Reintroduce into_future in .await desugaring

This is a reintroduction of the remaining parts from
https://github.com/rust-lang/rust/pull/65244 that have not been relanded
yet.

Issues GH-67644, GH-67982

src/test/ui/async-await/async-fn-size-moved-locals.rs

@@ -112,7 +112,7 @@ async fn mixed_sizes() {
 fn main() {
     assert_eq!(1025, std::mem::size_of_val(&single()));
     assert_eq!(1026, std::mem::size_of_val(&single_with_noop()));
-    assert_eq!(3078, std::mem::size_of_val(&joined()));
-    assert_eq!(3079, std::mem::size_of_val(&joined_with_noop()));
-    assert_eq!(7181, std::mem::size_of_val(&mixed_sizes()));
+    assert_eq!(3076, std::mem::size_of_val(&joined()));
+    assert_eq!(3076, std::mem::size_of_val(&joined_with_noop()));
+    assert_eq!(6157, std::mem::size_of_val(&mixed_sizes()));
 }

src/test/ui/async-await/await-into-future.rs

@@ -0,0 +1,28 @@
+// check-pass
+
+// edition:2021
+
+#![feature(into_future)]
+
+use std::{future::{Future, IntoFuture}, pin::Pin};
+
+struct AwaitMe;
+
+impl IntoFuture for AwaitMe {
+    type Output = i32;
+    type Future = Pin<Box<dyn Future<Output = i32>>>;
+
+    fn into_future(self) -> Self::Future {
+        Box::pin(me())
+    }
+}
+
+async fn me() -> i32 {
+    41
+}
+
+async fn run() {
+    assert_eq!(AwaitMe.await, 41);
+}
+
+fn main() {}

src/test/ui/async-await/unresolved_type_param.rs

@@ -10,12 +10,20 @@ async fn foo() {
     //~^ ERROR type inside `async fn` body must be known in this context
     //~| ERROR type inside `async fn` body must be known in this context
     //~| ERROR type inside `async fn` body must be known in this context
+    //~| ERROR type inside `async fn` body must be known in this context
+    //~| ERROR type inside `async fn` body must be known in this context
+    //~| NOTE cannot infer type for type parameter `T`
+    //~| NOTE cannot infer type for type parameter `T`
     //~| NOTE cannot infer type for type parameter `T`
     //~| NOTE cannot infer type for type parameter `T`
     //~| NOTE cannot infer type for type parameter `T`
     //~| NOTE the type is part of the `async fn` body because of this `await`
     //~| NOTE the type is part of the `async fn` body because of this `await`
     //~| NOTE the type is part of the `async fn` body because of this `await`
+    //~| NOTE the type is part of the `async fn` body because of this `await`
+    //~| NOTE the type is part of the `async fn` body because of this `await`
+    //~| NOTE in this expansion of desugaring of `await`
+    //~| NOTE in this expansion of desugaring of `await`
     //~| NOTE in this expansion of desugaring of `await`
     //~| NOTE in this expansion of desugaring of `await`
     //~| NOTE in this expansion of desugaring of `await`

src/test/ui/async-await/unresolved_type_param.stderr

@@ -34,6 +34,30 @@ note: the type is part of the `async fn` body because of this `await`
 LL |     bar().await;
    |     ^^^^^^^^^^^
 
-error: aborting due to 3 previous errors
+error[E0698]: type inside `async fn` body must be known in this context
+  --> $DIR/unresolved_type_param.rs:9:5
+   |
+LL |     bar().await;
+   |     ^^^ cannot infer type for type parameter `T` declared on the function `bar`
+   |
+note: the type is part of the `async fn` body because of this `await`
+  --> $DIR/unresolved_type_param.rs:9:5
+   |
+LL |     bar().await;
+   |     ^^^^^^^^^^^
+
+error[E0698]: type inside `async fn` body must be known in this context
+  --> $DIR/unresolved_type_param.rs:9:5
+   |
+LL |     bar().await;
+   |     ^^^ cannot infer type for type parameter `T` declared on the function `bar`
+   |
+note: the type is part of the `async fn` body because of this `await`
+  --> $DIR/unresolved_type_param.rs:9:5
+   |
+LL |     bar().await;
+   |     ^^^^^^^^^^^
+
+error: aborting due to 5 previous errors
 
 For more information about this error, try `rustc --explain E0698`.