Function mz_expr::relation::func::order_aggregate_datums

pub fn order_aggregate_datums<'a: 'b, 'b, I>(
    datums: I,
    order_by: &[ColumnOrder],
) -> impl Iterator<Item = Datum<'b>>
where
    I: IntoIterator<Item = Datum<'a>>,

Assuming datums is a List, sort them by the 2nd through Nth elements corresponding to order_by, then return the 1st element.

Near the usages of this function, we sometimes want to produce Datums with a shorter lifetime than ’a. We have to actually perform the shortening of the lifetime here, inside this function, because if we were to simply return impl Iterator<Item = Datum<'a>>, that wouldn’t be covariant in the item type, because opaque types are always invariant. (Contrast this with how we perform the shortening inside this function: the input of the map is known to specifically be std::vec::IntoIter, which is known to be covariant.)