error: method `foo` should be async because the method from the trait is async
  --> $DIR/async-example-desugared-boxed.rs:15:5
   |
LL |     async fn foo(&self) -> i32;
   |     --------------------------- required because the trait method is async
...
LL |     fn foo(&self) -> Pin<Box<dyn Future<Output = i32> + '_>> {
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to previous error