Note that `impl Foo` return types don't actually cost anything extra with regards to code-size, the compiler knows what the actual type is and there is no dynamic dispatch. Only actual generics have an impact here, and `impl` in a return position doesn't count.
The code size cost doesn't live in my code, but in yours.
Because I didn't admit you were getting a Vec, if you actually need a Vec you actually can't just use the one I gave you. You must jump though hoops to turn whatever I gave you into a Vec, bloating your code.
The implementation is pretty clever, it is probably not going to meticulously take my Vec to pieces, throw it away and make you a new one, instead just giving the same Vec. But this trick is fragile, so much better not to even need it.
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.
