Auto merge of rust-lang#124662 - zetanumbers:needs_async_drop, r=oli-obk

Implement `needs_async_drop` in rustc and optimize async drop glue

This PR expands on rust-lang#121801 and implements `Ty::needs_async_drop` which works almost exactly the same as `Ty::needs_drop`, which is needed for rust-lang#123948.

Also made compiler's async drop code to look more like compiler's regular drop code, which enabled me to write an optimization where types which do not use `AsyncDrop` can simply forward async drop glue to `drop_in_place`. This made size of the async block from the [async_drop test](https://github.com/zetanumbers/rust/blob/67980dd6fb11917d23d01a19c2cf4cfc3978aac8/tests/ui/async-await/async-drop.rs) to decrease by 12%.

Author: bors

core/src/future/async_drop.rs

@@ -161,6 +161,11 @@ async unsafe fn surface_drop_in_place<T: Drop + ?Sized>(ptr: *mut T) {
 /// 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>,
 }
@@ -251,6 +256,14 @@ async unsafe fn either<O: IntoFuture<Output = ()>, M: IntoFuture<Output = ()>, T
     }
 }
 
+#[cfg(not(bootstrap))]
+#[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.