Function spawn_blocking 

pub fn spawn_blocking<F, R>(func: F) -> Handle<F::Output>
where
    F: FnOnce() -> R + Send + 'static,
    R: Send + 'static,

Available on crate features coroutine-heavy and coroutine only.

Spawn a task onto the blocking pool of the background runtime

Since the light context only has one background thread, this is only enabled in heavy context to prevent misuse.