Function build_halfjoin_op 

fn build_halfjoin_op<'scope, T, Tr>(
    updates: VecCollection<'scope, T, (Row, Row, T), Diff>,
    trace: Arranged<'scope, Tr>,
    strict: bool,
    closure: JoinClosure,
    datums: DatumVec,
    errs: VecCollection<'scope, T, DataflowErrorSer, Diff>,
) -> (VecCollection<'scope, T, (Row, T), Diff>, VecCollection<'scope, T, DataflowErrorSer, Diff>)
where
    T: RenderTimestamp,
    Tr: TraceReader<Batch: Navigable, Time = T> + Clone + 'static,
    for<'a> BatchCursor<Tr>: Cursor<Val<'a>: ExtendDatums, KeyContainer: BatchContainer<Owned = Row>, Time = T, Diff = Diff>,

Renders the half join of updates against trace, splitting errors from the closure off.