fix size_hint

Author: kdn36

crates/polars-expr/src/expressions/apply.rs

@@ -137,7 +137,7 @@ impl ApplyExpr {
 
         // In case of overlapping (rolling) groups, we build groups in a lazy manner to avoid
         // memory explosion.
-        // kdn TODO: TBD - do we want to follow this path for *all* Slice
+        // kdn TODO: TBD - do we want to follow this path for *all* Slice groupstype?
         if matches!(ac.agg_state(), AggState::NotAggregated(_))
             && let GroupsType::Slice { rolling: true, .. } = ac.groups.as_ref().as_ref()
         {

crates/polars-expr/src/expressions/group_iter.rs

@@ -264,6 +264,7 @@ impl<'a, I: Iterator<Item = GroupsIndicator<'a>>> Iterator for NotAggLazyIter<'a
 
     fn next(&mut self) -> Option<Self::Item> {
         if let Some(g) = self.iter.next() {
+            self.groups_idx += 1;
             match g {
                 GroupsIndicator::Idx(_) => todo!(), //kdn TODO
                 GroupsIndicator::Slice(s) => {