Function crawl 

async fn crawl<S>(
    crawl_service: RateLimitBySkipping<CrawlFanout<S>>,
    demand_tx: Sender<MorePeers>,
) -> Result<(), BoxError>
where
    S: Service<Request, Response = Response, Error = BoxError> + Clone + Send + Sync + 'static,
    S::Future: Send + 'static,

Try to get more peers using crawl_service, then queue a connection attempt using demand_tx. If there were no new peers, the connection attempt is skipped.