Pre-feature request: suppress unused variable warning for unit-type arguments

zackw:

You might have to pass it down to other functions for API contract reasons

This is exactly why I wouldn't want to suppress it: a unit type may still exist for the purposes of passing to some other function you call as a token (e.g. to prove you have a capability), and if you're not using it, it's very easy to write _: MyUnitType.