ListItem<$num> for $t {
             // GUARANTEES:
             // This implementation of `ListItem` will not give out exclusive access to the same
             // `ListLinks` several times because calls to `prepare_to_insert` and `post_remove`
             // must alternate and exclusive access is given up when `post_remove` is called.
             //
             // Other invocations of `impl_list_item!` also cannot give out exclusive access to the
             // same `ListLinks` because you can only implement `ListItem` once for each value of
             // `ID`, and the `ListLinks` fields only work with the specified `ID`.
             unsafe fn prepare_to_insert(me: *const Self) -> *mut $crate::list::ListLinks<$num> {
                 // SAFETY: The caller promises that `me` points at a valid value of type `Self`.
                 let links_field = unsafe { <Self as $crate::list::ListItem<$num>>::view_links(me) };

                 let spoff = $crate::list::ListLinksSelfPtr::<Self, $num>::LIST_LINKS_SELF_PTR_OFFSET;
                 // Goes via the offset as the field is private.
                 //
                 // SAFETY: The constant is equal to `offset_of!(ListLinksSelfPtr, self_ptr)`, so
                 // the pointer stays in bounds of the allocation.
                 let self_ptr = unsafe { (links_field as *const u8).add(spoff) }
                     as *const $crate::types::Opaque<*const Self>;
                 let cell_inner = $crate::types::Opaque::raw_get(self_ptr);

                 // SAFETY: This value is not accessed in any other places than `prepare_to_insert`,
                 // `post_remove`, or `view_value`. By the safety requirements of those methods,
                 // none of these three methods may be called in parallel with this call to
                 // `prepare_to_insert`, so this write will not race with any other access to the
                 // value.
                 unsafe { ::core::ptr::write(cell_inner, me) };

                 links_field
             }

             // GUARANTEES:
             // * This returns the same pointer as `prepare_to_insert` because `prepare_to_insert`
             //   returns the return value of `view_links`.
             // * By the type invariants of `ListLinks`, the `ListLinks` has two null pointers when
             //   this value is not in a list.
             unsafe fn view_links(me: *const Self) -> *mut $crate::list::ListLinks<$num> {
                 // SAFETY: The caller promises that `me` points at a valid value of type `Self`.
                 unsafe { <Self as HasListLinks<$num>>::raw_get_list_links(me.cast_mut()) }
             }

             // This function is also used as the implementation of `post_remove`, so the caller
             // may choose to satisfy the safety requirements of `post_remove` instead of the safety
             // requirements for `view_value`.
             //
             // GUARANTEES: (always)
             // * This returns the same pointer as the one passed to the most recent call to
             //   `prepare_to_insert` since that call wrote that pointer to this location. The value
             //   is only modified in `prepare_to_insert`, so it has not been modified since the
             //   most recent call.
             //
             // GUARANTEES: (only when using the `view_value` safety requirements)
             // * The pointer remains valid until the next call to `post_remove` because the caller
             //   of the most recent call to `prepare_to_insert` promised to retain ownership of the
             //   `ListArc` containing `Self` until the next call to `post_remove`. The value cannot
             //   be destroyed while a `ListArc` reference exists.
             unsafe fn view_value(links_field: *mut $crate::list::ListLinks<$num>) -> *const Self {
                 let spoff = $crate::list::ListLinksSelfPtr::<Self, $num>::LIST_LINKS_SELF_PTR_OFFSET;
                 // SAFETY: The constant is equal to `offset_of!(ListLinksSelfPtr, self_ptr)`, so
                 // the pointer stays in bounds of the allocation.
                 let self_ptr = unsafe { (links_field as *const u8).add(spoff) }
                     as *const ::core::cell::UnsafeCell<*const Self>;
                 let cell_inner = ::core::cell::UnsafeCell::raw_get(self_ptr);
                 // SAFETY: This is not a data race, because the only function that writes to this
                 // value is `prepare_to_insert`, but by the safety requirements the
                 // `prepare_to_insert` method may not be called in parallel with `view_value` or
                 // `post_remove`.
                 unsafe { ::core::ptr::read(cell_inner) }
             }

             // GUARANTEES:
             // The first guarantee of `view_value` is exactly what `post_remove` guarantees.
             unsafe fn post_remove(me: *mut $crate::list::ListLinks<$num>) -> *const Self {
                 // SAFETY: This specific implementation of `view_value` allows the caller to
                 // promise the safety requirements of `post_remove` instead of the safety
                 // requirements for `view_value`.
                 unsafe { <Self as $crate::list::ListItem<$num>>::view_value(me) }
             }
         }
     )*};
 }
 pub use impl_list_item;
	// SPDX-License-Identifier: GPL-2.0

	// Copyright (C) 2024 Google LLC.

	//! Helpers for implementing list traits safely.

	use crate::list::ListLinks;

	/// Declares that this type has a `ListLinks<ID>` field at a fixed offset.
	///
	/// This trait is only used to help implement `ListItem` safely. If `ListItem` is implemented
	/// manually, then this trait is not needed. Use the [`impl_has_list_links!`] macro to implement
	/// this trait.
	///
	/// # Safety
	///
	/// All values of this type must have a `ListLinks<ID>` field at the given offset.
	///
	/// The behavior of `raw_get_list_links` must not be changed.
	pub unsafe trait HasListLinks<const ID: u64 = 0> {
	/// The offset of the `ListLinks` field.
	const OFFSET: usize;

	/// Returns a pointer to the [`ListLinks<T, ID>`] field.
	///
	/// # Safety
	///
	/// The provided pointer must point at a valid struct of type `Self`.
	///
	/// [`ListLinks<T, ID>`]: ListLinks
	// We don't really need this method, but it's necessary for the implementation of
	// `impl_has_list_links!` to be correct.
	#[inline]
	unsafe fn raw_get_list_links(ptr: mut Self) -> mut ListLinks<ID> {
	// SAFETY: The caller promises that the pointer is valid. The implementer promises that the
	// `OFFSET` constant is correct.
	unsafe { (ptr as mut u8).add(Self::OFFSET) as mut ListLinks<ID> }
	}
	}

	/// Implements the [`HasListLinks`] trait for the given type.
	#[macro_export]
	macro_rules! impl_has_list_links {
	($(impl$(<$($implarg:ident),*>)?
	HasListLinks$(<$id:tt>)?
	for $self:ident $(<$($selfarg:ty),*>)?
	{ self$(.$field:ident)* }
	)*) => {$(
	// SAFETY: The implementation of `raw_get_list_links` only compiles if the field has the
	// right type.
	//
	// The behavior of `raw_get_list_links` is not changed since the `addr_of_mut!` macro is
	// equivalent to the pointer offset operation in the trait definition.
	unsafe impl$(<$($implarg),*>)? $crate::list::HasListLinks$(<$id>)? for
	$self $(<$($selfarg),*>)?
	{
	const OFFSET: usize = ::core::mem::offset_of!(Self, $($field).*) as usize;

	#[inline]
	unsafe fn raw_get_list_links(ptr: mut Self) -> mut $crate::list::ListLinks$(<$id>)? {
	// SAFETY: The caller promises that the pointer is not dangling. We know that this
	// expression doesn't follow any pointers, as the `offset_of!` invocation above
	// would otherwise not compile.
	unsafe { ::core::ptr::addr_of_mut!((ptr)$(.$field)) }
	}
	}
	)*};
	}
	pub use impl_has_list_links;

	/// Declares that the `ListLinks<ID>` field in this struct is inside a `ListLinksSelfPtr<T, ID>`.
	///
	/// # Safety
	///
	/// The `ListLinks<ID>` field of this struct at the offset `HasListLinks<ID>::OFFSET` must be
	/// inside a `ListLinksSelfPtr<T, ID>`.
	pub unsafe trait HasSelfPtr<T: ?Sized, const ID: u64 = 0>
	where
	Self: HasListLinks<ID>,
	{
	}

	/// Implements the [`HasListLinks`] and [`HasSelfPtr`] traits for the given type.
	#[macro_export]
	macro_rules! impl_has_list_links_self_ptr {
	($(impl$({$($implarg:tt)*})?
	HasSelfPtr<$item_type:ty $(, $id:tt)?>
	for $self:ident $(<$($selfarg:ty),*>)?
	{ self.$field:ident }
	)*) => {$(
	// SAFETY: The implementation of `raw_get_list_links` only compiles if the field has the
	// right type.
	unsafe impl$(<$($implarg)*>)? $crate::list::HasSelfPtr<$item_type $(, $id)?> for
	$self $(<$($selfarg),*>)?
	{}

	unsafe impl$(<$($implarg)*>)? $crate::list::HasListLinks$(<$id>)? for
	$self $(<$($selfarg),*>)?
	{
	const OFFSET: usize = ::core::mem::offset_of!(Self, $field) as usize;

	#[inline]
	unsafe fn raw_get_list_links(ptr: mut Self) -> mut $crate::list::ListLinks$(<$id>)? {
	// SAFETY: The caller promises that the pointer is not dangling.
	let ptr: *mut $crate::list::ListLinksSelfPtr<$item_type $(, $id)?> =
	unsafe { ::core::ptr::addr_of_mut!((*ptr).$field) };
	ptr.cast()
	}
	}
	)*};
	}
	pub use impl_has_list_links_self_ptr;

	/// Implements the [`ListItem`] trait for the given type.
	///
	/// Requires that the type implements [`HasListLinks`]. Use the [`impl_has_list_links!`] macro to
	/// implement that trait.
	///
	/// [`ListItem`]: crate::list::ListItem
	#[macro_export]
	macro_rules! impl_list_item {
	(
	$(impl$({$($generics:tt)*})? ListItem<$num:tt> for $t:ty {
	using ListLinks;
	})*
	) => {$(
	// SAFETY: See GUARANTEES comment on each method.
	unsafe impl$(<$($generics)*>)? $crate::list::ListItem<$num> for $t {
	// GUARANTEES:
	// * This returns the same pointer as `prepare_to_insert` because `prepare_to_insert`
	// is implemented in terms of `view_links`.
	// * By the type invariants of `ListLinks`, the `ListLinks` has two null pointers when
	// this value is not in a list.
	unsafe fn view_links(me: const Self) -> mut $crate::list::ListLinks<$num> {
	// SAFETY: The caller guarantees that `me` points at a valid value of type `Self`.
	unsafe {
	<Self as $crate::list::HasListLinks<$num>>::raw_get_list_links(me.cast_mut())
	}
	}

	// GUARANTEES:
	// * `me` originates from the most recent call to `prepare_to_insert`, which just added
	// `offset` to the pointer passed to `prepare_to_insert`. This method subtracts
	// `offset` from `me` so it returns the pointer originally passed to
	// `prepare_to_insert`.
	// * The pointer remains valid until the next call to `post_remove` because the caller
	// of the most recent call to `prepare_to_insert` promised to retain ownership of the
	// `ListArc` containing `Self` until the next call to `post_remove`. The value cannot
	// be destroyed while a `ListArc` reference exists.
	unsafe fn view_value(me: mut $crate::list::ListLinks<$num>) -> const Self {
	let offset = <Self as $crate::list::HasListLinks<$num>>::OFFSET;
	// SAFETY: `me` originates from the most recent call to `prepare_to_insert`, so it
	// points at the field at offset `offset` in a value of type `Self`. Thus,
	// subtracting `offset` from `me` is still in-bounds of the allocation.
	unsafe { (me as const u8).sub(offset) as const Self }
	}

	// GUARANTEES:
	// This implementation of `ListItem` will not give out exclusive access to the same
	// `ListLinks` several times because calls to `prepare_to_insert` and `post_remove`
	// must alternate and exclusive access is given up when `post_remove` is called.
	//
	// Other invocations of `impl_list_item!` also cannot give out exclusive access to the
	// same `ListLinks` because you can only implement `ListItem` once for each value of
	// `ID`, and the `ListLinks` fields only work with the specified `ID`.
	unsafe fn prepare_to_insert(me: const Self) -> mut $crate::list::ListLinks<$num> {
	// SAFETY: The caller promises that `me` points at a valid value.
	unsafe { <Self as $crate::list::ListItem<$num>>::view_links(me) }
	}

	// GUARANTEES:
	// * `me` originates from the most recent call to `prepare_to_insert`, which just added
	// `offset` to the pointer passed to `prepare_to_insert`. This method subtracts
	// `offset` from `me` so it returns the pointer originally passed to
	// `prepare_to_insert`.
	unsafe fn post_remove(me: mut $crate::list::ListLinks<$num>) -> const Self {
	let offset = <Self as $crate::list::HasListLinks<$num>>::OFFSET;
	// SAFETY: `me` originates from the most recent call to `prepare_to_insert`, so it
	// points at the field at offset `offset` in a value of type `Self`. Thus,
	// subtracting `offset` from `me` is still in-bounds of the allocation.
	unsafe { (me as const u8).sub(offset) as const Self }
	}
	}
	)*};

	(
	$(impl$({$($generics:tt)*})? ListItem<$num:tt> for $t:ty {
	using ListLinksSelfPtr;
	})*
	) => {$(
	// SAFETY: See GUARANTEES comment on each method.
	unsafe impl$(<$($generics)*>)? $crate::list::ListItem<$num> for $t {
	// GUARANTEES:
	// This implementation of `ListItem` will not give out exclusive access to the same
	// `ListLinks` several times because calls to `prepare_to_insert` and `post_remove`
	// must alternate and exclusive access is given up when `post_remove` is called.
	//
	// Other invocations of `impl_list_item!` also cannot give out exclusive access to the
	// same `ListLinks` because you can only implement `ListItem` once for each value of
	// `ID`, and the `ListLinks` fields only work with the specified `ID`.
	unsafe fn prepare_to_insert(me: const Self) -> mut $crate::list::ListLinks<$num> {
	// SAFETY: The caller promises that `me` points at a valid value of type `Self`.
	let links_field = unsafe { <Self as $crate::list::ListItem<$num>>::view_links(me) };

	let spoff = $crate::list::ListLinksSelfPtr::<Self, $num>::LIST_LINKS_SELF_PTR_OFFSET;
	// Goes via the offset as the field is private.
	//
	// SAFETY: The constant is equal to `offset_of!(ListLinksSelfPtr, self_ptr)`, so
	// the pointer stays in bounds of the allocation.
	let self_ptr = unsafe { (links_field as *const u8).add(spoff) }
	as const $crate::types::Opaque<const Self>;
	let cell_inner = $crate::types::Opaque::raw_get(self_ptr);

	// SAFETY: This value is not accessed in any other places than `prepare_to_insert`,
	// `post_remove`, or `view_value`. By the safety requirements of those methods,
	// none of these three methods may be called in parallel with this call to
	// `prepare_to_insert`, so this write will not race with any other access to the
	// value.
	unsafe { ::core::ptr::write(cell_inner, me) };

	links_field
	}

	// GUARANTEES:
	// * This returns the same pointer as `prepare_to_insert` because `prepare_to_insert`
	// returns the return value of `view_links`.
	// * By the type invariants of `ListLinks`, the `ListLinks` has two null pointers when
	// this value is not in a list.
	unsafe fn view_links(me: const Self) -> mut $crate::list::ListLinks<$num> {
	// SAFETY: The caller promises that `me` points at a valid value of type `Self`.
	unsafe { <Self as HasListLinks<$num>>::raw_get_list_links(me.cast_mut()) }
	}

	// This function is also used as the implementation of `post_remove`, so the caller
	// may choose to satisfy the safety requirements of `post_remove` instead of the safety
	// requirements for `view_value`.
	//
	// GUARANTEES: (always)
	// * This returns the same pointer as the one passed to the most recent call to
	// `prepare_to_insert` since that call wrote that pointer to this location. The value
	// is only modified in `prepare_to_insert`, so it has not been modified since the
	// most recent call.
	//
	// GUARANTEES: (only when using the `view_value` safety requirements)
	// * The pointer remains valid until the next call to `post_remove` because the caller
	// of the most recent call to `prepare_to_insert` promised to retain ownership of the
	// `ListArc` containing `Self` until the next call to `post_remove`. The value cannot
	// be destroyed while a `ListArc` reference exists.
	unsafe fn view_value(links_field: mut $crate::list::ListLinks<$num>) -> const Self {
	let spoff = $crate::list::ListLinksSelfPtr::<Self, $num>::LIST_LINKS_SELF_PTR_OFFSET;
	// SAFETY: The constant is equal to `offset_of!(ListLinksSelfPtr, self_ptr)`, so
	// the pointer stays in bounds of the allocation.
	let self_ptr = unsafe { (links_field as *const u8).add(spoff) }
	as const ::core::cell::UnsafeCell<const Self>;
	let cell_inner = ::core::cell::UnsafeCell::raw_get(self_ptr);
	// SAFETY: This is not a data race, because the only function that writes to this
	// value is `prepare_to_insert`, but by the safety requirements the
	// `prepare_to_insert` method may not be called in parallel with `view_value` or
	// `post_remove`.
	unsafe { ::core::ptr::read(cell_inner) }
	}

	// GUARANTEES:
	// The first guarantee of `view_value` is exactly what `post_remove` guarantees.
	unsafe fn post_remove(me: mut $crate::list::ListLinks<$num>) -> const Self {
	// SAFETY: This specific implementation of `view_value` allows the caller to
	// promise the safety requirements of `post_remove` instead of the safety
	// requirements for `view_value`.
	unsafe { <Self as $crate::list::ListItem<$num>>::view_value(me) }
	}
	}
	)*};
	}
	pub use impl_list_item;