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#![unstable(feature = "async_drop", issue = "126482")]
use crate::fmt;
use crate::future::{Future, IntoFuture};
use crate::intrinsics::discriminant_value;
use crate::marker::{DiscriminantKind, PhantomPinned};
use crate::mem::MaybeUninit;
use crate::pin::Pin;
use crate::task::{ready, Context, Poll};
/// Asynchronously drops a value by running `AsyncDrop::async_drop`
/// on a value and its fields recursively.
#[unstable(feature = "async_drop", issue = "126482")]
pub fn async_drop<T>(value: T) -> AsyncDropOwning<T> {
AsyncDropOwning { value: MaybeUninit::new(value), dtor: None, _pinned: PhantomPinned }
}
/// A future returned by the [`async_drop`].
#[unstable(feature = "async_drop", issue = "126482")]
pub struct AsyncDropOwning<T> {
value: MaybeUninit<T>,
dtor: Option<AsyncDropInPlace<T>>,
_pinned: PhantomPinned,
}
#[unstable(feature = "async_drop", issue = "126482")]
impl<T> fmt::Debug for AsyncDropOwning<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("AsyncDropOwning").finish_non_exhaustive()
}
}
#[unstable(feature = "async_drop", issue = "126482")]
impl<T> Future for AsyncDropOwning<T> {
type Output = ();
fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
// SAFETY: Self is pinned thus it is ok to store references to self
unsafe {
let this = self.get_unchecked_mut();
let dtor = Pin::new_unchecked(
this.dtor.get_or_insert_with(|| async_drop_in_place(this.value.as_mut_ptr())),
);
// AsyncDestuctors are idempotent so Self gets idempotency as well
dtor.poll(cx)
}
}
}
#[lang = "async_drop_in_place"]
#[allow(unconditional_recursion)]
// FIXME: Consider if `#[rustc_diagnostic_item = "ptr_drop_in_place"]` is needed?
unsafe fn async_drop_in_place_raw<T: ?Sized>(
to_drop: *mut T,
) -> <T as AsyncDestruct>::AsyncDestructor {
// Code here does not matter - this is replaced by the
// real async drop glue constructor by the compiler.
// SAFETY: see comment above
unsafe { async_drop_in_place_raw(to_drop) }
}
/// Creates the asynchronous destructor of the pointed-to value.
///
/// # Safety
///
/// Behavior is undefined if any of the following conditions are violated:
///
/// * `to_drop` must be [valid](crate::ptr#safety) for both reads and writes.
///
/// * `to_drop` must be properly aligned, even if `T` has size 0.
///
/// * `to_drop` must be nonnull, even if `T` has size 0.
///
/// * The value `to_drop` points to must be valid for async dropping,
/// which may mean it must uphold additional invariants. These
/// invariants depend on the type of the value being dropped. For
/// instance, when dropping a Box, the box's pointer to the heap must
/// be valid.
///
/// * While `async_drop_in_place` is executing or the returned async
/// destructor is alive, the only way to access parts of `to_drop`
/// is through the `self: Pin<&mut Self>` references supplied to
/// the `AsyncDrop::async_drop` methods that `async_drop_in_place`
/// or `AsyncDropInPlace<T>::poll` invokes. This usually means the
/// returned future stores the `to_drop` pointer and user is required
/// to guarantee that dropped value doesn't move.
///
#[unstable(feature = "async_drop", issue = "126482")]
pub unsafe fn async_drop_in_place<T: ?Sized>(to_drop: *mut T) -> AsyncDropInPlace<T> {
// SAFETY: `async_drop_in_place_raw` has the same safety requirements
unsafe { AsyncDropInPlace(async_drop_in_place_raw(to_drop)) }
}
/// A future returned by the [`async_drop_in_place`].
#[unstable(feature = "async_drop", issue = "126482")]
pub struct AsyncDropInPlace<T: ?Sized>(<T as AsyncDestruct>::AsyncDestructor);
#[unstable(feature = "async_drop", issue = "126482")]
impl<T: ?Sized> fmt::Debug for AsyncDropInPlace<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("AsyncDropInPlace").finish_non_exhaustive()
}
}
#[unstable(feature = "async_drop", issue = "126482")]
impl<T: ?Sized> Future for AsyncDropInPlace<T> {
type Output = ();
#[inline(always)]
fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
// SAFETY: This code simply forwards poll call to the inner future
unsafe { Pin::new_unchecked(&mut self.get_unchecked_mut().0) }.poll(cx)
}
}
// FIXME(zetanumbers): Add same restrictions on AsyncDrop impls as
// with Drop impls
/// Custom code within the asynchronous destructor.
#[unstable(feature = "async_drop", issue = "126482")]
#[lang = "async_drop"]
pub trait AsyncDrop {
/// A future returned by the [`AsyncDrop::async_drop`] to be part
/// of the async destructor.
#[unstable(feature = "async_drop", issue = "126482")]
type Dropper<'a>: Future<Output = ()>
where
Self: 'a;
/// Constructs the asynchronous destructor for this type.
#[unstable(feature = "async_drop", issue = "126482")]
fn async_drop(self: Pin<&mut Self>) -> Self::Dropper<'_>;
}
#[lang = "async_destruct"]
#[rustc_deny_explicit_impl(implement_via_object = false)]
trait AsyncDestruct {
type AsyncDestructor: Future<Output = ()>;
}
/// Basically calls `AsyncDrop::async_drop` with pointer. Used to simplify
/// generation of the code for `async_drop_in_place_raw`
#[lang = "surface_async_drop_in_place"]
async unsafe fn surface_async_drop_in_place<T: AsyncDrop + ?Sized>(ptr: *mut T) {
// SAFETY: We call this from async drop `async_drop_in_place_raw`
// which has the same safety requirements
unsafe { <T as AsyncDrop>::async_drop(Pin::new_unchecked(&mut *ptr)).await }
}
/// Basically calls `Drop::drop` with pointer. Used to simplify generation
/// of the code for `async_drop_in_place_raw`
#[allow(drop_bounds)]
#[lang = "async_drop_surface_drop_in_place"]
async unsafe fn surface_drop_in_place<T: Drop + ?Sized>(ptr: *mut T) {
// SAFETY: We call this from async drop `async_drop_in_place_raw`
// which has the same safety requirements
unsafe { crate::ops::fallback_surface_drop(&mut *ptr) }
}
/// Wraps a future to continue outputing `Poll::Ready(())` once after
/// wrapped future completes by returning `Poll::Ready(())` on poll. This
/// is useful for constructing async destructors to guarantee this
/// "fuse" property
//
// FIXME: Consider optimizing combinators to not have to use fuse in majority
// of cases, perhaps by adding `#[(rustc_)idempotent(_future)]` attribute for
// async functions and blocks with the unit return type. However current layout
// optimizations currently encode `None` case into the async block's discriminant.
struct Fuse<T> {
inner: Option<T>,
}
#[lang = "async_drop_fuse"]
fn fuse<T>(inner: T) -> Fuse<T> {
Fuse { inner: Some(inner) }
}
impl<T> Future for Fuse<T>
where
T: Future<Output = ()>,
{
type Output = ();
fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
// SAFETY: pin projection into `self.inner`
unsafe {
let this = self.get_unchecked_mut();
if let Some(inner) = &mut this.inner {
ready!(Pin::new_unchecked(inner).poll(cx));
this.inner = None;
}
}
Poll::Ready(())
}
}
/// Async destructor for arrays and slices.
#[lang = "async_drop_slice"]
async unsafe fn slice<T>(s: *mut [T]) {
let len = s.len();
let ptr = s.as_mut_ptr();
for i in 0..len {
// SAFETY: we iterate over elements of `s` slice
unsafe { async_drop_in_place_raw(ptr.add(i)).await }
}
}
/// Constructs a chain of two futures, which awaits them sequentially as
/// a future.
#[lang = "async_drop_chain"]
async fn chain<F, G>(first: F, last: G)
where
F: IntoFuture<Output = ()>,
G: IntoFuture<Output = ()>,
{
first.await;
last.await;
}
/// Basically a lazy version of `async_drop_in_place`. Returns a future
/// that would call `AsyncDrop::async_drop` on a first poll.
///
/// # Safety
///
/// Same as `async_drop_in_place` except is lazy to avoid creating
/// multiple mutable refernces.
#[lang = "async_drop_defer"]
async unsafe fn defer<T: ?Sized>(to_drop: *mut T) {
// SAFETY: same safety requirements as `async_drop_in_place`
unsafe { async_drop_in_place(to_drop) }.await
}
/// If `T`'s discriminant is equal to the stored one then awaits `M`
/// otherwise awaits the `O`.
///
/// # Safety
///
/// Users should carefully manage the returned future, since it would
/// try creating an immutable reference from `this` and get pointee's
/// discriminant.
// FIXME(zetanumbers): Send and Sync impls
#[lang = "async_drop_either"]
async unsafe fn either<O: IntoFuture<Output = ()>, M: IntoFuture<Output = ()>, T>(
other: O,
matched: M,
this: *mut T,
discr: <T as DiscriminantKind>::Discriminant,
) {
// SAFETY: Guaranteed by the safety section of this funtion's documentation
if unsafe { discriminant_value(&*this) } == discr {
drop(other);
matched.await
} else {
drop(matched);
other.await
}
}
#[lang = "async_drop_deferred_drop_in_place"]
async unsafe fn deferred_drop_in_place<T>(to_drop: *mut T) {
// SAFETY: same safety requirements as with drop_in_place (implied by
// function's name)
unsafe { crate::ptr::drop_in_place(to_drop) }
}
/// Used for noop async destructors. We don't use [`core::future::Ready`]
/// because it panics after its second poll, which could be potentially
/// bad if that would happen during the cleanup.
#[derive(Clone, Copy)]
struct Noop;
#[lang = "async_drop_noop"]
fn noop() -> Noop {
Noop
}
impl Future for Noop {
type Output = ();
fn poll(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<Self::Output> {
Poll::Ready(())
}
}