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](rust-lang/compiler-team#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 (rust-lang/rust#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

tests/pass/async-drop.rs

@@ -0,0 +1,191 @@
+//@revisions: stack tree
+//@compile-flags: -Zmiri-strict-provenance
+//@[tree]compile-flags: -Zmiri-tree-borrows
+#![feature(async_drop, impl_trait_in_assoc_type, noop_waker, async_closure)]
+#![allow(incomplete_features, dead_code)]
+
+// FIXME(zetanumbers): consider AsyncDestruct::async_drop cleanup tests
+use core::future::{async_drop_in_place, AsyncDrop, Future};
+use core::hint::black_box;
+use core::mem::{self, ManuallyDrop};
+use core::pin::{pin, Pin};
+use core::task::{Context, Poll, Waker};
+
+async fn test_async_drop<T>(x: T) {
+    let mut x = mem::MaybeUninit::new(x);
+    let dtor = pin!(unsafe { async_drop_in_place(x.as_mut_ptr()) });
+    test_idempotency(dtor).await;
+}
+
+fn test_idempotency<T>(mut x: Pin<&mut T>) -> impl Future<Output = ()> + '_
+where
+    T: Future<Output = ()>,
+{
+    core::future::poll_fn(move |cx| {
+        assert_eq!(x.as_mut().poll(cx), Poll::Ready(()));
+        assert_eq!(x.as_mut().poll(cx), Poll::Ready(()));
+        Poll::Ready(())
+    })
+}
+
+fn main() {
+    let waker = Waker::noop();
+    let mut cx = Context::from_waker(&waker);
+
+    let i = 13;
+    let fut = pin!(async {
+        test_async_drop(Int(0)).await;
+        test_async_drop(AsyncInt(0)).await;
+        test_async_drop([AsyncInt(1), AsyncInt(2)]).await;
+        test_async_drop((AsyncInt(3), AsyncInt(4))).await;
+        test_async_drop(5).await;
+        let j = 42;
+        test_async_drop(&i).await;
+        test_async_drop(&j).await;
+        test_async_drop(AsyncStruct { b: AsyncInt(8), a: AsyncInt(7), i: 6 }).await;
+        test_async_drop(ManuallyDrop::new(AsyncInt(9))).await;
+
+        let foo = AsyncInt(10);
+        test_async_drop(AsyncReference { foo: &foo }).await;
+
+        let foo = AsyncInt(11);
+        test_async_drop(|| {
+            black_box(foo);
+            let foo = AsyncInt(10);
+            foo
+        })
+        .await;
+
+        test_async_drop(AsyncEnum::A(AsyncInt(12))).await;
+        test_async_drop(AsyncEnum::B(SyncInt(13))).await;
+
+        test_async_drop(SyncInt(14)).await;
+        test_async_drop(SyncThenAsync { i: 15, a: AsyncInt(16), b: SyncInt(17), c: AsyncInt(18) })
+            .await;
+
+        let async_drop_fut = pin!(core::future::async_drop(AsyncInt(19)));
+        test_idempotency(async_drop_fut).await;
+
+        let foo = AsyncInt(20);
+        test_async_drop(async || {
+            black_box(foo);
+            let foo = AsyncInt(19);
+            // Await point there, but this is async closure so it's fine
+            black_box(core::future::ready(())).await;
+            foo
+        })
+        .await;
+
+        test_async_drop(AsyncUnion { signed: 21 }).await;
+    });
+    let res = fut.poll(&mut cx);
+    assert_eq!(res, Poll::Ready(()));
+}
+
+struct AsyncInt(i32);
+
+impl AsyncDrop for AsyncInt {
+    type Dropper<'a> = impl Future<Output = ()>;
+
+    fn async_drop(self: Pin<&mut Self>) -> Self::Dropper<'_> {
+        async move {
+            println!("AsyncInt::Dropper::poll: {}", self.0);
+        }
+    }
+}
+
+struct SyncInt(i32);
+
+impl Drop for SyncInt {
+    fn drop(&mut self) {
+        println!("SyncInt::drop: {}", self.0);
+    }
+}
+
+struct SyncThenAsync {
+    i: i32,
+    a: AsyncInt,
+    b: SyncInt,
+    c: AsyncInt,
+}
+
+impl Drop for SyncThenAsync {
+    fn drop(&mut self) {
+        println!("SyncThenAsync::drop: {}", self.i);
+    }
+}
+
+struct AsyncReference<'a> {
+    foo: &'a AsyncInt,
+}
+
+impl AsyncDrop for AsyncReference<'_> {
+    type Dropper<'a> = impl Future<Output = ()> where Self: 'a;
+
+    fn async_drop(self: Pin<&mut Self>) -> Self::Dropper<'_> {
+        async move {
+            println!("AsyncReference::Dropper::poll: {}", self.foo.0);
+        }
+    }
+}
+
+struct Int(i32);
+
+struct AsyncStruct {
+    i: i32,
+    a: AsyncInt,
+    b: AsyncInt,
+}
+
+impl AsyncDrop for AsyncStruct {
+    type Dropper<'a> = impl Future<Output = ()>;
+
+    fn async_drop(self: Pin<&mut Self>) -> Self::Dropper<'_> {
+        async move {
+            println!("AsyncStruct::Dropper::poll: {}", self.i);
+        }
+    }
+}
+
+enum AsyncEnum {
+    A(AsyncInt),
+    B(SyncInt),
+}
+
+impl AsyncDrop for AsyncEnum {
+    type Dropper<'a> = impl Future<Output = ()>;
+
+    fn async_drop(mut self: Pin<&mut Self>) -> Self::Dropper<'_> {
+        async move {
+            let new_self = match &*self {
+                AsyncEnum::A(foo) => {
+                    println!("AsyncEnum(A)::Dropper::poll: {}", foo.0);
+                    AsyncEnum::B(SyncInt(foo.0))
+                }
+                AsyncEnum::B(foo) => {
+                    println!("AsyncEnum(B)::Dropper::poll: {}", foo.0);
+                    AsyncEnum::A(AsyncInt(foo.0))
+                }
+            };
+            mem::forget(mem::replace(&mut *self, new_self));
+        }
+    }
+}
+
+// FIXME(zetanumbers): Disallow types with `AsyncDrop` in unions
+union AsyncUnion {
+    signed: i32,
+    unsigned: u32,
+}
+
+impl AsyncDrop for AsyncUnion {
+    type Dropper<'a> = impl Future<Output = ()>;
+
+    fn async_drop(self: Pin<&mut Self>) -> Self::Dropper<'_> {
+        async move {
+            println!("AsyncUnion::Dropper::poll: {}, {}", unsafe { self.signed }, unsafe {
+                self.unsigned
+            });
+        }
+    }
+}

tests/pass/async-drop.stack.stdout

@@ -0,0 +1,22 @@
+AsyncInt::Dropper::poll: 0
+AsyncInt::Dropper::poll: 1
+AsyncInt::Dropper::poll: 2
+AsyncInt::Dropper::poll: 3
+AsyncInt::Dropper::poll: 4
+AsyncStruct::Dropper::poll: 6
+AsyncInt::Dropper::poll: 7
+AsyncInt::Dropper::poll: 8
+AsyncReference::Dropper::poll: 10
+AsyncInt::Dropper::poll: 11
+AsyncEnum(A)::Dropper::poll: 12
+SyncInt::drop: 12
+AsyncEnum(B)::Dropper::poll: 13
+AsyncInt::Dropper::poll: 13
+SyncInt::drop: 14
+SyncThenAsync::drop: 15
+AsyncInt::Dropper::poll: 16
+SyncInt::drop: 17
+AsyncInt::Dropper::poll: 18
+AsyncInt::Dropper::poll: 19
+AsyncInt::Dropper::poll: 20
+AsyncUnion::Dropper::poll: 21, 21

tests/pass/async-drop.tree.stdout

@@ -0,0 +1,22 @@
+AsyncInt::Dropper::poll: 0
+AsyncInt::Dropper::poll: 1
+AsyncInt::Dropper::poll: 2
+AsyncInt::Dropper::poll: 3
+AsyncInt::Dropper::poll: 4
+AsyncStruct::Dropper::poll: 6
+AsyncInt::Dropper::poll: 7
+AsyncInt::Dropper::poll: 8
+AsyncReference::Dropper::poll: 10
+AsyncInt::Dropper::poll: 11
+AsyncEnum(A)::Dropper::poll: 12
+SyncInt::drop: 12
+AsyncEnum(B)::Dropper::poll: 13
+AsyncInt::Dropper::poll: 13
+SyncInt::drop: 14
+SyncThenAsync::drop: 15
+AsyncInt::Dropper::poll: 16
+SyncInt::drop: 17
+AsyncInt::Dropper::poll: 18
+AsyncInt::Dropper::poll: 19
+AsyncInt::Dropper::poll: 20
+AsyncUnion::Dropper::poll: 21, 21