error: unused `async` for function with no await statements
  --> $DIR/unused_async.rs:3:1
   |
LL | / async fn foo() -> i32 {
LL | |     4
LL | | }
   | |_^
   |
   = note: `-D clippy::unused-async` implied by `-D warnings`
   = help: consider removing the `async` from this function

error: aborting due to previous error