Rust

To do it 100% probably isn't possible, something something halting problem. However, you'd catch a lot of basic mistakes with proper typing. In your example, the first function should be typed like this: fn third_planet_from_sun() -> Planet, where Planet is an enum. De/serializing it still has the same problem of interpreting an arbitrary string, but at least for deserializing it, you can be loose in what you allow and just lowercase it before matching it to the enum.