Maybe a more specific way to put it is: you only pay for the (combinations of) types you actually use, whether that's in argument position, return position, or

Maybe a more specific way to put it is: you only pay for the (combinations of) types you actually use, whether that's in argument position, return position, or even a local binding. So if it's always Vec<T> it's not costing much more in compile time or code size, but if it's sometimes another type then you do now pay for both.