Function user_display_name 

pub async fn user_display_name(
    host: &dyn SlackHost,
    api: &dyn SlackApi,
    workspace: &str,
    user: &str,
) -> Option<String>

The display name for a user id, from the host’s cache or a users.info lookup that is then remembered. None when the id cannot be named - the caller draws the id rather than a guess. A failure that is not settled is not remembered, so a later ask tries again.