Function apply_lag 

pub(crate) fn apply_lag(
    n: i32,
    unit: SettlementLagUnit,
    business: &dyn Calendar,
    settlement: Option<&dyn Calendar>,
    convention: BusinessDayConvention,
    from: NaiveDate,
) -> NaiveDate

SettlementLag::apply over borrowed calendars, for a caller that holds the lag’s parts and would otherwise clone them into one per call.