error: method `foo` should be async because the method from the trait is async
  --> $DIR/async-example-desugared-manual.rs:19:5
   |
LL |     async fn foo(&self) -> i32;
   |     --------------------------- required because the trait method is async
...
LL |     fn foo(&self) -> MyFuture {
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to 1 previous error