message: Give that panic! a message. Also, make it an unreachable!.
diff: @@ -182,7 +182,7 @@ impl BitAnd for ExtraInfo {
  (false, false) => info,
  (true, false) => info | ExtraInfo::pending_f32_nan(),
  (false, true) => info | ExtraInfo::pending_f64_nan(),
  - (true, true) => panic!(""),
  + (true, true) => unreachable!("Can't form ExtraInfo with two pending canonicalizations"),
  };
  info
  }
  