// SPDX-License-Identifier: GPL-2.0
//! A reference-counted pointer.
//!
//! This module implements a way for users to create reference-counted objects and pointers to
//! them. Such a pointer automatically increments and decrements the count, and drops the
//! underlying object when it reaches zero. It is also safe to use concurrently from multiple
//! threads.
//!
//! It is different from the standard library's [`Arc`] in a few ways:
//! 1. It is backed by the kernel's `refcount_t` type.
//! 2. It does not support weak references, which allows it to be half the size.
//! 3. It saturates the reference count instead of aborting when it goes over a threshold.
//! 4. It does not provide a `get_mut` method, so the ref counted object is pinned.
//!
//! [`Arc`]: https://doc.rust-lang.org/std/sync/struct.Arc.html
use crate::{
alloc::{box_ext::BoxExt, AllocError, Flags},
error::{self, Error},
init::{self, InPlaceInit, Init, PinInit},
try_init,
types::{ForeignOwnable, Opaque},
};
use alloc::boxed::Box;
use core::{
alloc::Layout,
fmt,
marker::{PhantomData, Unsize},
mem::{ManuallyDrop, MaybeUninit},
ops::{Deref, DerefMut},
pin::Pin,
ptr::NonNull,
};
use macros::pin_data;
mod std_vendor;
/// A reference-counted pointer to an instance of `T`.
///
/// The reference count is incremented when new instances of [`Arc`] are created, and decremented
/// when they are dropped. When the count reaches zero, the underlying `T` is also dropped.
///
/// # Invariants
///
/// The reference count on an instance of [`Arc`] is always non-zero.
/// The object pointed to by [`Arc`] is always pinned.
///
/// # Examples
///
/// ```
/// use kernel::sync::Arc;
///
/// struct Example {
/// a: u32,
/// b: u32,
/// }
///
/// // Create a refcounted instance of `Example`.
/// let obj = Arc::new(Example { a: 10, b: 20 }, GFP_KERNEL)?;
///
/// // Get a new pointer to `obj` and increment the refcount.
/// let cloned = obj.clone();
///
/// // Assert that both `obj` and `cloned` point to the same underlying object.
/// assert!(core::ptr::eq(&*obj, &*cloned));
///
/// // Destroy `obj` and decrement its refcount.
/// drop(obj);
///
/// // Check that the values are still accessible through `cloned`.
/// assert_eq!(cloned.a, 10);
/// assert_eq!(cloned.b, 20);
///
/// // The refcount drops to zero when `cloned` goes out of scope, and the memory is freed.
/// # Ok::<(), Error>(())
/// ```
///
/// Using `Arc<T>` as the type of `self`:
///
/// ```
/// use kernel::sync::Arc;
///
/// struct Example {
/// a: u32,
/// b: u32,
/// }
///
/// impl Example {
/// fn take_over(self: Arc<Self>) {
/// // ...
/// }
///
/// fn use_reference(self: &Arc<Self>) {
/// // ...
/// }
/// }
///
/// let obj = Arc::new(Example { a: 10, b: 20 }, GFP_KERNEL)?;
/// obj.use_reference();
/// obj.take_over();
/// # Ok::<(), Error>(())
/// ```
///
/// Coercion from `Arc<Example>` to `Arc<dyn MyTrait>`:
///
/// ```
/// use kernel::sync::{Arc, ArcBorrow};
///
/// trait MyTrait {
/// // Trait has a function whose `self` type is `Arc<Self>`.
/// fn example1(self: Arc<Self>) {}
///
/// // Trait has a function whose `self` type is `ArcBorrow<'_, Self>`.
/// fn example2(self: ArcBorrow<'_, Self>) {}
/// }
///
/// struct Example;
/// impl MyTrait for Example {}
///
/// // `obj` has type `Arc<Example>`.
/// let obj: Arc<Example> = Arc::new(Example, GFP_KERNEL)?;
///
/// // `coerced` has type `Arc<dyn MyTrait>`.
/// let coerced: Arc<dyn MyTrait> = obj;
/// # Ok::<(), Error>(())
/// ```
pub struct Arc<T: ?Sized> {
ptr: NonNull<ArcInner<T>>,
_p: PhantomData<ArcInner<T>>,
}
#[pin_data]
#[repr(C)]
struct ArcInner<T: ?Sized> {
refcount: Opaque<bindings::refcount_t>,
data: T,
}
impl<T: ?Sized> ArcInner<T> {
/// Converts a pointer to the contents of an [`Arc`] into a pointer to the [`ArcInner`].
///
/// # Safety
///
/// `ptr` must have been returned by a previous call to [`Arc::into_raw`], and the `Arc` must
/// not yet have been destroyed.
unsafe fn container_of(ptr: *const T) -> NonNull<ArcInner<T>> {
let refcount_layout = Layout::new::<bindings::refcount_t>();
// SAFETY: The caller guarantees that the pointer is valid.
let val_layout = Layout::for_value(unsafe { &*ptr });
// SAFETY: We're computing the layout of a real struct that existed when compiling this
// binary, so its layout is not so large that it can trigger arithmetic overflow.
let val_offset = unsafe { refcount_layout.extend(val_layout).unwrap_unchecked().1 };
// Pointer casts leave the metadata unchanged. This is okay because the metadata of `T` and
// `ArcInner<T>` is the same since `ArcInner` is a struct with `T` as its last field.
//
// This is documented at:
// <https://doc.rust-lang.org/std/ptr/trait.Pointee.html>.
let ptr = ptr as *const ArcInner<T>;
// SAFETY: The pointer is in-bounds of an allocation both before and after offsetting the
// pointer, since it originates from a previous call to `Arc::into_raw` on an `Arc` that is
// still valid.
let ptr = unsafe { ptr.byte_sub(val_offset) };
// SAFETY: The pointer can't be null since you can't have an `ArcInner<T>` value at the null
// address.
unsafe { NonNull::new_unchecked(ptr.cast_mut()) }
}
}
// This is to allow [`Arc`] (and variants) to be used as the type of `self`.
impl<T: ?Sized> core::ops::Receiver for Arc<T> {}
// This is to allow coercion from `Arc<T>` to `Arc<U>` if `T` can be converted to the
// dynamically-sized type (DST) `U`.
impl<T: ?Sized + Unsize<U>, U: ?Sized> core::ops::CoerceUnsized<Arc<U>> for Arc<T> {}
// This is to allow `Arc<U>` to be dispatched on when `Arc<T>` can be coerced into `Arc<U>`.
impl<T: ?Sized + Unsize<U>, U: ?Sized> core::ops::DispatchFromDyn<Arc<U>> for Arc<T> {}
// SAFETY: It is safe to send `Arc<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 `Arc<T>` may ultimately access `T` using a
// mutable reference when the reference count reaches zero and `T` is dropped.
unsafe impl<T: ?Sized + Sync + Send> Send for Arc<T> {}
// SAFETY: It is safe to send `&Arc<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 `&Arc<T>` may clone it and get an
// `Arc<T>` on that thread, so the thread may ultimately access `T` using a mutable reference when
// the reference count reaches zero and `T` is dropped.
unsafe impl<T: ?Sized + Sync + Send> Sync for Arc<T> {}
impl<T> Arc<T> {
/// Constructs a new reference counted instance of `T`.
pub fn new(contents: T, flags: Flags) -> Result<Self, AllocError> {
// INVARIANT: The refcount is initialised to a non-zero value.
let value = ArcInner {
// SAFETY: There are no safety requirements for this FFI call.
refcount: Opaque::new(unsafe { bindings::REFCOUNT_INIT(1) }),
data: contents,
};
let inner = <Box<_> as BoxExt<_>>::new(value, flags)?;
// SAFETY: We just created `inner` with a reference count of 1, which is owned by the new
// `Arc` object.
Ok(unsafe { Self::from_inner(Box::leak(inner).into()) })
}
/// Use the given initializer to in-place initialize a `T`.
///
/// If `T: !Unpin` it will not be able to move afterwards.
#[inline]
pub fn pin_init<E>(init: impl PinInit<T, E>, flags: Flags) -> error::Result<Self>
where
Error: From<E>,
{
UniqueArc::pin_init(init, flags).map(|u| u.into())
}
/// Use the given initializer to in-place initialize a `T`.
///
/// This is equivalent to [`Arc<T>::pin_init`], since an [`Arc`] is always pinned.
#[inline]
pub fn init<E>(init: impl Init<T, E>, flags: Flags) -> error::Result<Self>
where
Error: From<E>,
{
UniqueArc::init(init, flags).map(|u| u.into())
}
}
impl<T: ?Sized> Arc<T> {
/// Constructs a new [`Arc`] from an existing [`ArcInner`].
///
/// # Safety
///
/// The caller must ensure that `inner` points to a valid location and has a non-zero reference
/// count, one of which will be owned by the new [`Arc`] instance.
unsafe fn from_inner(inner: NonNull<ArcInner<T>>) -> Self {
// INVARIANT: By the safety requirements, the invariants hold.
Arc {
ptr: inner,
_p: PhantomData,
}
}
/// Convert the [`Arc`] into a raw pointer.
///
/// The raw pointer has ownership of the refcount that this Arc object owned.
pub fn into_raw(self) -> *const T {
let ptr = self.ptr.as_ptr();
core::mem::forget(self);
// SAFETY: The pointer is valid.
unsafe { core::ptr::addr_of!((*ptr).data) }
}
/// Recreates an [`Arc`] instance previously deconstructed via [`Arc::into_raw`].
///
/// # Safety
///
/// `ptr` must have been returned by a previous call to [`Arc::into_raw`]. Additionally, it
/// must not be called more than once for each previous call to [`Arc::into_raw`].
pub unsafe fn from_raw(ptr: *const T) -> Self {
// SAFETY: The caller promises that this pointer originates from a call to `into_raw` on an
// `Arc` that is still valid.
let ptr = unsafe { ArcInner::container_of(ptr) };
// SAFETY: By the safety requirements we know that `ptr` came from `Arc::into_raw`, so the
// reference count held then will be owned by the new `Arc` object.
unsafe { Self::from_inner(ptr) }
}
/// Returns an [`ArcBorrow`] from the given [`Arc`].
///
/// This is useful when the argument of a function call is an [`ArcBorrow`] (e.g., in a method
/// receiver), but we have an [`Arc`] instead. Getting an [`ArcBorrow`] is free when optimised.
#[inline]
pub fn as_arc_borrow(&self) -> ArcBorrow<'_, T> {
// SAFETY: The constraint that the lifetime of the shared reference must outlive that of
// the returned `ArcBorrow` ensures that the object remains alive and that no mutable
// reference can be created.
unsafe { ArcBorrow::new(self.ptr) }
}
/// Compare whether two [`Arc`] pointers reference the same underlying object.
pub fn ptr_eq(this: &Self, other: &Self) -> bool {
core::ptr::eq(this.ptr.as_ptr(), other.ptr.as_ptr())
}
/// Converts this [`Arc`] into a [`UniqueArc`], or destroys it if it is not unique.
///
/// When this destroys the `Arc`, it does so while properly avoiding races. This means that
/// this method will never call the destructor of the value.
///
/// # Examples
///
/// ```
/// use kernel::sync::{Arc, UniqueArc};
///
/// let arc = Arc::new(42, GFP_KERNEL)?;
/// let unique_arc = arc.into_unique_or_drop();
///
/// // The above conversion should succeed since refcount of `arc` is 1.
/// assert!(unique_arc.is_some());
///
/// assert_eq!(*(unique_arc.unwrap()), 42);
///
/// # Ok::<(), Error>(())
/// ```
///
/// ```
/// use kernel::sync::{Arc, UniqueArc};
///
/// let arc = Arc::new(42, GFP_KERNEL)?;
/// let another = arc.clone();
///
/// let unique_arc = arc.into_unique_or_drop();
///
/// // The above conversion should fail since refcount of `arc` is >1.
/// assert!(unique_arc.is_none());
///
/// # Ok::<(), Error>(())
/// ```
pub fn into_unique_or_drop(self) -> Option<Pin<UniqueArc<T>>> {
// We will manually manage the refcount in this method, so we disable the destructor.
let me = ManuallyDrop::new(self);
// SAFETY: We own a refcount, so the pointer is still valid.
let refcount = unsafe { me.ptr.as_ref() }.refcount.get();
// If the refcount reaches a non-zero value, then we have destroyed this `Arc` and will
// return without further touching the `Arc`. If the refcount reaches zero, then there are
// no other arcs, and we can create a `UniqueArc`.
//
// SAFETY: We own a refcount, so the pointer is not dangling.
let is_zero = unsafe { bindings::refcount_dec_and_test(refcount) };
if is_zero {
// SAFETY: We have exclusive access to the arc, so we can perform unsynchronized
// accesses to the refcount.
unsafe { core::ptr::write(refcount, bindings::REFCOUNT_INIT(1)) };
// INVARIANT: We own the only refcount to this arc, so we may create a `UniqueArc`. We
// must pin the `UniqueArc` because the values was previously in an `Arc`, and they pin
// their values.
Some(Pin::from(UniqueArc {
inner: ManuallyDrop::into_inner(me),
}))
} else {
None
}
}
}
impl<T: 'static> ForeignOwnable for Arc<T> {
type Borrowed<'a> = ArcBorrow<'a, T>;
fn into_foreign(self) -> *const core::ffi::c_void {
ManuallyDrop::new(self).ptr.as_ptr() as _
}
unsafe fn borrow<'a>(ptr: *const core::ffi::c_void) -> ArcBorrow<'a, T> {
// SAFETY: By the safety requirement of this function, we know that `ptr` came from
// a previous call to `Arc::into_foreign`.
let inner = NonNull::new(ptr as *mut ArcInner<T>).unwrap();
// SAFETY: The safety requirements of `from_foreign` ensure that the object remains alive
// for the lifetime of the returned value.
unsafe { ArcBorrow::new(inner) }
}
unsafe fn from_foreign(ptr: *const core::ffi::c_void) -> Self {
// SAFETY: By the safety requirement of this function, we know that `ptr` came from
// a previous call to `Arc::into_foreign`, which guarantees that `ptr` is valid and
// holds a reference count increment that is transferrable to us.
unsafe { Self::from_inner(NonNull::new(ptr as _).unwrap()) }
}
}
impl<T: ?Sized> Deref for Arc<T> {
type Target = T;
fn deref(&self) -> &Self::Target {
// SAFETY: By the type invariant, there is necessarily a reference to the object, so it is
// safe to dereference it.
unsafe { &self.ptr.as_ref().data }
}
}
impl<T: ?Sized> AsRef<T> for Arc<T> {
fn as_ref(&self) -> &T {
self.deref()
}
}
impl<T: ?Sized> Clone for Arc<T> {
fn clone(&self) -> Self {
// INVARIANT: C `refcount_inc` saturates the refcount, so it cannot overflow to zero.
// SAFETY: By the type invariant, there is necessarily a reference to the object, so it is
// safe to increment the refcount.
unsafe { bindings::refcount_inc(self.ptr.as_ref().refcount.get()) };
// SAFETY: We just incremented the refcount. This increment is now owned by the new `Arc`.
unsafe { Self::from_inner(self.ptr) }
}
}
impl<T: ?Sized> Drop for Arc<T> {
fn drop(&mut self) {
// SAFETY: By the type invariant, there is necessarily a reference to the object. We cannot
// touch `refcount` after it's decremented to a non-zero value because another thread/CPU
// may concurrently decrement it to zero and free it. It is ok to have a raw pointer to
// freed/invalid memory as long as it is never dereferenced.
let refcount = unsafe { self.ptr.as_ref() }.refcount.get();
// INVARIANT: If the refcount reaches zero, there are no other instances of `Arc`, and
// this instance is being dropped, so the broken invariant is not observable.
// SAFETY: Also by the type invariant, we are allowed to decrement the refcount.
let is_zero = unsafe { bindings::refcount_dec_and_test(refcount) };
if is_zero {
// The count reached zero, we must free the memory.
//
// SAFETY: The pointer was initialised from the result of `Box::leak`.
unsafe { drop(Box::from_raw(self.ptr.as_ptr())) };
}
}
}
impl<T: ?Sized> From<UniqueArc<T>> for Arc<T> {
fn from(item: UniqueArc<T>) -> Self {
item.inner
}
}
impl<T: ?Sized> From<Pin<UniqueArc<T>>> for Arc<T> {
fn from(item: Pin<UniqueArc<T>>) -> Self {
// SAFETY: The type invariants of `Arc` guarantee that the data is pinned.
unsafe { Pin::into_inner_unchecked(item).inner }
}
}
/// A borrowed reference to an [`Arc`] instance.
///
/// For cases when one doesn't ever need to increment the refcount on the allocation, it is simpler
/// to use just `&T`, which we can trivially get from an [`Arc<T>`] instance.
///
/// However, when one may need to increment the refcount, it is preferable to use an `ArcBorrow<T>`
/// over `&Arc<T>` because the latter results in a double-indirection: a pointer (shared reference)
/// to a pointer ([`Arc<T>`]) to the object (`T`). An [`ArcBorrow`] eliminates this double
/// indirection while still allowing one to increment the refcount and getting an [`Arc<T>`] when/if
/// needed.
///
/// # Invariants
///
/// There are no mutable references to the underlying [`Arc`], and it remains valid for the
/// lifetime of the [`ArcBorrow`] instance.
///
/// # Example
///
/// ```
/// use kernel::sync::{Arc, ArcBorrow};
///
/// struct Example;
///
/// fn do_something(e: ArcBorrow<'_, Example>) -> Arc<Example> {
/// e.into()
/// }
///
/// let obj = Arc::new(Example, GFP_KERNEL)?;
/// let cloned = do_something(obj.as_arc_borrow());
///
/// // Assert that both `obj` and `cloned` point to the same underlying object.
/// assert!(core::ptr::eq(&*obj, &*cloned));
/// # Ok::<(), Error>(())
/// ```
///
/// Using `ArcBorrow<T>` as the type of `self`:
///
/// ```
/// use kernel::sync::{Arc, ArcBorrow};
///
/// struct Example {
/// a: u32,
/// b: u32,
/// }
///
/// impl Example {
/// fn use_reference(self: ArcBorrow<'_, Self>) {
/// // ...
/// }
/// }
///
/// let obj = Arc::new(Example { a: 10, b: 20 }, GFP_KERNEL)?;
/// obj.as_arc_borrow().use_reference();
/// # Ok::<(), Error>(())
/// ```
pub struct ArcBorrow<'a, T: ?Sized + 'a> {
inner: NonNull<ArcInner<T>>,
_p: PhantomData<&'a ()>,
}
// This is to allow [`ArcBorrow`] (and variants) to be used as the type of `self`.
impl<T: ?Sized> core::ops::Receiver for ArcBorrow<'_, T> {}
// This is to allow `ArcBorrow<U>` to be dispatched on when `ArcBorrow<T>` can be coerced into
// `ArcBorrow<U>`.
impl<T: ?Sized + Unsize<U>, U: ?Sized> core::ops::DispatchFromDyn<ArcBorrow<'_, U>>
for ArcBorrow<'_, T>
{
}
impl<T: ?Sized> Clone for ArcBorrow<'_, T> {
fn clone(&self) -> Self {
*self
}
}
impl<T: ?Sized> Copy for ArcBorrow<'_, T> {}
impl<T: ?Sized> ArcBorrow<'_, T> {
/// Creates a new [`ArcBorrow`] instance.
///
/// # Safety
///
/// Callers must ensure the following for the lifetime of the returned [`ArcBorrow`] instance:
/// 1. That `inner` remains valid;
/// 2. That no mutable references to `inner` are created.
unsafe fn new(inner: NonNull<ArcInner<T>>) -> Self {
// INVARIANT: The safety requirements guarantee the invariants.
Self {
inner,
_p: PhantomData,
}
}
/// Creates an [`ArcBorrow`] to an [`Arc`] that has previously been deconstructed with
/// [`Arc::into_raw`].
///
/// # Safety
///
/// * The provided pointer must originate from a call to [`Arc::into_raw`].
/// * For the duration of the lifetime annotated on this `ArcBorrow`, the reference count must
/// not hit zero.
/// * For the duration of the lifetime annotated on this `ArcBorrow`, there must not be a
/// [`UniqueArc`] reference to this value.
pub unsafe fn from_raw(ptr: *const T) -> Self {
// SAFETY: The caller promises that this pointer originates from a call to `into_raw` on an
// `Arc` that is still valid.
let ptr = unsafe { ArcInner::container_of(ptr) };
// SAFETY: The caller promises that the value remains valid since the reference count must
// not hit zero, and no mutable reference will be created since that would involve a
// `UniqueArc`.
unsafe { Self::new(ptr) }
}
}
impl<T: ?Sized> From<ArcBorrow<'_, T>> for Arc<T> {
fn from(b: ArcBorrow<'_, T>) -> Self {
// SAFETY: The existence of `b` guarantees that the refcount is non-zero. `ManuallyDrop`
// guarantees that `drop` isn't called, so it's ok that the temporary `Arc` doesn't own the
// increment.
ManuallyDrop::new(unsafe { Arc::from_inner(b.inner) })
.deref()
.clone()
}
}
impl<T: ?Sized> Deref for ArcBorrow<'_, T> {
type Target = T;
fn deref(&self) -> &Self::Target {
// SAFETY: By the type invariant, the underlying object is still alive with no mutable
// references to it, so it is safe to create a shared reference.
unsafe { &self.inner.as_ref().data }
}
}
/// A refcounted object that is known to have a refcount of 1.
///
/// It is mutable and can be converted to an [`Arc`] so that it can be shared.
///
/// # Invariants
///
/// `inner` always has a reference count of 1.
///
/// # Examples
///
/// In the following example, we make changes to the inner object before turning it into an
/// `Arc<Test>` object (after which point, it cannot be mutated directly). Note that `x.into()`
/// cannot fail.
///
/// ```
/// use kernel::sync::{Arc, UniqueArc};
///
/// struct Example {
/// a: u32,
/// b: u32,
/// }
///
/// fn test() -> Result<Arc<Example>> {
/// let mut x = UniqueArc::new(Example { a: 10, b: 20 }, GFP_KERNEL)?;
/// x.a += 1;
/// x.b += 1;
/// Ok(x.into())
/// }
///
/// # test().unwrap();
/// ```
///
/// In the following example we first allocate memory for a refcounted `Example` but we don't
/// initialise it on allocation. We do initialise it later with a call to [`UniqueArc::write`],
/// followed by a conversion to `Arc<Example>`. This is particularly useful when allocation happens
/// in one context (e.g., sleepable) and initialisation in another (e.g., atomic):
///
/// ```
/// use kernel::sync::{Arc, UniqueArc};
///
/// struct Example {
/// a: u32,
/// b: u32,
/// }
///
/// fn test() -> Result<Arc<Example>> {
/// let x = UniqueArc::new_uninit(GFP_KERNEL)?;
/// Ok(x.write(Example { a: 10, b: 20 }).into())
/// }
///
/// # test().unwrap();
/// ```
///
/// In the last example below, the caller gets a pinned instance of `Example` while converting to
/// `Arc<Example>`; this is useful in scenarios where one needs a pinned reference during
/// initialisation, for example, when initialising fields that are wrapped in locks.
///
/// ```
/// use kernel::sync::{Arc, UniqueArc};
///
/// struct Example {
/// a: u32,
/// b: u32,
/// }
///
/// fn test() -> Result<Arc<Example>> {
/// let mut pinned = Pin::from(UniqueArc::new(Example { a: 10, b: 20 }, GFP_KERNEL)?);
/// // We can modify `pinned` because it is `Unpin`.
/// pinned.as_mut().a += 1;
/// Ok(pinned.into())
/// }
///
/// # test().unwrap();
/// ```
pub struct UniqueArc<T: ?Sized> {
inner: Arc<T>,
}
impl<T> UniqueArc<T> {
/// Tries to allocate a new [`UniqueArc`] instance.
pub fn new(value: T, flags: Flags) -> Result<Self, AllocError> {
Ok(Self {
// INVARIANT: The newly-created object has a refcount of 1.
inner: Arc::new(value, flags)?,
})
}
/// Tries to allocate a new [`UniqueArc`] instance whose contents are not initialised yet.
pub fn new_uninit(flags: Flags) -> Result<UniqueArc<MaybeUninit<T>>, AllocError> {
// INVARIANT: The refcount is initialised to a non-zero value.
let inner = Box::try_init::<AllocError>(
try_init!(ArcInner {
// SAFETY: There are no safety requirements for this FFI call.
refcount: Opaque::new(unsafe { bindings::REFCOUNT_INIT(1) }),
data <- init::uninit::<T, AllocError>(),
}? AllocError),
flags,
)?;
Ok(UniqueArc {
// INVARIANT: The newly-created object has a refcount of 1.
// SAFETY: The pointer from the `Box` is valid.
inner: unsafe { Arc::from_inner(Box::leak(inner).into()) },
})
}
}
impl<T> UniqueArc<MaybeUninit<T>> {
/// Converts a `UniqueArc<MaybeUninit<T>>` into a `UniqueArc<T>` by writing a value into it.
pub fn write(mut self, value: T) -> UniqueArc<T> {
self.deref_mut().write(value);
// SAFETY: We just wrote the value to be initialized.
unsafe { self.assume_init() }
}
/// Unsafely assume that `self` is initialized.
///
/// # Safety
///
/// The caller guarantees that the value behind this pointer has been initialized. It is
/// *immediate* UB to call this when the value is not initialized.
pub unsafe fn assume_init(self) -> UniqueArc<T> {
let inner = ManuallyDrop::new(self).inner.ptr;
UniqueArc {
// SAFETY: The new `Arc` is taking over `ptr` from `self.inner` (which won't be
// dropped). The types are compatible because `MaybeUninit<T>` is compatible with `T`.
inner: unsafe { Arc::from_inner(inner.cast()) },
}
}
/// Initialize `self` using the given initializer.
pub fn init_with<E>(mut self, init: impl Init<T, E>) -> core::result::Result<UniqueArc<T>, E> {
// SAFETY: The supplied pointer is valid for initialization.
match unsafe { init.__init(self.as_mut_ptr()) } {
// SAFETY: Initialization completed successfully.
Ok(()) => Ok(unsafe { self.assume_init() }),
Err(err) => Err(err),
}
}
/// Pin-initialize `self` using the given pin-initializer.
pub fn pin_init_with<E>(
mut self,
init: impl PinInit<T, E>,
) -> core::result::Result<Pin<UniqueArc<T>>, E> {
// SAFETY: The supplied pointer is valid for initialization and we will later pin the value
// to ensure it does not move.
match unsafe { init.__pinned_init(self.as_mut_ptr()) } {
// SAFETY: Initialization completed successfully.
Ok(()) => Ok(unsafe { self.assume_init() }.into()),
Err(err) => Err(err),
}
}
}
impl<T: ?Sized> From<UniqueArc<T>> for Pin<UniqueArc<T>> {
fn from(obj: UniqueArc<T>) -> Self {
// SAFETY: It is not possible to move/replace `T` inside a `Pin<UniqueArc<T>>` (unless `T`
// is `Unpin`), so it is ok to convert it to `Pin<UniqueArc<T>>`.
unsafe { Pin::new_unchecked(obj) }
}
}
impl<T: ?Sized> Deref for UniqueArc<T> {
type Target = T;
fn deref(&self) -> &Self::Target {
self.inner.deref()
}
}
impl<T: ?Sized> DerefMut for UniqueArc<T> {
fn deref_mut(&mut self) -> &mut Self::Target {
// SAFETY: By the `Arc` type invariant, there is necessarily a reference to the object, so
// it is safe to dereference it. Additionally, we know there is only one reference when
// it's inside a `UniqueArc`, so it is safe to get a mutable reference.
unsafe { &mut self.inner.ptr.as_mut().data }
}
}
impl<T: fmt::Display + ?Sized> fmt::Display for UniqueArc<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt::Display::fmt(self.deref(), f)
}
}
impl<T: fmt::Display + ?Sized> fmt::Display for Arc<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt::Display::fmt(self.deref(), f)
}
}
impl<T: fmt::Debug + ?Sized> fmt::Debug for UniqueArc<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt::Debug::fmt(self.deref(), f)
}
}
impl<T: fmt::Debug + ?Sized> fmt::Debug for Arc<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt::Debug::fmt(self.deref(), f)
}
}
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