The entry point of the program was marked as async.
Erroneous code example:
async fn main() -> Result<(), ()> { // error!
Ok(())
}fn main() or the specified start function is not allowed to be async. Not
having a correct async runtime library setup may cause this error. To fix it,
declare the entry point without async:
fn main() -> Result<(), ()> { // ok!
Ok(())
}