pub fn spawn_ok<Fut>(&self, future: Fut)
    where
        Fut: Future<Output = ()> + Send + 'static,
    {
        self.spawn_obj_ok(FutureObj::new(Box::new(future)))
    }
Spawns a task that polls the given future with output `()` to
completion.

```
# {
use futures::executor::ThreadPool;

let pool = ThreadPool::new().unwrap();

let future = async { /* ... */ };
pool.spawn_ok(future);
# }
# std::thread::sleep(std::time::Duration::from_millis(500)); // wait for background threads closed: https://github.com/rust-lang/miri/issues/1371
```

> **Note**: This method is similar to `SpawnExt::spawn`, except that
>           it is guaranteed to always succeed.
futures-executor/src/thread_pool.rs