You can protect both devices from each other with a USB condom [1] which only connects the power pins. This should be the solution for trying to charge from untrusted slots, or for when an untrusted device wants to charge from you.
Know of any USB condoms that can filter for device types? Given BAD USB type of exploits there really no easy way for me to know that when I stuck my USB stick in the printer at the library it wasn't reprogrammed to be a keyboard or something else and when I then go plug it into my computer it now powns my computer
You mean, before we started using USB for charging...?
It wouldn't be hard at all to make a convincing looking power adapter with something like PoisonTap baked in.