Function block_on 

pub fn block_on<F: Future>(root_fut: F) -> F::Output

We block_on on a special future that we refer to as the root_future. It is guaranteed to be polled on the current thread, and is central in deciding how and when the runtime returns. This is why it has looser bounds (!Send and !Sync).

It can stay on the stack if it is small enough, otherwise it gets heap allocated.