error: functions cannot be both `async` and C-variadic
  --> $DIR/not-async.rs:5:1
   |
LL | async unsafe extern "C" fn cannot_be_async(x: isize, ...) {}
   | ^^^^^ `async` because of this                        ^^^ C-variadic because of this

error[E0700]: hidden type for `impl Future<Output = ()>` captures lifetime that does not appear in bounds
  --> $DIR/not-async.rs:5:59
   |
LL | async unsafe extern "C" fn cannot_be_async(x: isize, ...) {}
   | --------------------------------------------------------- ^^
   | |
   | opaque type defined here
   |
   = note: hidden type `{async fn body of cannot_be_async()}` captures lifetime `'_`

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0700`.