future that is spawened with `spawn_with_handle` is not executed if handle is dropped

[SrTobi]

Not sure if this is a bug or only not mentioned in the documentation. The following code,

let mut pool = LocalPool::new();
let mut spawner = pool.spawner();

let f = async {
    println!("first");
    3
};

let handle = spawner.spawn_with_handle(f).unwrap();

//drop(handle);

println!("Start run");
pool.run();

as expected, outputs:

Start run
first

If the line drop(handle) is uncommented, f is not executed anymore and only he following output is produced:

Start run

[Nemo157]

It's not documented very thoroughly, but this is intended behaviour to give you a way to easily cancel the future if you no longer care about the result. Use JoinHandle::forget to abandon the future and allow it to keep running independently.

Definitely should be mentioned in the main JoinHandle documentation, and maybe spawn_with_handle's documentation.