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Refactor sized Rc/Arc allocation/initialization #152600

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198 changes: 90 additions & 108 deletions library/alloc/src/rc.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -411,16 +411,16 @@ impl<T> Rc<T> {
#[cfg(not(no_global_oom_handling))]
#[stable(feature = "rust1", since = "1.0.0")]
pub fn new(value: T) -> Rc<T> {
// There is an implicit weak pointer owned by all the strong
// pointers, which ensures that the weak destructor never frees
// the allocation while the strong destructor is running, even
// if the weak pointer is stored inside the strong one.
let mut rc = Rc::new_uninit();

// SAFETY: this is a freshly allocated `Rc` so it's guaranteed there are
// no other strong or weak pointers other than `rc` itself.
unsafe {
Self::from_inner(
Box::leak(Box::new(RcInner { strong: Cell::new(1), weak: Cell::new(1), value }))
.into(),
)
Rc::get_mut_unchecked(&mut rc).write(value);
}

// SAFETY: this allocation was just initialized above.
unsafe { rc.assume_init() }
}

/// Constructs a new `Rc<T>` while giving you a `Weak<T>` to the allocation,
Expand Down Expand Up @@ -503,13 +503,7 @@ impl<T> Rc<T> {
#[stable(feature = "new_uninit", since = "1.82.0")]
#[must_use]
pub fn new_uninit() -> Rc<mem::MaybeUninit<T>> {
unsafe {
Rc::from_ptr(Rc::allocate_for_layout(
Layout::new::<T>(),
|layout| Global.allocate(layout),
<*mut u8>::cast,
))
}
Rc::from_box_uninit_inner(Box::new_uninit())
}

/// Constructs a new `Rc` with uninitialized contents, with the memory
Expand All @@ -534,13 +528,7 @@ impl<T> Rc<T> {
#[stable(feature = "new_zeroed_alloc", since = "1.92.0")]
#[must_use]
pub fn new_zeroed() -> Rc<mem::MaybeUninit<T>> {
unsafe {
Rc::from_ptr(Rc::allocate_for_layout(
Layout::new::<T>(),
|layout| Global.allocate_zeroed(layout),
<*mut u8>::cast,
))
}
Rc::from_box_uninit_inner(Box::new_zeroed())
}

/// Constructs a new `Rc<T>`, returning an error if the allocation fails
Expand All @@ -556,20 +544,16 @@ impl<T> Rc<T> {
/// ```
#[unstable(feature = "allocator_api", issue = "32838")]
pub fn try_new(value: T) -> Result<Rc<T>, AllocError> {
// There is an implicit weak pointer owned by all the strong
// pointers, which ensures that the weak destructor never frees
// the allocation while the strong destructor is running, even
// if the weak pointer is stored inside the strong one.
let mut rc = Rc::try_new_uninit()?;

// SAFETY: this is a freshly allocated `Rc` so it's guaranteed there are
// no other strong or weak pointers other than `rc` itself.
unsafe {
Ok(Self::from_inner(
Box::leak(Box::try_new(RcInner {
strong: Cell::new(1),
weak: Cell::new(1),
value,
})?)
.into(),
))
Rc::get_mut_unchecked(&mut rc).write(value);
}

// SAFETY: this allocation was just initialized above.
Ok(unsafe { rc.assume_init() })
}

/// Constructs a new `Rc` with uninitialized contents, returning an error if the allocation fails
Expand All @@ -593,13 +577,7 @@ impl<T> Rc<T> {
/// ```
#[unstable(feature = "allocator_api", issue = "32838")]
pub fn try_new_uninit() -> Result<Rc<mem::MaybeUninit<T>>, AllocError> {
unsafe {
Ok(Rc::from_ptr(Rc::try_allocate_for_layout(
Layout::new::<T>(),
|layout| Global.allocate(layout),
<*mut u8>::cast,
)?))
}
Ok(Rc::from_box_uninit_inner(Box::try_new_uninit()?))
}

/// Constructs a new `Rc` with uninitialized contents, with the memory
Expand All @@ -625,14 +603,9 @@ impl<T> Rc<T> {
/// [zeroed]: mem::MaybeUninit::zeroed
#[unstable(feature = "allocator_api", issue = "32838")]
pub fn try_new_zeroed() -> Result<Rc<mem::MaybeUninit<T>>, AllocError> {
unsafe {
Ok(Rc::from_ptr(Rc::try_allocate_for_layout(
Layout::new::<T>(),
|layout| Global.allocate_zeroed(layout),
<*mut u8>::cast,
)?))
}
Ok(Rc::from_box_uninit_inner(Box::try_new_zeroed()?))
}

/// Constructs a new `Pin<Rc<T>>`. If `T` does not implement `Unpin`, then
/// `value` will be pinned in memory and unable to be moved.
#[cfg(not(no_global_oom_handling))]
Expand Down Expand Up @@ -780,16 +753,7 @@ impl<T, A: Allocator> Rc<T, A> {
#[unstable(feature = "allocator_api", issue = "32838")]
#[inline]
pub fn new_uninit_in(alloc: A) -> Rc<mem::MaybeUninit<T>, A> {
unsafe {
Rc::from_ptr_in(
Rc::allocate_for_layout(
Layout::new::<T>(),
|layout| alloc.allocate(layout),
<*mut u8>::cast,
),
alloc,
)
}
Rc::from_box_uninit_inner(Box::new_uninit_in(alloc))
}

/// Constructs a new `Rc` with uninitialized contents, with the memory
Expand Down Expand Up @@ -817,16 +781,7 @@ impl<T, A: Allocator> Rc<T, A> {
#[unstable(feature = "allocator_api", issue = "32838")]
#[inline]
pub fn new_zeroed_in(alloc: A) -> Rc<mem::MaybeUninit<T>, A> {
unsafe {
Rc::from_ptr_in(
Rc::allocate_for_layout(
Layout::new::<T>(),
|layout| alloc.allocate_zeroed(layout),
<*mut u8>::cast,
),
alloc,
)
}
Rc::from_box_uninit_inner(Box::new_zeroed_in(alloc))
}

/// Constructs a new `Rc<T, A>` in the given allocator while giving you a `Weak<T, A>` to the allocation,
Expand Down Expand Up @@ -923,15 +878,16 @@ impl<T, A: Allocator> Rc<T, A> {
#[unstable(feature = "allocator_api", issue = "32838")]
#[inline]
pub fn try_new_in(value: T, alloc: A) -> Result<Self, AllocError> {
// There is an implicit weak pointer owned by all the strong
// pointers, which ensures that the weak destructor never frees
// the allocation while the strong destructor is running, even
// if the weak pointer is stored inside the strong one.
let (ptr, alloc) = Box::into_unique(Box::try_new_in(
RcInner { strong: Cell::new(1), weak: Cell::new(1), value },
alloc,
)?);
Ok(unsafe { Self::from_inner_in(ptr.into(), alloc) })
let mut rc = Rc::try_new_uninit_in(alloc)?;

// SAFETY: this is a freshly allocated `Rc` so it's guaranteed there are
// no other strong or weak pointers other than `rc` itself.
unsafe {
Rc::get_mut_unchecked(&mut rc).write(value);
}

// SAFETY: this allocation was just initialized above.
unsafe { Ok(rc.assume_init()) }
}

/// Constructs a new `Rc` with uninitialized contents, in the provided allocator, returning an
Expand Down Expand Up @@ -961,16 +917,7 @@ impl<T, A: Allocator> Rc<T, A> {
#[unstable(feature = "allocator_api", issue = "32838")]
#[inline]
pub fn try_new_uninit_in(alloc: A) -> Result<Rc<mem::MaybeUninit<T>, A>, AllocError> {
unsafe {
Ok(Rc::from_ptr_in(
Rc::try_allocate_for_layout(
Layout::new::<T>(),
|layout| alloc.allocate(layout),
<*mut u8>::cast,
)?,
alloc,
))
}
Ok(Rc::from_box_uninit_inner(Box::try_new_uninit_in(alloc)?))
}

/// Constructs a new `Rc` with uninitialized contents, with the memory
Expand Down Expand Up @@ -999,16 +946,7 @@ impl<T, A: Allocator> Rc<T, A> {
#[unstable(feature = "allocator_api", issue = "32838")]
#[inline]
pub fn try_new_zeroed_in(alloc: A) -> Result<Rc<mem::MaybeUninit<T>, A>, AllocError> {
unsafe {
Ok(Rc::from_ptr_in(
Rc::try_allocate_for_layout(
Layout::new::<T>(),
|layout| alloc.allocate_zeroed(layout),
<*mut u8>::cast,
)?,
alloc,
))
}
Ok(Rc::from_box_uninit_inner(Box::try_new_zeroed_in(alloc)?))
}

/// Constructs a new `Pin<Rc<T>>` in the provided allocator. If `T` does not implement `Unpin`, then
Expand Down Expand Up @@ -1098,6 +1036,40 @@ impl<T, A: Allocator> Rc<T, A> {
pub fn into_inner(this: Self) -> Option<T> {
Rc::try_unwrap(this).ok()
}

fn from_box_uninit_inner(
boxed: Box<mem::MaybeUninit<RcInner<T>>, A>,
) -> Rc<mem::MaybeUninit<T>, A> {
let (inner, alloc) = Box::into_non_null_with_allocator(boxed);

// This translates `NonNull<mem::MaybeUninit<RcInner<T>>>`
// to `NonNull<RcInner<mem::MaybeUninit<T>>>`. It's not yet safe to
// turn that into an `Rc` because the strong/weak fields need
// initialization. That's done next.
let inner = inner.cast::<RcInner<mem::MaybeUninit<T>>>();

// SAFETY: `inner` is a valid pointer to an uninitialized allocation
// which means that it is safe to initialize. Here of the three fields
// that `RcInner` contains, weak/strong/value, two are initialized. The
// `value` field is left uninitialized which is modeled through the
// return value of this function where `MaybeUninit` is used to
// represent the payload of the `Rc`.
//
// There is an implicit weak pointer owned by all the strong pointers,
// which ensures that the weak destructor never frees the allocation
// while the strong destructor is running, even if the weak pointer is
// stored inside the strong one. Hence both the strong and the weak
// count are both initialized to one.
unsafe {
(&raw mut (*inner.as_ptr()).strong).write(Cell::new(1));
(&raw mut (*inner.as_ptr()).weak).write(Cell::new(1));
}

// SAFETY: `inner` is allocated by `alloc`, it's strong/weak counts are
// initialized, and `value` is modeled as uninitialized in the return
// value to represent its lack of initialization.
unsafe { Rc::from_inner_in(inner, alloc) }
}
}

impl<T> Rc<[T]> {
Expand Down Expand Up @@ -4300,17 +4272,27 @@ impl<T, A: Allocator> UniqueRc<T, A> {
#[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(
RcInner {
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,
},
alloc,
));
Self { ptr: ptr.into(), _marker: PhantomData, _marker2: PhantomData, alloc }
let mut rc = Self::from_box_uninit_inner(Box::new_uninit_in(alloc));
unsafe {
rc.write(value);
rc.assume_init()
}
}

fn from_box_uninit_inner(
boxed: Box<mem::MaybeUninit<RcInner<T>>, A>,
) -> UniqueRc<mem::MaybeUninit<T>, A> {
let (inner, alloc) = Box::into_non_null_with_allocator(boxed);
let inner = inner.cast::<RcInner<mem::MaybeUninit<T>>>();

// SAFETY: same as above for `Rc`
unsafe {
(&raw mut (*inner.as_ptr()).strong).write(Cell::new(0));
// keep one weak reference so if all the weak pointers that are
// created are dropped the UniqueRc still stays valid.
(&raw mut (*inner.as_ptr()).weak).write(Cell::new(1));
}
unsafe { UniqueRc::from_inner_in(inner, alloc) }
}
}

Expand Down
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