it also doesn't require a rule for every instance of each enum to be defined at "compile" time, e.g. if the above didn't have a

Platform.darwin

override, it would still work as long as pants always ran on linux -- if run requesting

Platform.darwin

, it would fail at "runtime" but that just means saying e.g.:

Exception message: No installed @rules can compute CToolchain for input Params((NativeToolchain+Variant<Platform=darwin>+Variant<ToolchainVariant=gnu>)).

which is exceedingly readable imho. it all works out of the box because it's just converted into `UnionRule`s