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 [thir_body] building THIR for `foo`
#2 [check_match] match-checking `foo`
#3 [mir_built] building MIR for `foo`
#4 [unsafety_check_result] unsafety-checking `foo`
#5 [mir_const] preparing `foo` for borrow checking
#6 [mir_promoted] processing MIR for `foo`
#7 [mir_borrowck] borrow-checking `foo`
#8 [type_of] computing type of `foo::{opaque#0}`
#9 [check_mod_item_types] checking item types in top-level module
#10 [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`.