Function on_a_deep_stack 

pub fn on_a_deep_stack<Body, Fut>(body: Body)
where
    Body: FnOnce() -> Fut + Send + 'static,
    Fut: Future<Output = ()>,

Run a joining test body on a thread with room for the flow’s poll frames.

The joining futures are not Send, so the current-thread runtime moves to the configured thread rather than moving the future to a Tokio worker.