For a quick example, Bevy needs to implement some trait for functions with a variable number of inputs. However it doesn't stop there, it also needs to store so

For a quick example, Bevy needs to implement some trait for functions with a variable number of inputs. However it doesn't stop there, it also needs to store some state for each input, in other words it need to map that `fn(...T)` into a `(...State<T>)`. How do you model this with generics? When I tried to think about this every way I found to express this didn't allow the consumer code to call any methods on those `State`s, essentially making this useless.