Function soak 

pub async fn soak<L, Load, O, Count>(
    settle: Duration,
    outstanding: O,
    load: L,
) -> Soak
where
    L: FnOnce() -> Load,
    Load: Future<Output = ()>,
    O: Fn() -> Count,
    Count: Future<Output = usize>,

Run load, then wait for things to settle, and report what grew.

outstanding is asked twice — before and after — for whatever the stack under test counts as work in progress. settle should be at least SETTLE_PAST_TIMERS for anything driving SIP — see the module documentation for why a shorter one reports RFC-mandated state as a leak.