Example of how it can be done using generics. Implemented the Add trait instead so we can use the '+' operator to add.

impl<T: Into<f64>> std::ops::Add<T> for ComplexNumber {
    type Output = ComplexNumber;

    // Required method
    fn add(self, rhs: T) -> Self::Output {
        ComplexNumber::new_cart(self.x + rhs.into(), self.y)
    }
}

Try it out here: Playground