error[E0277]: `[async fn body@$DIR/async.rs:23:29: 25:2]` is not a future
  --> $DIR/async.rs:23:25
   |
LL | async fn foo(x: u32) -> u32 {
   |                         ^^^ `[async fn body@$DIR/async.rs:23:29: 25:2]` is not a future
   |
   = help: the trait `Future` is not implemented for `[async fn body@$DIR/async.rs:23:29: 25:2]`
   = note: [async fn body@$DIR/async.rs:23:29: 25:2] must be a future or must implement `IntoFuture` to be awaited

error: internal compiler error: projection clauses should be implied from elsewhere. obligation: `Obligation(predicate=Binder(ProjectionPredicate(AliasTy { substs: [[async fn body@$DIR/async.rs:23:29: 25:2]], def_id: ... }, Term::Ty(u32)), []), depth=0)`
  --> $DIR/async.rs:23:25
   |
LL | async fn foo(x: u32) -> u32 {
   |                         ^^^query stack during panic:
#0 [typeck] type-checking `foo`
#1 [type_of] computing type of `foo::{opaque#0}`
#2 [check_mod_item_types] checking item types in top-level module
#3 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0277`.