Auto merge of rust-lang#90737 - eholk:intofuture, r=tmandry

Reintroduce `into_future` in `.await` desugaring

This is a reintroduction of the remaining parts from rust-lang#65244 that have not been relanded yet.

This isn't quite ready to merge yet. The last attempt was reverting due to performance regressions, so we need to make sure this does not introduce those issues again.

Issues rust-lang#67644, rust-lang#67982

/cc `@yoshuawuyts`

Author: bors

core/src/future/future.rs

@@ -28,7 +28,11 @@ use crate::task::{Context, Poll};
 #[must_use = "futures do nothing unless you `.await` or poll them"]
 #[stable(feature = "futures_api", since = "1.36.0")]
 #[lang = "future_trait"]
-#[rustc_on_unimplemented(label = "`{Self}` is not a future", message = "`{Self}` is not a future")]
+#[rustc_on_unimplemented(
+    label = "`{Self}` is not a future",
+    message = "`{Self}` is not a future",
+    note = "{Self} must be a future or must implement `IntoFuture` to be awaited"
+)]
 pub trait Future {
     /// The type of value produced on completion.
     #[stable(feature = "futures_api", since = "1.36.0")]

core/src/future/into_future.rs

@@ -13,6 +13,7 @@ pub trait IntoFuture {
 
     /// Creates a future from a value.
     #[unstable(feature = "into_future", issue = "67644")]
+    #[cfg_attr(not(bootstrap), lang = "into_future")]
     fn into_future(self) -> Self::Future;
 }