The result type in rust does not return a true/false but a type. More importantly though, it doesn't return err if both values are set but simply returns the first value:

So... It's not only not mapping your input to truth values, it also behaves more like I'd expect an "or" to behave, which is not "xor" or, if there's more than two inputs, "exactly one", but succeeding if any input is set.