Function spawn 

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

Spawns a task on the global task runner.

If this actually spawns a task on a background task or just spawns a local task, depends on the TaskRunner type that is in use.