error[E0277]: `[async fn body@$DIR/async.rs:23:29: 25:2]` is not a future
  --> $DIR/async.rs:23:29
   |
LL |   async fn foo(x: u32) -> u32 {
   |  _____________________________-
LL | |     x
LL | | }
   | | ^
   | | |
   | |_`[async fn body@$DIR/async.rs:23:29: 25:2]` is not a future
   |   required by a bound introduced by this call
   |
   = 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
note: required by a bound in `identity_future`
  --> $SRC_DIR/core/src/future/mod.rs:LL:COL

error[E0277]: the size for values of type `<[async fn body@$DIR/async.rs:23:29: 25:2] as Future>::Output` cannot be known at compilation time
  --> $DIR/async.rs:23:29
   |
LL |   async fn foo(x: u32) -> u32 {
   |  _____________________________^
LL | |     x
LL | | }
   | |_^ doesn't have a size known at compile-time
   |
   = help: the trait `Sized` is not implemented for `<[async fn body@$DIR/async.rs:23:29: 25:2] as Future>::Output`
note: required by a bound in `identity_future`
  --> $SRC_DIR/core/src/future/mod.rs:LL:COL

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 [mir_built] building MIR for `foo`
#3 [unsafety_check_result] unsafety-checking `foo`
#4 [mir_const] preparing `foo` for borrow checking
#5 [mir_promoted] processing MIR for `foo`
#6 [mir_borrowck] borrow-checking `foo`
#7 [type_of] computing type of `foo::{opaque#0}`
#8 [check_mod_item_types] checking item types in top-level module
#9 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to 4 previous errors

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