Function rank_actions_fields 

pub fn rank_actions_fields<'a, R>(
    needle: &str,
    rows: impl IntoIterator<Item = R>,
) -> Vec<Option<i32>>
where
    R: IntoIterator<Item = (&'a str, FieldWeight)>,

rank_action_fields for every action of a list at once, row for row: the list is indexed once, so ranking a session’s hundreds of published actions costs one index, not one per action.