Function tidb_query_expr::impl_cast::get_cast_fn_rpn_node

pub fn get_cast_fn_rpn_node(
    is_from_constant: bool, 
    from_field_type: &FieldType, 
    to_field_type: FieldType
) -> Result<RpnExpressionNode>

Gets the cast function between specified data types.

TODO: This function supports some internal casts performed by TiKV. However it would be better to be done in TiDB.