message: bigint: remove an unused mut
diff: @@ -1757,7 +1757,7 @@ fn twos_complement<'a, I>(digits: I)
  where I: IntoIterator<Item = &'a mut u8>
  {
  let mut carry = true;
  - for mut d in digits {
  + for d in digits {
  *d = d.not();
  if carry {
  *d = d.wrapping_add(1);
  