message: Remove `#[must_use]` on `__add2`
  It doesn't actually work on functions yet, and nightly now warns that it
  is experimental, behind `#[feature(fn_must_use)]`.
diff: @@ -123,7 +123,6 @@ pub fn div_rem_digit(mut a: BigUint, b: BigDigit) -> (BigUint, BigDigit) {
  }
  // Only for the Add impl:
  -#[must_use]
  #[inline]
  pub fn __add2(a: &mut [BigDigit], b: &[BigDigit]) -> BigDigit {
  debug_assert!(a.len() >= b.len());
  