Function spawn 

pub fn spawn<F>(future: F) -> Task<F::Output> 
where
    F: Future + Send + 'static,
    F::Output: Send + 'static,

Spawns a task on the runtime this thread is inside, or on the default runtime. Spawned on a worker, the task stays on that worker unless it holds more live tasks than another shard by the runtime’s local_spawn_margin; spawned anywhere else, it goes to the spawning thread’s home shard by the same rule (the first spawn takes the less loaded of two shards picked at random). The task stays on its shard.

Dropping the returned Task cancels it; call Task::detach to let it run on its own. Awaiting a task that was cancelled (through an AbortHandle, or by its runtime’s shutdown) panics; Task::join and Task::fallible answer it instead.

Panics

When the default runtime or this thread’s shard cannot start (its poller cannot be made); task::Builder::spawn answers the error instead.