Function spawn_blocking 

pub fn spawn_blocking<F, T>(f: F) -> Task<T> 
where
    F: FnOnce() -> T + Send + 'static,
    T: Send + 'static,

Runs f on the blocking pool of the runtime this thread is inside, or of the default runtime, so it never holds a shard up.

Panics

When the default runtime cannot start.