Function union_child_rank 

fn union_child_rank(value: &Variant<'_, '_>, data_type: &DataType) -> Option<u8>

Ranks how exactly a union child of type data_type can represent a variant value’s runtime type: 0 is the value’s natural Arrow type, higher ranks are lossless widenings, and None means the child cannot represent the value losslessly. Every pair admitted here must be convertible by the corresponding row builder.