Function apply_join_closure 

fn apply_join_closure<K, V1, V2>(
    closure: &JoinClosure,
    datums: &mut DatumVec,
    key: K,
    old: V1,
    new: V2,
) -> Result<Option<Row>, EvalError>
where
    K: ExtendDatums,
    V1: ExtendDatums,
    V2: ExtendDatums,

Unpacks one join match into datums and applies closure to it.

None means the closure filtered the match out. The output row is owned because the row closure writes borrows the shared row builder, which the caller must not hold past this call.