Function insert_retain_history 

fn insert_retain_history<'a, N, T>(
    info: &mut BTreeMap<&str, Value>,
    options: impl DoubleEndedIterator<Item = (&'a N, &'a Option<WithOptionValue<T>>)>,
    retain_history: N,
) -> Result<(), String>
where
    N: PartialEq + 'a,
    T: AstInfo + 'a,

Records the window of the last RETAIN HISTORY option among options, the one CatalogItem::update_retain_history keeps, as retain_history_millis in info.

Each statement type has its own option type, so options yields each option’s name and value, and retain_history is that type’s RetainHistory name.