message: mark serialization as unimplemented for now
diff: @@ -2614,6 +2614,7 @@ fn u32_from_u128(n: u128) -> (u32, u32, u32, u32) {
  }
  #[cfg(feature = "serde")]
  +#[cfg(not(feature = "u64_digit"))]
  impl serde::Serialize for BigUint {
  fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
  where
  @@ -2628,6 +2629,18 @@ impl serde::Serialize for BigUint {
  }
  #[cfg(feature = "serde")]
  +#[cfg(feature = "u64_digit")]
  +impl serde::Serialize for BigUint {
  + fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
  + where
  + S: serde::Serializer,
  + {
  + unimplemented!()
  + }
  +}
  +
  +#[cfg(feature = "serde")]
  +#[cfg(not(feature = "u64_digit"))]
  impl<'de> serde::Deserialize<'de> for BigUint {
  fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
  where
  @@ -2638,6 +2651,17 @@ impl<'de> serde::Deserialize<'de> for BigUint {
  }
  }
  +#[cfg(feature = "serde")]
  +#[cfg(feature = "u64_digit")]
  +impl<'de> serde::Deserialize<'de> for BigUint {
  + fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
  + where
  + D: serde::Deserializer<'de>,
  + {
  + unimplemented!()
  + }
  +}
  +
  /// Returns the greatest power of the radix <= big_digit::BASE
  #[inline]
  fn get_radix_base(radix: u32) -> (BigDigit, usize) {
  