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

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.

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 #67644, #67982

/cc `@yoshuawuyts`

9 files changed, 74 insertions, 4 deletions

diff --git a/src/test/ui/async-await/async-error-span.stderr b/src/test/ui/async-await/async-error-span.stderr
index 994bfd33ba4..2e3f8bb5256 100644
--- a/src/test/ui/async-await/async-error-span.stderr
+++ b/src/test/ui/async-await/async-error-span.stderr
@@ -5,6 +5,7 @@ LL | fn get_future() -> impl Future<Output = ()> {
    |                    ^^^^^^^^^^^^^^^^^^^^^^^^ `()` is not a future
    |
    = help: the trait `Future` is not implemented for `()`
+   = note: () must be a future or must implement `IntoFuture` to be awaited
 
 error[E0698]: type inside `async fn` body must be known in this context
   --> $DIR/async-error-span.rs:13:9
diff --git a/src/test/ui/async-await/async-fn-size-moved-locals.rs b/src/test/ui/async-await/async-fn-size-moved-locals.rs
index 636fafc2bc4..a603ebd6e85 100644
--- a/src/test/ui/async-await/async-fn-size-moved-locals.rs
+++ b/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()));
 }
diff --git a/src/test/ui/async-await/await-into-future.rs b/src/test/ui/async-await/await-into-future.rs
new file mode 100644
index 00000000000..b74b1684440
--- /dev/null
+++ b/src/test/ui/async-await/await-into-future.rs
@@ -0,0 +1,30 @@
+// run-pass
+// aux-build: issue-72470-lib.rs
+// edition:2021
+#![feature(into_future)]
+
+extern crate issue_72470_lib;
+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() {
+    issue_72470_lib::run(run());
+}
diff --git a/src/test/ui/async-await/issue-70594.stderr b/src/test/ui/async-await/issue-70594.stderr
index eb24040404b..ab05251526b 100644
--- a/src/test/ui/async-await/issue-70594.stderr
+++ b/src/test/ui/async-await/issue-70594.stderr
@@ -25,6 +25,8 @@ LL |     [1; ().await];
    |         ^^^^^^^^ `()` is not a future
    |
    = help: the trait `Future` is not implemented for `()`
+   = note: () must be a future or must implement `IntoFuture` to be awaited
+   = note: required because of the requirements on the impl of `IntoFuture` for `()`
 
 error: aborting due to 4 previous errors
 
diff --git a/src/test/ui/async-await/issues/issue-62009-1.stderr b/src/test/ui/async-await/issues/issue-62009-1.stderr
index e2ea72a1e61..19d6f9bc438 100644
--- a/src/test/ui/async-await/issues/issue-62009-1.stderr
+++ b/src/test/ui/async-await/issues/issue-62009-1.stderr
@@ -34,6 +34,8 @@ LL |     (|_| 2333).await;
    |     ^^^^^^^^^^^^^^^^ `[closure@$DIR/issue-62009-1.rs:12:5: 12:15]` is not a future
    |
    = help: the trait `Future` is not implemented for `[closure@$DIR/issue-62009-1.rs:12:5: 12:15]`
+   = note: [closure@$DIR/issue-62009-1.rs:12:5: 12:15] must be a future or must implement `IntoFuture` to be awaited
+   = note: required because of the requirements on the impl of `IntoFuture` for `[closure@$DIR/issue-62009-1.rs:12:5: 12:15]`
 
 error: aborting due to 4 previous errors
 
diff --git a/src/test/ui/async-await/unresolved_type_param.rs b/src/test/ui/async-await/unresolved_type_param.rs
index d313691b388..85d868c2703 100644
--- a/src/test/ui/async-await/unresolved_type_param.rs
+++ b/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`
diff --git a/src/test/ui/async-await/unresolved_type_param.stderr b/src/test/ui/async-await/unresolved_type_param.stderr
index 6b9e960ca1a..130667a49c5 100644
--- a/src/test/ui/async-await/unresolved_type_param.stderr
+++ b/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`.
diff --git a/src/test/ui/issues-71798.stderr b/src/test/ui/issues-71798.stderr
index bbcdd3e37a9..bc4dc9ebf9e 100644
--- a/src/test/ui/issues-71798.stderr
+++ b/src/test/ui/issues-71798.stderr
@@ -11,6 +11,7 @@ LL | fn test_ref(x: &u32) -> impl std::future::Future<Output = u32> + '_ {
    |                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `u32` is not a future
    |
    = help: the trait `Future` is not implemented for `u32`
+   = note: u32 must be a future or must implement `IntoFuture` to be awaited
 
 error: aborting due to 2 previous errors
 
diff --git a/src/test/ui/suggestions/async-fn-ctor-passed-as-arg-where-it-should-have-been-called.stderr b/src/test/ui/suggestions/async-fn-ctor-passed-as-arg-where-it-should-have-been-called.stderr
index 4759ebea0e9..766db2a8356 100644
--- a/src/test/ui/suggestions/async-fn-ctor-passed-as-arg-where-it-should-have-been-called.stderr
+++ b/src/test/ui/suggestions/async-fn-ctor-passed-as-arg-where-it-should-have-been-called.stderr
@@ -10,6 +10,7 @@ LL |     bar(foo);
    |     required by a bound introduced by this call
    |
    = help: the trait `Future` is not implemented for `fn() -> impl Future<Output = ()> {foo}`
+   = note: fn() -> impl Future<Output = ()> {foo} must be a future or must implement `IntoFuture` to be awaited
 note: required by a bound in `bar`
   --> $DIR/async-fn-ctor-passed-as-arg-where-it-should-have-been-called.rs:7:16
    |
@@ -31,6 +32,7 @@ LL |     bar(async_closure);
    |     required by a bound introduced by this call
    |
    = help: the trait `Future` is not implemented for `[closure@$DIR/async-fn-ctor-passed-as-arg-where-it-should-have-been-called.rs:11:25: 11:36]`
+   = note: [closure@$DIR/async-fn-ctor-passed-as-arg-where-it-should-have-been-called.rs:11:25: 11:36] must be a future or must implement `IntoFuture` to be awaited
 note: required by a bound in `bar`
   --> $DIR/async-fn-ctor-passed-as-arg-where-it-should-have-been-called.rs:7:16
    |