Function columnar_to_vec 

pub fn columnar_to_vec<'scope, T>(
    collection: Collection<'scope, T, Column<(Row, T, Diff)>>,
) -> VecCollection<'scope, T, Row, Diff>
where
    T: RenderTimestamp,

Decodes columnar batches into a row-based collection.

The leaf decode described in the module docs, named ColumnarToVec in a rendered dataflow. It allocates an owned Row per record, which is why it stays at those boundaries.