&Counter implementing Add<i32> and returning Result<i32, String>

counter.rs

impl Add<i32> for &Counter {
  type Output = Result<i32, String>;

  fn add(self, rhs: i32) -> Self::Output {
    let sum = self.0 + rhs; 
    if sum > MAX_COUNTER {
      Err("Counter overflow".to_string())
    } else {
       Ok(i32::try_from(sum).unwrap())
    }
  }
}