message: Use assignment instead of replace
  The line moves f2 into f1 using `replace` but that can be done with assignment
diff: //! # fn main() {
  //! use num_bigint::BigUint;
  //! use num_traits::{Zero, One};
  -//! use std::mem::replace;
  //!
  //! // Calculate large fibonacci numbers.
  //! fn fib(n: usize) -> BigUint {
  //! let mut f1: BigUint = One::one();
  //! for _ in 0..n {
  //! let f2 = f0 + &f1;
  -//! // This is a low cost way of swapping f0 with f1 and f1 with f2.
  -//! f0 = replace(&mut f1, f2);
  +//! f0 = f1;
  +//! f1 = f2;
  //! }
  //! f0
  //! }
  