Function arrange_join_collection 

fn arrange_join_collection<'s, T>(
    collection: VecCollection<'s, T, Row, Diff>,
    stream_key: Vec<LirScalarExpr>,
    stream_thinning: Vec<usize>,
    batcher: ArrangementBatcher,
) -> (Arranged<'s, TraceAgent<RowRowSpine<T, Diff>>>, VecCollection<'s, T, DataflowErrorSer, Diff>)
where
    T: Lattice + RenderTimestamp,

Forms the arrangement for the intra-operator Vec accumulator of a linear join. Unlike arrange_join_input, the accumulator is a bare VecCollection rather than a collection edge: mz_join_core is Vec-internal, so the accumulator never carries the collection edge type.