rust/kernel/types.rs - linux - Git at Google

 // SPDX-License-Identifier: GPL-2.0

 //! Kernel types.

 use crate::init::{self, PinInit};
 use alloc::boxed::Box;
 use core::{
     cell::UnsafeCell,
     marker::{PhantomData, PhantomPinned},
     mem::MaybeUninit,
     ops::{Deref, DerefMut},
     ptr::NonNull,
 };

 /// Used to transfer ownership to and from foreign (non-Rust) languages.
 ///
 /// Ownership is transferred from Rust to a foreign language by calling [`Self::into_foreign`] and
 /// later may be transferred back to Rust by calling [`Self::from_foreign`].
 ///
 /// This trait is meant to be used in cases when Rust objects are stored in C objects and
 /// eventually "freed" back to Rust.
 pub trait ForeignOwnable: Sized {
     /// Type of values borrowed between calls to [`ForeignOwnable::into_foreign`] and
     /// [`ForeignOwnable::from_foreign`].
     type Borrowed<'a>;

     /// Converts a Rust-owned object to a foreign-owned one.
     ///
     /// The foreign representation is a pointer to void.
     fn into_foreign(self) -> *const core::ffi::c_void;

     /// Borrows a foreign-owned object.
     ///
     /// # Safety
     ///
     /// `ptr` must have been returned by a previous call to [`ForeignOwnable::into_foreign`] for
     /// which a previous matching [`ForeignOwnable::from_foreign`] hasn't been called yet.
     unsafe fn borrow<'a>(ptr: *const core::ffi::c_void) -> Self::Borrowed<'a>;

     /// Converts a foreign-owned object back to a Rust-owned one.
     ///
     /// # Safety
     ///
     /// `ptr` must have been returned by a previous call to [`ForeignOwnable::into_foreign`] for
     /// which a previous matching [`ForeignOwnable::from_foreign`] hasn't been called yet.
     /// Additionally, all instances (if any) of values returned by [`ForeignOwnable::borrow`] for
     /// this object must have been dropped.
     unsafe fn from_foreign(ptr: *const core::ffi::c_void) -> Self;
 }

 impl<T: 'static> ForeignOwnable for Box<T> {
     type Borrowed<'a> = &'a T;

     fn into_foreign(self) -> *const core::ffi::c_void {
         Box::into_raw(self) as _
     }

     unsafe fn borrow<'a>(ptr: *const core::ffi::c_void) -> &'a T {
         // SAFETY: The safety requirements for this function ensure that the object is still alive,
         // so it is safe to dereference the raw pointer.
         // The safety requirements of `from_foreign` also ensure that the object remains alive for
         // the lifetime of the returned value.
         unsafe { &*ptr.cast() }
     }

     unsafe fn from_foreign(ptr: *const core::ffi::c_void) -> Self {
         // SAFETY: The safety requirements of this function ensure that `ptr` comes from a previous
         // call to `Self::into_foreign`.
         unsafe { Box::from_raw(ptr as _) }
     }
 }

 impl ForeignOwnable for () {
     type Borrowed<'a> = ();

     fn into_foreign(self) -> *const core::ffi::c_void {
         core::ptr::NonNull::dangling().as_ptr()
     }

     unsafe fn borrow<'a>(_: *const core::ffi::c_void) -> Self::Borrowed<'a> {}

     unsafe fn from_foreign(_: *const core::ffi::c_void) -> Self {}
 }

 /// Runs a cleanup function/closure when dropped.
 ///
 /// The [`ScopeGuard::dismiss`] function prevents the cleanup function from running.
 ///
 /// # Examples
 ///
 /// In the example below, we have multiple exit paths and we want to log regardless of which one is
 /// taken:
 /// ```
 /// # use kernel::types::ScopeGuard;
 /// fn example1(arg: bool) {
 ///     let _log = ScopeGuard::new(|| pr_info!("example1 completed\n"));
 ///
 ///     if arg {
 ///         return;
 ///     }
 ///
 ///     pr_info!("Do something...\n");
 /// }
 ///
 /// # example1(false);
 /// # example1(true);
 /// ```
 ///
 /// In the example below, we want to log the same message on all early exits but a different one on
 /// the main exit path:
 /// ```
 /// # use kernel::types::ScopeGuard;
 /// fn example2(arg: bool) {
 ///     let log = ScopeGuard::new(|| pr_info!("example2 returned early\n"));
 ///
 ///     if arg {
 ///         return;
 ///     }
 ///
 ///     // (Other early returns...)
 ///
 ///     log.dismiss();
 ///     pr_info!("example2 no early return\n");
 /// }
 ///
 /// # example2(false);
 /// # example2(true);
 /// ```
 ///
 /// In the example below, we need a mutable object (the vector) to be accessible within the log
 /// function, so we wrap it in the [`ScopeGuard`]:
 /// ```
 /// # use kernel::types::ScopeGuard;
 /// fn example3(arg: bool) -> Result {
 ///     let mut vec =
 ///         ScopeGuard::new_with_data(Vec::new(), |v| pr_info!("vec had {} elements\n", v.len()));
 ///
 ///     vec.try_push(10u8)?;
 ///     if arg {
 ///         return Ok(());
 ///     }
 ///     vec.try_push(20u8)?;
 ///     Ok(())
 /// }
 ///
 /// # assert_eq!(example3(false), Ok(()));
 /// # assert_eq!(example3(true), Ok(()));
 /// ```
 ///
 /// # Invariants
 ///
 /// The value stored in the struct is nearly always `Some(_)`, except between
 /// [`ScopeGuard::dismiss`] and [`ScopeGuard::drop`]: in this case, it will be `None` as the value
 /// will have been returned to the caller. Since  [`ScopeGuard::dismiss`] consumes the guard,
 /// callers won't be able to use it anymore.
 pub struct ScopeGuard<T, F: FnOnce(T)>(Option<(T, F)>);

 impl<T, F: FnOnce(T)> ScopeGuard<T, F> {
     /// Creates a new guarded object wrapping the given data and with the given cleanup function.
     pub fn new_with_data(data: T, cleanup_func: F) -> Self {
         // INVARIANT: The struct is being initialised with `Some(_)`.
         Self(Some((data, cleanup_func)))
     }

     /// Prevents the cleanup function from running and returns the guarded data.
     pub fn dismiss(mut self) -> T {
         // INVARIANT: This is the exception case in the invariant; it is not visible to callers
         // because this function consumes `self`.
         self.0.take().unwrap().0
     }
 }

 impl ScopeGuard<(), fn(())> {
     /// Creates a new guarded object with the given cleanup function.
     pub fn new(cleanup: impl FnOnce()) -> ScopeGuard<(), impl FnOnce(())> {
         ScopeGuard::new_with_data((), move |_| cleanup())
     }
 }

 impl<T, F: FnOnce(T)> Deref for ScopeGuard<T, F> {
     type Target = T;

     fn deref(&self) -> &T {
         // The type invariants guarantee that `unwrap` will succeed.
         &self.0.as_ref().unwrap().0
     }
 }

 impl<T, F: FnOnce(T)> DerefMut for ScopeGuard<T, F> {
     fn deref_mut(&mut self) -> &mut T {
         // The type invariants guarantee that `unwrap` will succeed.
         &mut self.0.as_mut().unwrap().0
     }
 }

 impl<T, F: FnOnce(T)> Drop for ScopeGuard<T, F> {
     fn drop(&mut self) {
         // Run the cleanup function if one is still present.
         if let Some((data, cleanup)) = self.0.take() {
             cleanup(data)
         }
     }
 }

 /// Stores an opaque value.
 ///
 /// This is meant to be used with FFI objects that are never interpreted by Rust code.
 #[repr(transparent)]
 pub struct Opaque<T> {
     value: UnsafeCell<MaybeUninit<T>>,
     _pin: PhantomPinned,
 }

 impl<T> Opaque<T> {
     /// Creates a new opaque value.
     pub const fn new(value: T) -> Self {
         Self {
             value: UnsafeCell::new(MaybeUninit::new(value)),
             _pin: PhantomPinned,
         }
     }

     /// Creates an uninitialised value.
     pub const fn uninit() -> Self {
         Self {
             value: UnsafeCell::new(MaybeUninit::uninit()),
             _pin: PhantomPinned,
         }
     }

     /// Creates a pin-initializer from the given initializer closure.
     ///
     /// The returned initializer calls the given closure with the pointer to the inner `T` of this
     /// `Opaque`. Since this memory is uninitialized, the closure is not allowed to read from it.
     ///
     /// This function is safe, because the `T` inside of an `Opaque` is allowed to be
     /// uninitialized. Additionally, access to the inner `T` requires `unsafe`, so the caller needs
     /// to verify at that point that the inner value is valid.
     pub fn ffi_init(init_func: impl FnOnce(*mut T)) -> impl PinInit<Self> {
         // SAFETY: We contain a `MaybeUninit`, so it is OK for the `init_func` to not fully
         // initialize the `T`.
         unsafe {
             init::pin_init_from_closure::<_, ::core::convert::Infallible>(move |slot| {
                 init_func(Self::raw_get(slot));
                 Ok(())
             })
         }
     }

     /// Returns a raw pointer to the opaque data.
     pub fn get(&self) -> *mut T {
         UnsafeCell::get(&self.value).cast::<T>()
     }

     /// Gets the value behind `this`.
     ///
     /// This function is useful to get access to the value without creating intermediate
     /// references.
     pub const fn raw_get(this: *const Self) -> *mut T {
         UnsafeCell::raw_get(this.cast::<UnsafeCell<MaybeUninit<T>>>()).cast::<T>()
     }
 }

 /// Types that are _always_ reference counted.
 ///
 /// It allows such types to define their own custom ref increment and decrement functions.
 /// Additionally, it allows users to convert from a shared reference `&T` to an owned reference
 /// [`ARef<T>`].
 ///
 /// This is usually implemented by wrappers to existing structures on the C side of the code. For
 /// Rust code, the recommendation is to use [`Arc`](crate::sync::Arc) to create reference-counted
 /// instances of a type.
 ///
 /// # Safety
 ///
 /// Implementers must ensure that increments to the reference count keep the object alive in memory
 /// at least until matching decrements are performed.
 ///
 /// Implementers must also ensure that all instances are reference-counted. (Otherwise they
 /// won't be able to honour the requirement that [`AlwaysRefCounted::inc_ref`] keep the object
 /// alive.)
 pub unsafe trait AlwaysRefCounted {
     /// Increments the reference count on the object.
     fn inc_ref(&self);

     /// Decrements the reference count on the object.
     ///
     /// Frees the object when the count reaches zero.
     ///
     /// # Safety
     ///
     /// Callers must ensure that there was a previous matching increment to the reference count,
     /// and that the object is no longer used after its reference count is decremented (as it may
     /// result in the object being freed), unless the caller owns another increment on the refcount
     /// (e.g., it calls [`AlwaysRefCounted::inc_ref`] twice, then calls
     /// [`AlwaysRefCounted::dec_ref`] once).
     unsafe fn dec_ref(obj: NonNull<Self>);
 }

 /// An owned reference to an always-reference-counted object.
 ///
 /// The object's reference count is automatically decremented when an instance of [`ARef`] is
 /// dropped. It is also automatically incremented when a new instance is created via
 /// [`ARef::clone`].
 ///
 /// # Invariants
 ///
 /// The pointer stored in `ptr` is non-null and valid for the lifetime of the [`ARef`] instance. In
 /// particular, the [`ARef`] instance owns an increment on the underlying object's reference count.
 pub struct ARef<T: AlwaysRefCounted> {
     ptr: NonNull<T>,
     _p: PhantomData<T>,
 }

 // SAFETY: It is safe to send `ARef<T>` to another thread when the underlying `T` is `Sync` because
 // it effectively means sharing `&T` (which is safe because `T` is `Sync`); additionally, it needs
 // `T` to be `Send` because any thread that has an `ARef<T>` may ultimately access `T` using a
 // mutable reference, for example, when the reference count reaches zero and `T` is dropped.
 unsafe impl<T: AlwaysRefCounted + Sync + Send> Send for ARef<T> {}

 // SAFETY: It is safe to send `&ARef<T>` to another thread when the underlying `T` is `Sync`
 // because it effectively means sharing `&T` (which is safe because `T` is `Sync`); additionally,
 // it needs `T` to be `Send` because any thread that has a `&ARef<T>` may clone it and get an
 // `ARef<T>` on that thread, so the thread may ultimately access `T` using a mutable reference, for
 // example, when the reference count reaches zero and `T` is dropped.
 unsafe impl<T: AlwaysRefCounted + Sync + Send> Sync for ARef<T> {}

 impl<T: AlwaysRefCounted> ARef<T> {
     /// Creates a new instance of [`ARef`].
     ///
     /// It takes over an increment of the reference count on the underlying object.
     ///
     /// # Safety
     ///
     /// Callers must ensure that the reference count was incremented at least once, and that they
     /// are properly relinquishing one increment. That is, if there is only one increment, callers
     /// must not use the underlying object anymore -- it is only safe to do so via the newly
     /// created [`ARef`].
     pub unsafe fn from_raw(ptr: NonNull<T>) -> Self {
         // INVARIANT: The safety requirements guarantee that the new instance now owns the
         // increment on the refcount.
         Self {
             ptr,
             _p: PhantomData,
         }
     }
 }

 impl<T: AlwaysRefCounted> Clone for ARef<T> {
     fn clone(&self) -> Self {
         self.inc_ref();
         // SAFETY: We just incremented the refcount above.
         unsafe { Self::from_raw(self.ptr) }
     }
 }

 impl<T: AlwaysRefCounted> Deref for ARef<T> {
     type Target = T;

     fn deref(&self) -> &Self::Target {
         // SAFETY: The type invariants guarantee that the object is valid.
         unsafe { self.ptr.as_ref() }
     }
 }

 impl<T: AlwaysRefCounted> From<&T> for ARef<T> {
     fn from(b: &T) -> Self {
         b.inc_ref();
         // SAFETY: We just incremented the refcount above.
         unsafe { Self::from_raw(NonNull::from(b)) }
     }
 }

 impl<T: AlwaysRefCounted> Drop for ARef<T> {
     fn drop(&mut self) {
         // SAFETY: The type invariants guarantee that the `ARef` owns the reference we're about to
         // decrement.
         unsafe { T::dec_ref(self.ptr) };
     }
 }

 /// A sum type that always holds either a value of type `L` or `R`.
 pub enum Either<L, R> {
     /// Constructs an instance of [`Either`] containing a value of type `L`.
     Left(L),

     /// Constructs an instance of [`Either`] containing a value of type `R`.
     Right(R),
 }
	// SPDX-License-Identifier: GPL-2.0

	//! Kernel types.

	use crate::init::{self, PinInit};
	use alloc::boxed::Box;
	use core::{
	cell::UnsafeCell,
	marker::{PhantomData, PhantomPinned},
	mem::MaybeUninit,
	ops::{Deref, DerefMut},
	ptr::NonNull,
	};

	/// Used to transfer ownership to and from foreign (non-Rust) languages.
	///
	/// Ownership is transferred from Rust to a foreign language by calling [`Self::into_foreign`] and
	/// later may be transferred back to Rust by calling [`Self::from_foreign`].
	///
	/// This trait is meant to be used in cases when Rust objects are stored in C objects and
	/// eventually "freed" back to Rust.
	pub trait ForeignOwnable: Sized {
	/// Type of values borrowed between calls to [`ForeignOwnable::into_foreign`] and
	/// [`ForeignOwnable::from_foreign`].
	type Borrowed<'a>;

	/// Converts a Rust-owned object to a foreign-owned one.
	///
	/// The foreign representation is a pointer to void.
	fn into_foreign(self) -> *const core::ffi::c_void;

	/// Borrows a foreign-owned object.
	///
	/// # Safety
	///
	/// `ptr` must have been returned by a previous call to [`ForeignOwnable::into_foreign`] for
	/// which a previous matching [`ForeignOwnable::from_foreign`] hasn't been called yet.
	unsafe fn borrow<'a>(ptr: *const core::ffi::c_void) -> Self::Borrowed<'a>;

	/// Converts a foreign-owned object back to a Rust-owned one.
	///
	/// # Safety
	///
	/// `ptr` must have been returned by a previous call to [`ForeignOwnable::into_foreign`] for
	/// which a previous matching [`ForeignOwnable::from_foreign`] hasn't been called yet.
	/// Additionally, all instances (if any) of values returned by [`ForeignOwnable::borrow`] for
	/// this object must have been dropped.
	unsafe fn from_foreign(ptr: *const core::ffi::c_void) -> Self;
	}

	impl<T: 'static> ForeignOwnable for Box<T> {
	type Borrowed<'a> = &'a T;

	fn into_foreign(self) -> *const core::ffi::c_void {
	Box::into_raw(self) as _
	}

	unsafe fn borrow<'a>(ptr: *const core::ffi::c_void) -> &'a T {
	// SAFETY: The safety requirements for this function ensure that the object is still alive,
	// so it is safe to dereference the raw pointer.
	// The safety requirements of `from_foreign` also ensure that the object remains alive for
	// the lifetime of the returned value.
	unsafe { &*ptr.cast() }
	}

	unsafe fn from_foreign(ptr: *const core::ffi::c_void) -> Self {
	// SAFETY: The safety requirements of this function ensure that `ptr` comes from a previous
	// call to `Self::into_foreign`.
	unsafe { Box::from_raw(ptr as _) }
	}
	}

	impl ForeignOwnable for () {
	type Borrowed<'a> = ();

	fn into_foreign(self) -> *const core::ffi::c_void {
	core::ptr::NonNull::dangling().as_ptr()
	}

	unsafe fn borrow<'a>(_: *const core::ffi::c_void) -> Self::Borrowed<'a> {}

	unsafe fn from_foreign(_: *const core::ffi::c_void) -> Self {}
	}

	/// Runs a cleanup function/closure when dropped.
	///
	/// The [`ScopeGuard::dismiss`] function prevents the cleanup function from running.
	///
	/// # Examples
	///
	/// In the example below, we have multiple exit paths and we want to log regardless of which one is
	/// taken:
	/// ```
	/// # use kernel::types::ScopeGuard;
	/// fn example1(arg: bool) {
	/// let _log = ScopeGuard::new(\|\| pr_info!("example1 completed\n"));
	///
	/// if arg {
	/// return;
	/// }
	///
	/// pr_info!("Do something...\n");
	/// }
	///
	/// # example1(false);
	/// # example1(true);
	/// ```
	///
	/// In the example below, we want to log the same message on all early exits but a different one on
	/// the main exit path:
	/// ```
	/// # use kernel::types::ScopeGuard;
	/// fn example2(arg: bool) {
	/// let log = ScopeGuard::new(\|\| pr_info!("example2 returned early\n"));
	///
	/// if arg {
	/// return;
	/// }
	///
	/// // (Other early returns...)
	///
	/// log.dismiss();
	/// pr_info!("example2 no early return\n");
	/// }
	///
	/// # example2(false);
	/// # example2(true);
	/// ```
	///
	/// In the example below, we need a mutable object (the vector) to be accessible within the log
	/// function, so we wrap it in the [`ScopeGuard`]:
	/// ```
	/// # use kernel::types::ScopeGuard;
	/// fn example3(arg: bool) -> Result {
	/// let mut vec =
	/// ScopeGuard::new_with_data(Vec::new(), \|v\| pr_info!("vec had {} elements\n", v.len()));
	///
	/// vec.try_push(10u8)?;
	/// if arg {
	/// return Ok(());
	/// }
	/// vec.try_push(20u8)?;
	/// Ok(())
	/// }
	///
	/// # assert_eq!(example3(false), Ok(()));
	/// # assert_eq!(example3(true), Ok(()));
	/// ```
	///
	/// # Invariants
	///
	/// The value stored in the struct is nearly always `Some(_)`, except between
	/// [`ScopeGuard::dismiss`] and [`ScopeGuard::drop`]: in this case, it will be `None` as the value
	/// will have been returned to the caller. Since [`ScopeGuard::dismiss`] consumes the guard,
	/// callers won't be able to use it anymore.
	pub struct ScopeGuard<T, F: FnOnce(T)>(Option<(T, F)>);

	impl<T, F: FnOnce(T)> ScopeGuard<T, F> {
	/// Creates a new guarded object wrapping the given data and with the given cleanup function.
	pub fn new_with_data(data: T, cleanup_func: F) -> Self {
	// INVARIANT: The struct is being initialised with `Some(_)`.
	Self(Some((data, cleanup_func)))
	}

	/// Prevents the cleanup function from running and returns the guarded data.
	pub fn dismiss(mut self) -> T {
	// INVARIANT: This is the exception case in the invariant; it is not visible to callers
	// because this function consumes `self`.
	self.0.take().unwrap().0
	}
	}

	impl ScopeGuard<(), fn(())> {
	/// Creates a new guarded object with the given cleanup function.
	pub fn new(cleanup: impl FnOnce()) -> ScopeGuard<(), impl FnOnce(())> {
	ScopeGuard::new_with_data((), move \|_\| cleanup())
	}
	}

	impl<T, F: FnOnce(T)> Deref for ScopeGuard<T, F> {
	type Target = T;

	fn deref(&self) -> &T {
	// The type invariants guarantee that `unwrap` will succeed.
	&self.0.as_ref().unwrap().0
	}
	}

	impl<T, F: FnOnce(T)> DerefMut for ScopeGuard<T, F> {
	fn deref_mut(&mut self) -> &mut T {
	// The type invariants guarantee that `unwrap` will succeed.
	&mut self.0.as_mut().unwrap().0
	}
	}

	impl<T, F: FnOnce(T)> Drop for ScopeGuard<T, F> {
	fn drop(&mut self) {
	// Run the cleanup function if one is still present.
	if let Some((data, cleanup)) = self.0.take() {
	cleanup(data)
	}
	}
	}

	/// Stores an opaque value.
	///
	/// This is meant to be used with FFI objects that are never interpreted by Rust code.
	#[repr(transparent)]
	pub struct Opaque<T> {
	value: UnsafeCell<MaybeUninit<T>>,
	_pin: PhantomPinned,
	}

	impl<T> Opaque<T> {
	/// Creates a new opaque value.
	pub const fn new(value: T) -> Self {
	Self {
	value: UnsafeCell::new(MaybeUninit::new(value)),
	_pin: PhantomPinned,
	}
	}

	/// Creates an uninitialised value.
	pub const fn uninit() -> Self {
	Self {
	value: UnsafeCell::new(MaybeUninit::uninit()),
	_pin: PhantomPinned,
	}
	}

	/// Creates a pin-initializer from the given initializer closure.
	///
	/// The returned initializer calls the given closure with the pointer to the inner `T` of this
	/// `Opaque`. Since this memory is uninitialized, the closure is not allowed to read from it.
	///
	/// This function is safe, because the `T` inside of an `Opaque` is allowed to be
	/// uninitialized. Additionally, access to the inner `T` requires `unsafe`, so the caller needs
	/// to verify at that point that the inner value is valid.
	pub fn ffi_init(init_func: impl FnOnce(*mut T)) -> impl PinInit<Self> {
	// SAFETY: We contain a `MaybeUninit`, so it is OK for the `init_func` to not fully
	// initialize the `T`.
	unsafe {
	init::pin_init_from_closure::<_, ::core::convert::Infallible>(move \|slot\| {
	init_func(Self::raw_get(slot));
	Ok(())
	})
	}
	}

	/// Returns a raw pointer to the opaque data.
	pub fn get(&self) -> *mut T {
	UnsafeCell::get(&self.value).cast::<T>()
	}

	/// Gets the value behind `this`.
	///
	/// This function is useful to get access to the value without creating intermediate
	/// references.
	pub const fn raw_get(this: const Self) -> mut T {
	UnsafeCell::raw_get(this.cast::<UnsafeCell<MaybeUninit<T>>>()).cast::<T>()
	}
	}

	/// Types that are _always_ reference counted.
	///
	/// It allows such types to define their own custom ref increment and decrement functions.
	/// Additionally, it allows users to convert from a shared reference `&T` to an owned reference
	/// [`ARef<T>`].
	///
	/// This is usually implemented by wrappers to existing structures on the C side of the code. For
	/// Rust code, the recommendation is to use [`Arc`](crate::sync::Arc) to create reference-counted
	/// instances of a type.
	///
	/// # Safety
	///
	/// Implementers must ensure that increments to the reference count keep the object alive in memory
	/// at least until matching decrements are performed.
	///
	/// Implementers must also ensure that all instances are reference-counted. (Otherwise they
	/// won't be able to honour the requirement that [`AlwaysRefCounted::inc_ref`] keep the object
	/// alive.)
	pub unsafe trait AlwaysRefCounted {
	/// Increments the reference count on the object.
	fn inc_ref(&self);

	/// Decrements the reference count on the object.
	///
	/// Frees the object when the count reaches zero.
	///
	/// # Safety
	///
	/// Callers must ensure that there was a previous matching increment to the reference count,
	/// and that the object is no longer used after its reference count is decremented (as it may
	/// result in the object being freed), unless the caller owns another increment on the refcount
	/// (e.g., it calls [`AlwaysRefCounted::inc_ref`] twice, then calls
	/// [`AlwaysRefCounted::dec_ref`] once).
	unsafe fn dec_ref(obj: NonNull<Self>);
	}

	/// An owned reference to an always-reference-counted object.
	///
	/// The object's reference count is automatically decremented when an instance of [`ARef`] is
	/// dropped. It is also automatically incremented when a new instance is created via
	/// [`ARef::clone`].
	///
	/// # Invariants
	///
	/// The pointer stored in `ptr` is non-null and valid for the lifetime of the [`ARef`] instance. In
	/// particular, the [`ARef`] instance owns an increment on the underlying object's reference count.
	pub struct ARef<T: AlwaysRefCounted> {
	ptr: NonNull<T>,
	_p: PhantomData<T>,
	}

	// SAFETY: It is safe to send `ARef<T>` to another thread when the underlying `T` is `Sync` because
	// it effectively means sharing `&T` (which is safe because `T` is `Sync`); additionally, it needs
	// `T` to be `Send` because any thread that has an `ARef<T>` may ultimately access `T` using a
	// mutable reference, for example, when the reference count reaches zero and `T` is dropped.
	unsafe impl<T: AlwaysRefCounted + Sync + Send> Send for ARef<T> {}

	// SAFETY: It is safe to send `&ARef<T>` to another thread when the underlying `T` is `Sync`
	// because it effectively means sharing `&T` (which is safe because `T` is `Sync`); additionally,
	// it needs `T` to be `Send` because any thread that has a `&ARef<T>` may clone it and get an
	// `ARef<T>` on that thread, so the thread may ultimately access `T` using a mutable reference, for
	// example, when the reference count reaches zero and `T` is dropped.
	unsafe impl<T: AlwaysRefCounted + Sync + Send> Sync for ARef<T> {}

	impl<T: AlwaysRefCounted> ARef<T> {
	/// Creates a new instance of [`ARef`].
	///
	/// It takes over an increment of the reference count on the underlying object.
	///
	/// # Safety
	///
	/// Callers must ensure that the reference count was incremented at least once, and that they
	/// are properly relinquishing one increment. That is, if there is only one increment, callers
	/// must not use the underlying object anymore -- it is only safe to do so via the newly
	/// created [`ARef`].
	pub unsafe fn from_raw(ptr: NonNull<T>) -> Self {
	// INVARIANT: The safety requirements guarantee that the new instance now owns the
	// increment on the refcount.
	Self {
	ptr,
	_p: PhantomData,
	}
	}
	}

	impl<T: AlwaysRefCounted> Clone for ARef<T> {
	fn clone(&self) -> Self {
	self.inc_ref();
	// SAFETY: We just incremented the refcount above.
	unsafe { Self::from_raw(self.ptr) }
	}
	}

	impl<T: AlwaysRefCounted> Deref for ARef<T> {
	type Target = T;

	fn deref(&self) -> &Self::Target {
	// SAFETY: The type invariants guarantee that the object is valid.
	unsafe { self.ptr.as_ref() }
	}
	}

	impl<T: AlwaysRefCounted> From<&T> for ARef<T> {
	fn from(b: &T) -> Self {
	b.inc_ref();
	// SAFETY: We just incremented the refcount above.
	unsafe { Self::from_raw(NonNull::from(b)) }
	}
	}

	impl<T: AlwaysRefCounted> Drop for ARef<T> {
	fn drop(&mut self) {
	// SAFETY: The type invariants guarantee that the `ARef` owns the reference we're about to
	// decrement.
	unsafe { T::dec_ref(self.ptr) };
	}
	}

	/// A sum type that always holds either a value of type `L` or `R`.
	pub enum Either<L, R> {
	/// Constructs an instance of [`Either`] containing a value of type `L`.
	Left(L),

	/// Constructs an instance of [`Either`] containing a value of type `R`.
	Right(R),
	}