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UniqueRc: support allocators and T: ?Sized.

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Added the following (all unstable):

* Defaulted type pararameter `A: Allocator`.
* `UniqueRc::new_in()`.
* `T:  ?Sized` where possible.
* `impl CoerceUnsized for UniqueRc`.
* Drive-by doc polish: links and periods at the end of sentences.

These changes are motivated by supporting the implementation of unsized
`Rc::make_mut()` (PR #116113), but are also intended to be obvious
generalizations of `UniqueRc` to support the things `Rc` does.
This commit is contained in:
Kevin Reid 2024-06-11 15:16:47 -07:00
parent 0c960618b5
commit 27ecb71635
2 changed files with 91 additions and 30 deletions
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95
library/alloc/src/rc.rs
View File

@ -3516,7 +3516,7 @@ fn data_offset_align(align: usize) -> usize {
layout.size() + layout.padding_needed_for(align)
}
/// A uniquely owned `Rc`
/// A uniquely owned [`Rc`].
///
/// This represents an `Rc` 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
@ -3554,13 +3554,24 @@ fn data_offset_align(align: usize) -> usize {
/// including fallible or async constructors.
#[unstable(feature = "unique_rc_arc", issue = "112566")]
#[derive(Debug)]
pub struct UniqueRc<T> {
pub struct UniqueRc<
T: ?Sized,
#[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global,
> {
ptr: NonNull<RcBox<T>>,
phantom: PhantomData<RcBox<T>>,
alloc: A,
}
#[unstable(feature = "unique_rc_arc", issue = "112566")]
impl<T: ?Sized + Unsize<U>, U: ?Sized, A: Allocator> CoerceUnsized<UniqueRc<U, A>>
for UniqueRc<T, A>
{
}
// Depends on A = Global
impl<T> UniqueRc<T> {
/// Creates a new `UniqueRc`
/// Creates a new `UniqueRc`.
///
/// Weak references to this `UniqueRc` can be created with [`UniqueRc::downgrade`]. Upgrading
/// these weak references will fail before the `UniqueRc` has been converted into an [`Rc`].
@ -3569,34 +3580,36 @@ impl<T> UniqueRc<T> {
#[cfg(not(no_global_oom_handling))]
#[unstable(feature = "unique_rc_arc", issue = "112566")]
pub fn new(value: T) -> Self {
Self {
ptr: Box::leak(Box::new(RcBox {
Self::new_in(value, Global)
}
}
impl<T, A: Allocator> UniqueRc<T, A> {
/// Creates a new `UniqueRc` in the provided allocator.
///
/// Weak references to this `UniqueRc` can be created with [`UniqueRc::downgrade`]. Upgrading
/// these weak references will fail before the `UniqueRc` has been converted into an [`Rc`].
/// After converting the `UniqueRc` into an [`Rc`], any weak references created beforehand will
/// point to the new [`Rc`].
#[cfg(not(no_global_oom_handling))]
#[unstable(feature = "unique_rc_arc", issue = "112566")]
pub fn new_in(value: T, alloc: A) -> Self {
let (ptr, alloc) = Box::into_unique(Box::new_in(
RcBox {
strong: Cell::new(0),
// keep one weak reference so if all the weak pointers that are created are dropped
// the UniqueRc still stays valid.
weak: Cell::new(1),
value,
}))
.into(),
phantom: PhantomData,
}
},
alloc,
));
Self { ptr: ptr.into(), phantom: PhantomData, alloc }
}
}
/// Creates a new weak reference to the `UniqueRc`
///
/// Attempting to upgrade this weak reference will fail before the `UniqueRc` has been converted
/// to a [`Rc`] using [`UniqueRc::into_rc`].
#[unstable(feature = "unique_rc_arc", issue = "112566")]
pub fn downgrade(this: &Self) -> Weak<T> {
// SAFETY: This pointer was allocated at creation time and we guarantee that we only have
// one strong reference before converting to a regular Rc.
unsafe {
this.ptr.as_ref().inc_weak();
}
Weak { ptr: this.ptr, alloc: Global }
}
/// Converts the `UniqueRc` into a regular [`Rc`]
impl<T: ?Sized, A: Allocator> UniqueRc<T, A> {
/// Converts the `UniqueRc` into a regular [`Rc`].
///
/// This consumes the `UniqueRc` and returns a regular [`Rc`] that contains the `value` that
/// is passed to `into_rc`.
@ -3604,19 +3617,41 @@ impl<T> UniqueRc<T> {
/// Any weak references created before this method is called can now be upgraded to strong
/// references.
#[unstable(feature = "unique_rc_arc", issue = "112566")]
pub fn into_rc(this: Self) -> Rc<T> {
pub fn into_rc(this: Self) -> Rc<T, A> {
let mut 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_mut().strong.set(1);
Rc::from_inner(this.ptr)
Rc::from_inner_in(this.ptr, alloc)
}
}
}
impl<T: ?Sized, A: Allocator + Clone> UniqueRc<T, A> {
/// Creates a new weak reference to the `UniqueRc`.
///
/// Attempting to upgrade this weak reference will fail before the `UniqueRc` has been converted
/// to a [`Rc`] using [`UniqueRc::into_rc`].
#[unstable(feature = "unique_rc_arc", issue = "112566")]
pub fn downgrade(this: &Self) -> Weak<T, A> {
// SAFETY: This pointer was allocated at creation time and we guarantee that we only have
// one strong reference before converting to a regular Rc.
unsafe {
this.ptr.as_ref().inc_weak();
}
Weak { ptr: this.ptr, alloc: this.alloc.clone() }
}
}
#[unstable(feature = "unique_rc_arc", issue = "112566")]
impl<T> Deref for UniqueRc<T> {
impl<T: ?Sized, A: Allocator> Deref for UniqueRc<T, A> {
type Target = T;
fn deref(&self) -> &T {
@ -3626,7 +3661,7 @@ impl<T> Deref for UniqueRc<T> {
}
#[unstable(feature = "unique_rc_arc", issue = "112566")]
impl<T> DerefMut for UniqueRc<T> {
impl<T: ?Sized, A: Allocator> DerefMut for UniqueRc<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
@ -3636,7 +3671,7 @@ impl<T> DerefMut for UniqueRc<T> {
}
#[unstable(feature = "unique_rc_arc", issue = "112566")]
unsafe impl<#[may_dangle] T> Drop for UniqueRc<T> {
unsafe impl<#[may_dangle] T: ?Sized, A: Allocator> Drop for UniqueRc<T, A> {
fn drop(&mut self) {
unsafe {
// destroy the contained object
@ -3646,7 +3681,7 @@ unsafe impl<#[may_dangle] T> Drop for UniqueRc<T> {
self.ptr.as_ref().dec_weak();
if self.ptr.as_ref().weak() == 0 {
Global.deallocate(self.ptr.cast(), Layout::for_value_raw(self.ptr.as_ptr()));
self.alloc.deallocate(self.ptr.cast(), Layout::for_value_raw(self.ptr.as_ptr()));
}
}
}

26
library/alloc/src/rc/tests.rs
View File

@ -606,6 +606,23 @@ fn test_unique_rc_drops_contents() {
assert!(dropped);
}
/// Exercise the non-default allocator usage.
#[test]
fn test_unique_rc_with_alloc_drops_contents() {
let mut dropped = false;
struct DropMe<'a>(&'a mut bool);
impl Drop for DropMe<'_> {
fn drop(&mut self) {
*self.0 = true;
}
}
{
let rc = UniqueRc::new_in(DropMe(&mut dropped), std::alloc::System);
drop(rc);
}
assert!(dropped);
}
#[test]
fn test_unique_rc_weak_clone_holding_ref() {
let mut v = UniqueRc::new(0u8);
@ -614,3 +631,12 @@ fn test_unique_rc_weak_clone_holding_ref() {
let _ = w.clone(); // touch weak count
*r = 123;
}
#[test]
fn test_unique_rc_unsizing_coercion() {
let mut rc: UniqueRc<[u8]> = UniqueRc::new([0u8; 3]);
assert_eq!(rc.len(), 3);
rc[0] = 123;
let rc: Rc<[u8]> = UniqueRc::into_rc(rc);
assert_eq!(*rc, [123, 0, 0]);
}