There should be a way to generate code that works in 2015, and works in 2018, where the 2018 version produces warnings. So if you generate the code with #![allow(warnings)], that should work.
There should be a way to generate code that works in 2015, and works in 2018, where the 2018 version produces warnings. So if you generate the code with #![allow(warnings)], that should work.