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Auto merge of #133572 - frank-king:feature/unique_arc, r=Amanieu

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Implement `alloc::sync::UniqueArc`

This implements the `alloc::sync::UniqueArc` part of #112566.
This commit is contained in:
bors 2025-03-29 20:05:06 +00:00
commit 1799887bb2
6 changed files with 482 additions and 4 deletions
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412
library/alloc/src/sync.rs
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@ -20,7 +20,7 @@ use core::iter;
use core::marker::{PhantomData, Unsize};
use core::mem::{self, ManuallyDrop, align_of_val_raw};
use core::num::NonZeroUsize;
use core::ops::{CoerceUnsized, Deref, DerefPure, DispatchFromDyn, LegacyReceiver};
use core::ops::{CoerceUnsized, Deref, DerefMut, DerefPure, DispatchFromDyn, LegacyReceiver};
use core::panic::{RefUnwindSafe, UnwindSafe};
use core::pin::{Pin, PinCoerceUnsized};
use core::ptr::{self, NonNull};
@ -4066,3 +4066,413 @@ impl<T: core::error::Error + ?Sized> core::error::Error for Arc<T> {
core::error::Error::provide(&**self, req);
}
}
/// A uniquely owned [`Arc`].
///
/// This represents an `Arc` that is known to be uniquely owned -- that is, have exactly one strong
/// reference. Multiple weak pointers can be created, but attempts to upgrade those to strong
/// references will fail unless the `UniqueArc` they point to has been converted into a regular `Arc`.
///
/// Because it is uniquely owned, the contents of a `UniqueArc` can be freely mutated. A common
/// use case is to have an object be mutable during its initialization phase but then have it become
/// immutable and converted to a normal `Arc`.
///
/// This can be used as a flexible way to create cyclic data structures, as in the example below.
///
/// ```
/// #![feature(unique_rc_arc)]
/// use std::sync::{Arc, Weak, UniqueArc};
///
/// struct Gadget {
/// me: Weak<Gadget>,
/// }
///
/// fn create_gadget() -> Option<Arc<Gadget>> {
/// let mut rc = UniqueArc::new(Gadget {
/// me: Weak::new(),
/// });
/// rc.me = UniqueArc::downgrade(&rc);
/// Some(UniqueArc::into_arc(rc))
/// }
///
/// create_gadget().unwrap();
/// ```
///
/// An advantage of using `UniqueArc` over [`Arc::new_cyclic`] to build cyclic data structures is that
/// [`Arc::new_cyclic`]'s `data_fn` parameter cannot be async or return a [`Result`]. As shown in the
/// previous example, `UniqueArc` allows for more flexibility in the construction of cyclic data,
/// including fallible or async constructors.
#[unstable(feature = "unique_rc_arc", issue = "112566")]
pub struct UniqueArc<
T: ?Sized,
#[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global,
> {
ptr: NonNull<ArcInner<T>>,
// Define the ownership of `ArcInner<T>` for drop-check
_marker: PhantomData<ArcInner<T>>,
// Invariance is necessary for soundness: once other `Weak`
// references exist, we already have a form of shared mutability!
_marker2: PhantomData<*mut T>,
alloc: A,
}
#[unstable(feature = "unique_rc_arc", issue = "112566")]
unsafe impl<T: ?Sized + Sync + Send, A: Allocator + Send> Send for UniqueArc<T, A> {}
#[unstable(feature = "unique_rc_arc", issue = "112566")]
unsafe impl<T: ?Sized + Sync + Send, A: Allocator + Sync> Sync for UniqueArc<T, A> {}
#[unstable(feature = "unique_rc_arc", issue = "112566")]
// #[unstable(feature = "coerce_unsized", issue = "18598")]
impl<T: ?Sized + Unsize<U>, U: ?Sized, A: Allocator> CoerceUnsized<UniqueArc<U, A>>
for UniqueArc<T, A>
{
}
//#[unstable(feature = "unique_rc_arc", issue = "112566")]
#[unstable(feature = "dispatch_from_dyn", issue = "none")]
impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<UniqueArc<U>> for UniqueArc<T> {}
#[unstable(feature = "unique_rc_arc", issue = "112566")]
impl<T: ?Sized + fmt::Display, A: Allocator> fmt::Display for UniqueArc<T, A> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt::Display::fmt(&**self, f)
}
}
#[unstable(feature = "unique_rc_arc", issue = "112566")]
impl<T: ?Sized + fmt::Debug, A: Allocator> fmt::Debug for UniqueArc<T, A> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt::Debug::fmt(&**self, f)
}
}
#[unstable(feature = "unique_rc_arc", issue = "112566")]
impl<T: ?Sized, A: Allocator> fmt::Pointer for UniqueArc<T, A> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt::Pointer::fmt(&(&raw const **self), f)
}
}
#[unstable(feature = "unique_rc_arc", issue = "112566")]
impl<T: ?Sized, A: Allocator> borrow::Borrow<T> for UniqueArc<T, A> {
fn borrow(&self) -> &T {
&**self
}
}
#[unstable(feature = "unique_rc_arc", issue = "112566")]
impl<T: ?Sized, A: Allocator> borrow::BorrowMut<T> for UniqueArc<T, A> {
fn borrow_mut(&mut self) -> &mut T {
&mut **self
}
}
#[unstable(feature = "unique_rc_arc", issue = "112566")]
impl<T: ?Sized, A: Allocator> AsRef<T> for UniqueArc<T, A> {
fn as_ref(&self) -> &T {
&**self
}
}
#[unstable(feature = "unique_rc_arc", issue = "112566")]
impl<T: ?Sized, A: Allocator> AsMut<T> for UniqueArc<T, A> {
fn as_mut(&mut self) -> &mut T {
&mut **self
}
}
#[unstable(feature = "unique_rc_arc", issue = "112566")]
impl<T: ?Sized, A: Allocator> Unpin for UniqueArc<T, A> {}
#[unstable(feature = "unique_rc_arc", issue = "112566")]
impl<T: ?Sized + PartialEq, A: Allocator> PartialEq for UniqueArc<T, A> {
/// Equality for two `UniqueArc`s.
///
/// Two `UniqueArc`s are equal if their inner values are equal.
///
/// # Examples
///
/// ```
/// #![feature(unique_rc_arc)]
/// use std::sync::UniqueArc;
///
/// let five = UniqueArc::new(5);
///
/// assert!(five == UniqueArc::new(5));
/// ```
#[inline]
fn eq(&self, other: &Self) -> bool {
PartialEq::eq(&**self, &**other)
}
}
#[unstable(feature = "unique_rc_arc", issue = "112566")]
impl<T: ?Sized + PartialOrd, A: Allocator> PartialOrd for UniqueArc<T, A> {
/// Partial comparison for two `UniqueArc`s.
///
/// The two are compared by calling `partial_cmp()` on their inner values.
///
/// # Examples
///
/// ```
/// #![feature(unique_rc_arc)]
/// use std::sync::UniqueArc;
/// use std::cmp::Ordering;
///
/// let five = UniqueArc::new(5);
///
/// assert_eq!(Some(Ordering::Less), five.partial_cmp(&UniqueArc::new(6)));
/// ```
#[inline(always)]
fn partial_cmp(&self, other: &UniqueArc<T, A>) -> Option<Ordering> {
(**self).partial_cmp(&**other)
}
/// Less-than comparison for two `UniqueArc`s.
///
/// The two are compared by calling `<` on their inner values.
///
/// # Examples
///
/// ```
/// #![feature(unique_rc_arc)]
/// use std::sync::UniqueArc;
///
/// let five = UniqueArc::new(5);
///
/// assert!(five < UniqueArc::new(6));
/// ```
#[inline(always)]
fn lt(&self, other: &UniqueArc<T, A>) -> bool {
**self < **other
}
/// 'Less than or equal to' comparison for two `UniqueArc`s.
///
/// The two are compared by calling `<=` on their inner values.
///
/// # Examples
///
/// ```
/// #![feature(unique_rc_arc)]
/// use std::sync::UniqueArc;
///
/// let five = UniqueArc::new(5);
///
/// assert!(five <= UniqueArc::new(5));
/// ```
#[inline(always)]
fn le(&self, other: &UniqueArc<T, A>) -> bool {
**self <= **other
}
/// Greater-than comparison for two `UniqueArc`s.
///
/// The two are compared by calling `>` on their inner values.
///
/// # Examples
///
/// ```
/// #![feature(unique_rc_arc)]
/// use std::sync::UniqueArc;
///
/// let five = UniqueArc::new(5);
///
/// assert!(five > UniqueArc::new(4));
/// ```
#[inline(always)]
fn gt(&self, other: &UniqueArc<T, A>) -> bool {
**self > **other
}
/// 'Greater than or equal to' comparison for two `UniqueArc`s.
///
/// The two are compared by calling `>=` on their inner values.
///
/// # Examples
///
/// ```
/// #![feature(unique_rc_arc)]
/// use std::sync::UniqueArc;
///
/// let five = UniqueArc::new(5);
///
/// assert!(five >= UniqueArc::new(5));
/// ```
#[inline(always)]
fn ge(&self, other: &UniqueArc<T, A>) -> bool {
**self >= **other
}
}
#[unstable(feature = "unique_rc_arc", issue = "112566")]
impl<T: ?Sized + Ord, A: Allocator> Ord for UniqueArc<T, A> {
/// Comparison for two `UniqueArc`s.
///
/// The two are compared by calling `cmp()` on their inner values.
///
/// # Examples
///
/// ```
/// #![feature(unique_rc_arc)]
/// use std::sync::UniqueArc;
/// use std::cmp::Ordering;
///
/// let five = UniqueArc::new(5);
///
/// assert_eq!(Ordering::Less, five.cmp(&UniqueArc::new(6)));
/// ```
#[inline]
fn cmp(&self, other: &UniqueArc<T, A>) -> Ordering {
(**self).cmp(&**other)
}
}
#[unstable(feature = "unique_rc_arc", issue = "112566")]
impl<T: ?Sized + Eq, A: Allocator> Eq for UniqueArc<T, A> {}
#[unstable(feature = "unique_rc_arc", issue = "112566")]
impl<T: ?Sized + Hash, A: Allocator> Hash for UniqueArc<T, A> {
fn hash<H: Hasher>(&self, state: &mut H) {
(**self).hash(state);
}
}
impl<T> UniqueArc<T, Global> {
/// Creates a new `UniqueArc`.
///
/// Weak references to this `UniqueArc` can be created with [`UniqueArc::downgrade`]. Upgrading
/// these weak references will fail before the `UniqueArc` has been converted into an [`Arc`].
/// After converting the `UniqueArc` into an [`Arc`], any weak references created beforehand will
/// point to the new [`Arc`].
#[cfg(not(no_global_oom_handling))]
#[unstable(feature = "unique_rc_arc", issue = "112566")]
#[must_use]
pub fn new(value: T) -> Self {
Self::new_in(value, Global)
}
}
impl<T, A: Allocator> UniqueArc<T, A> {
/// Creates a new `UniqueArc` in the provided allocator.
///
/// Weak references to this `UniqueArc` can be created with [`UniqueArc::downgrade`]. Upgrading
/// these weak references will fail before the `UniqueArc` has been converted into an [`Arc`].
/// After converting the `UniqueArc` into an [`Arc`], any weak references created beforehand will
/// point to the new [`Arc`].
#[cfg(not(no_global_oom_handling))]
#[unstable(feature = "unique_rc_arc", issue = "112566")]
#[must_use]
// #[unstable(feature = "allocator_api", issue = "32838")]
pub fn new_in(data: T, alloc: A) -> Self {
let (ptr, alloc) = Box::into_unique(Box::new_in(
ArcInner {
strong: atomic::AtomicUsize::new(0),
// keep one weak reference so if all the weak pointers that are created are dropped
// the UniqueArc still stays valid.
weak: atomic::AtomicUsize::new(1),
data,
},
alloc,
));
Self { ptr: ptr.into(), _marker: PhantomData, _marker2: PhantomData, alloc }
}
}
impl<T: ?Sized, A: Allocator> UniqueArc<T, A> {
/// Converts the `UniqueArc` into a regular [`Arc`].
///
/// This consumes the `UniqueArc` and returns a regular [`Arc`] that contains the `value` that
/// is passed to `into_arc`.
///
/// Any weak references created before this method is called can now be upgraded to strong
/// references.
#[unstable(feature = "unique_rc_arc", issue = "112566")]
#[must_use]
pub fn into_arc(this: Self) -> Arc<T, A> {
let this = ManuallyDrop::new(this);
// Move the allocator out.
// SAFETY: `this.alloc` will not be accessed again, nor dropped because it is in
// a `ManuallyDrop`.
let alloc: A = unsafe { ptr::read(&this.alloc) };
// SAFETY: This pointer was allocated at creation time so we know it is valid.
unsafe {
// Convert our weak reference into a strong reference
(*this.ptr.as_ptr()).strong.store(1, Release);
Arc::from_inner_in(this.ptr, alloc)
}
}
}
impl<T: ?Sized, A: Allocator + Clone> UniqueArc<T, A> {
/// Creates a new weak reference to the `UniqueArc`.
///
/// Attempting to upgrade this weak reference will fail before the `UniqueArc` has been converted
/// to a [`Arc`] using [`UniqueArc::into_arc`].
#[unstable(feature = "unique_rc_arc", issue = "112566")]
#[must_use]
pub fn downgrade(this: &Self) -> Weak<T, A> {
// Using a relaxed ordering is alright here, as knowledge of the
// original reference prevents other threads from erroneously deleting
// the object or converting the object to a normal `Arc<T, A>`.
//
// Note that we don't need to test if the weak counter is locked because there
// are no such operations like `Arc::get_mut` or `Arc::make_mut` that will lock
// the weak counter.
//
// SAFETY: This pointer was allocated at creation time so we know it is valid.
let old_size = unsafe { (*this.ptr.as_ptr()).weak.fetch_add(1, Relaxed) };
// See comments in Arc::clone() for why we do this (for mem::forget).
if old_size > MAX_REFCOUNT {
abort();
}
Weak { ptr: this.ptr, alloc: this.alloc.clone() }
}
}
#[unstable(feature = "unique_rc_arc", issue = "112566")]
impl<T: ?Sized, A: Allocator> Deref for UniqueArc<T, A> {
type Target = T;
fn deref(&self) -> &T {
// SAFETY: This pointer was allocated at creation time so we know it is valid.
unsafe { &self.ptr.as_ref().data }
}
}
// #[unstable(feature = "unique_rc_arc", issue = "112566")]
#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
unsafe impl<T: ?Sized> PinCoerceUnsized for UniqueArc<T> {}
#[unstable(feature = "unique_rc_arc", issue = "112566")]
impl<T: ?Sized, A: Allocator> DerefMut for UniqueArc<T, A> {
fn deref_mut(&mut self) -> &mut T {
// SAFETY: This pointer was allocated at creation time so we know it is valid. We know we
// have unique ownership and therefore it's safe to make a mutable reference because
// `UniqueArc` owns the only strong reference to itself.
// We also need to be careful to only create a mutable reference to the `data` field,
// as a mutable reference to the entire `ArcInner` would assert uniqueness over the
// ref count fields too, invalidating any attempt by `Weak`s to access the ref count.
unsafe { &mut (*self.ptr.as_ptr()).data }
}
}
#[unstable(feature = "unique_rc_arc", issue = "112566")]
// #[unstable(feature = "deref_pure_trait", issue = "87121")]
unsafe impl<T: ?Sized, A: Allocator> DerefPure for UniqueArc<T, A> {}
#[unstable(feature = "unique_rc_arc", issue = "112566")]
unsafe impl<#[may_dangle] T: ?Sized, A: Allocator> Drop for UniqueArc<T, A> {
fn drop(&mut self) {
// See `Arc::drop_slow` which drops an `Arc` with a strong count of 0.
// SAFETY: This pointer was allocated at creation time so we know it is valid.
let _weak = Weak { ptr: self.ptr, alloc: &self.alloc };
unsafe { ptr::drop_in_place(&mut (*self.ptr.as_ptr()).data) };
}
}

28
library/alloctests/tests/arc.rs
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@ -1,7 +1,7 @@
use std::any::Any;
use std::cell::{Cell, RefCell};
use std::iter::TrustedLen;
use std::sync::{Arc, Weak};
use std::sync::{Arc, UniqueArc, Weak};
#[test]
fn uninhabited() {
@ -263,9 +263,30 @@ fn make_mut_unsized() {
assert_eq!(*other_data, [110, 20, 30]);
}
#[test]
fn test_unique_arc_weak() {
let data = UniqueArc::new(32);
// Test that `Weak` downgraded from `UniqueArc` cannot be upgraded.
let weak = UniqueArc::downgrade(&data);
assert_eq!(weak.strong_count(), 0);
assert_eq!(weak.weak_count(), 0);
assert!(weak.upgrade().is_none());
// Test that `Weak` can now be upgraded after the `UniqueArc` being converted to `Arc`.
let strong = UniqueArc::into_arc(data);
assert_eq!(*strong, 32);
assert_eq!(weak.strong_count(), 1);
assert_eq!(weak.weak_count(), 1);
let upgraded = weak.upgrade().unwrap();
assert_eq!(*upgraded, 32);
assert_eq!(weak.strong_count(), 2);
assert_eq!(weak.weak_count(), 1);
}
#[allow(unused)]
mod pin_coerce_unsized {
use alloc::sync::Arc;
use alloc::sync::{Arc, UniqueArc};
use core::pin::Pin;
pub trait MyTrait {}
@ -275,4 +296,7 @@ mod pin_coerce_unsized {
pub fn pin_arc(arg: Pin<Arc<String>>) -> Pin<Arc<dyn MyTrait>> {
arg
}
pub fn pin_unique_arc(arg: Pin<UniqueArc<String>>) -> Pin<UniqueArc<dyn MyTrait>> {
arg
}
}

2
library/std/src/sync/mod.rs
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@ -176,6 +176,8 @@ pub use core::sync::Exclusive;
#[stable(feature = "rust1", since = "1.0.0")]
pub use core::sync::atomic;
#[unstable(feature = "unique_rc_arc", issue = "112566")]
pub use alloc_crate::sync::UniqueArc;
#[stable(feature = "rust1", since = "1.0.0")]
pub use alloc_crate::sync::{Arc, Weak};

27
tests/ui/variance/variance-uniquearc.rs Normal file
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@ -0,0 +1,27 @@
// regression test of https://github.com/rust-lang/rust/pull/133572#issuecomment-2543007164
// see also the test for UniqueRc` in variance-uniquerc.rs
//
// inline comments explain how this code *would* compile if UniqueArc was still covariant
#![feature(unique_rc_arc)]
use std::sync::UniqueArc;
fn extend_lifetime<'a, 'b>(x: &'a str) -> &'b str {
let r = UniqueArc::new(""); // UniqueArc<&'static str>
let w = UniqueArc::downgrade(&r); // Weak<&'static str>
let mut r = r; // [IF COVARIANT]: ==>> UniqueArc<&'a str>
*r = x; // assign the &'a str
let _r = UniqueArc::into_arc(r); // Arc<&'a str>, but we only care to activate the weak
let r = w.upgrade().unwrap(); // Arc<&'static str>
*r // &'static str, coerces to &'b str
//~^ ERROR lifetime may not live long enough
}
fn main() {
let s = String::from("Hello World!");
let r = extend_lifetime(&s);
println!("{r}");
drop(s);
println!("{r}");
}

15
tests/ui/variance/variance-uniquearc.stderr Normal file
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@ -0,0 +1,15 @@
error: lifetime may not live long enough
--> $DIR/variance-uniquearc.rs:17:5
|
LL | fn extend_lifetime<'a, 'b>(x: &'a str) -> &'b str {
| -- -- lifetime `'b` defined here
| |
| lifetime `'a` defined here
...
LL | *r // &'static str, coerces to &'b str
| ^^ function was supposed to return data with lifetime `'b` but it is returning data with lifetime `'a`
|
= help: consider adding the following bound: `'a: 'b`
error: aborting due to 1 previous error

2
tests/ui/variance/variance-uniquerc.rs
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@ -1,5 +1,5 @@
// regression test of https://github.com/rust-lang/rust/pull/133572#issuecomment-2543007164
// we should also test UniqueArc once implemented
// see also the test for UniqueArc in variance-uniquearc.rs
//
// inline comments explain how this code *would* compile if UniqueRc was still covariant