In the code, the result type of FutureProducer::send is declared as OwnedDeliveryResult, which has documentation about the meaning of the members of the (i32, i64) tuple:

https://github.com/fede1024/rust-rdkafka/blob/770ac7419f9c2d0b8607a5614efc978dd1c6182b/src/producer/future_producer.rs#L128-L135

However, this type is not showing up in the generated documentation, so it's difficult to figure out how to interpret the result:

Looks great, thank you!