AsyncDrop implementation using shim codegen of async_drop_in_place::{closure}, scoped async drop added.

Author: azhogin

core/src/future/async_drop.rs

@@ -1,284 +1,51 @@
 #![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::{Context, Poll, ready};
-
-/// 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)
-        }
-    }
-}
+#[allow(unused_imports)]
+use core::future::Future;
 
-#[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) }
-}
+#[allow(unused_imports)]
+use crate::pin::Pin;
+#[allow(unused_imports)]
+use crate::task::{Context, Poll};
 
-/// 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.
+/// Async version of Drop trait.
 ///
-/// * `to_drop` must be nonnull, even if `T` has size 0.
+/// When a value is no longer needed, Rust will run a "destructor" on that value.
+/// The most common way that a value is no longer needed is when it goes out of
+/// scope. Destructors may still run in other circumstances, but we're going to
+/// focus on scope for the examples here. To learn about some of those other cases,
+/// please see [the reference] section on destructors.
 ///
-/// * 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.
+/// [the reference]: https://doc.rust-lang.org/reference/destructors.html
 ///
-/// * 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.
+/// ## `Copy` and ([`Drop`]|`AsyncDrop`) are exclusive
 ///
-#[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.
+/// You cannot implement both [`Copy`] and ([`Drop`]|`AsyncDrop`) on the same type. Types that
+/// are `Copy` get implicitly duplicated by the compiler, making it very
+/// hard to predict when, and how often destructors will be executed. As such,
+/// these types cannot have destructors.
+#[cfg(not(bootstrap))]
 #[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]
-#[rustc_do_not_implement_via_object]
-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 outputting `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 references.
-#[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(())
-    }
+    /// Executes the async destructor for this type.
+    ///
+    /// This method is called implicitly when the value goes out of scope,
+    /// and cannot be called explicitly.
+    ///
+    /// When this method has been called, `self` has not yet been deallocated.
+    /// That only happens after the method is over.
+    ///
+    /// # Panics
+    #[allow(async_fn_in_trait)]
+    async fn drop(self: Pin<&mut Self>);
+}
+
+/// Async drop.
+#[cfg(not(bootstrap))]
+#[unstable(feature = "async_drop", issue = "126482")]
+#[lang = "async_drop_in_place"]
+pub async unsafe fn async_drop_in_place<T: ?Sized>(_to_drop: *mut T) {
+    // Code here does not matter - this is replaced by the
+    // real implementation by the compiler.
 }

core/src/future/mod.rs

@@ -20,8 +20,9 @@ mod pending;
 mod poll_fn;
 mod ready;
 
+#[cfg(not(bootstrap))]
 #[unstable(feature = "async_drop", issue = "126482")]
-pub use async_drop::{AsyncDrop, AsyncDropInPlace, async_drop, async_drop_in_place};
+pub use async_drop::{AsyncDrop, async_drop_in_place};
 #[stable(feature = "into_future", since = "1.64.0")]
 pub use into_future::IntoFuture;
 #[stable(feature = "future_readiness_fns", since = "1.48.0")]

core/src/ops/drop.rs

@@ -240,10 +240,3 @@ pub trait Drop {
     #[stable(feature = "rust1", since = "1.0.0")]
     fn drop(&mut self);
 }
-
-/// Fallback function to call surface level `Drop::drop` function
-#[allow(drop_bounds)]
-#[lang = "fallback_surface_drop"]
-pub(crate) fn fallback_surface_drop<T: Drop + ?Sized>(x: &mut T) {
-    <T as Drop>::drop(x)
-}

core/src/ops/mod.rs

@@ -176,7 +176,6 @@ pub use self::deref::Receiver;
 pub use self::deref::{Deref, DerefMut};
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use self::drop::Drop;
-pub(crate) use self::drop::fallback_surface_drop;
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use self::function::{Fn, FnMut, FnOnce};
 #[stable(feature = "rust1", since = "1.0.0")]