Auto merge of #121801 - zetanumbers:async_drop_glue, r=oli-obk

Add simple async drop glue generation

This is a prototype of the async drop glue generation for some simple types. Async drop glue is intended to behave very similar to the regular drop glue except for being asynchronous. Currently it does not execute synchronous drops but only calls user implementations of `AsyncDrop::async_drop` associative function and awaits the returned future. It is not complete as it only recurses into arrays, slices, tuples, and structs and does not have same sensible restrictions as the old `Drop` trait implementation like having the same bounds as the type definition, while code assumes their existence (requires a future work).

This current design uses a workaround as it does not create any custom async destructor state machine types for ADTs, but instead uses types defined in the std library called future combinators (deferred_async_drop, chain, ready_unit).

Also I recommend reading my [explainer](https://zetanumbers.github.io/book/async-drop-design.html).

This is a part of the [MCP: Low level components for async drop](https://github.com/rust-lang/compiler-team/issues/727) work.

Feature completeness:

 - [x] `AsyncDrop` trait
 - [ ] `async_drop_in_place_raw`/async drop glue generation support for
   - [x] Trivially destructible types (integers, bools, floats, string slices, pointers, references, etc.)
   - [x] Arrays and slices (array pointer is unsized into slice pointer)
   - [x] ADTs (enums, structs, unions)
   - [x] tuple-like types (tuples, closures)
   - [ ] Dynamic types (`dyn Trait`, see explainer's [proposed design](https://github.com/zetanumbers/posts/blob/main/async-drop-design.md#async-drop-glue-for-dyn-trait))
   - [ ] coroutines (https://github.com/rust-lang/rust/pull/123948)
 - [x] Async drop glue includes sync drop glue code
 - [x] Cleanup branch generation for `async_drop_in_place_raw`
 - [ ] Union rejects non-trivially async destructible fields
 - [ ] `AsyncDrop` implementation requires same bounds as type definition
 - [ ] Skip trivially destructible fields (optimization)
 - [ ] New [`TyKind::AdtAsyncDestructor`](https://github.com/zetanumbers/posts/blob/main/async-drop-design.md#adt-async-destructor-types) and get rid of combinators
 - [ ] [Synchronously undroppable types](https://github.com/zetanumbers/posts/blob/main/async-drop-design.md#exclusively-async-drop)
 - [ ] Automatic async drop at the end of the scope in async context

4 files changed, 288 insertions, 0 deletions

diff --git a/library/core/src/future/async_drop.rs b/library/core/src/future/async_drop.rs
new file mode 100644
index 00000000000..0eb8d7bb328
--- /dev/null
+++ b/library/core/src/future/async_drop.rs
@@ -0,0 +1,271 @@
+#![unstable(feature = "async_drop", issue = "none")]
+
+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 = "none")]
+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 = "none")]
+pub struct AsyncDropOwning<T> {
+    value: MaybeUninit<T>,
+    dtor: Option<AsyncDropInPlace<T>>,
+    _pinned: PhantomPinned,
+}
+
+#[unstable(feature = "async_drop", issue = "none")]
+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 = "none")]
+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 = "none")]
+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 = "none")]
+pub struct AsyncDropInPlace<T: ?Sized>(<T as AsyncDestruct>::AsyncDestructor);
+
+#[unstable(feature = "async_drop", issue = "none")]
+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 = "none")]
+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 = "none")]
+#[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 = "none")]
+    type Dropper<'a>: Future<Output = ()>
+    where
+        Self: 'a;
+
+    /// Constructs the asynchronous destructor for this type.
+    #[unstable(feature = "async_drop", issue = "none")]
+    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
+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 }
+    }
+}
+
+/// Construct 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
+///
+/// User should carefully manage returned future, since it would
+/// try creating an immutable referece 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
+    }
+}
+
+/// 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(())
+    }
+}
diff --git a/library/core/src/future/mod.rs b/library/core/src/future/mod.rs
index 0f77a2d8343..c3bd18e30aa 100644
--- a/library/core/src/future/mod.rs
+++ b/library/core/src/future/mod.rs
@@ -12,6 +12,8 @@
 use crate::ptr::NonNull;
 use crate::task::Context;
 
+#[cfg(not(bootstrap))]
+mod async_drop;
 mod future;
 mod into_future;
 mod join;
@@ -36,6 +38,10 @@ pub use ready::{ready, Ready};
 #[stable(feature = "future_poll_fn", since = "1.64.0")]
 pub use poll_fn::{poll_fn, PollFn};
 
+#[cfg(not(bootstrap))]
+#[unstable(feature = "async_drop", issue = "none")]
+pub use async_drop::{async_drop, async_drop_in_place, AsyncDrop, AsyncDropInPlace};
+
 /// This type is needed because:
 ///
 /// a) Coroutines cannot implement `for<'a, 'b> Coroutine<&'a mut Context<'b>>`, so we need to pass
diff --git a/library/core/src/ops/drop.rs b/library/core/src/ops/drop.rs
index 34dfa9e4c51..1325d90e4f3 100644
--- a/library/core/src/ops/drop.rs
+++ b/library/core/src/ops/drop.rs
@@ -238,3 +238,11 @@ pub trait Drop {
     #[stable(feature = "rust1", since = "1.0.0")]
     fn drop(&mut self);
 }
+
+/// Fallback function to call surface level `Drop::drop` function
+#[cfg(not(bootstrap))]
+#[allow(drop_bounds)]
+#[lang = "fallback_surface_drop"]
+pub(crate) fn fallback_surface_drop<T: Drop + ?Sized>(x: &mut T) {
+    <T as Drop>::drop(x)
+}
diff --git a/library/core/src/ops/mod.rs b/library/core/src/ops/mod.rs
index ac808bec50e..81d5e5c949e 100644
--- a/library/core/src/ops/mod.rs
+++ b/library/core/src/ops/mod.rs
@@ -174,6 +174,9 @@ pub use self::deref::Receiver;
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use self::drop::Drop;
 
+#[cfg(not(bootstrap))]
+pub(crate) use self::drop::fallback_surface_drop;
+
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use self::function::{Fn, FnMut, FnOnce};