Function days_between_impl 

fn days_between_impl(
    cal: &(impl Calendar + ?Sized),
    from: NaiveDate,
    to: NaiveDate,
    include_first: bool,
    include_last: bool,
) -> i64

Counts business days for the forward case (from < to) with endpoint inclusion flags. The last endpoint is counted up front when included; the loop covers [from-or-from+1, to).