error: unsupported base type for `bitfields!`: only primitive unsigned types are supported
 --> $DIR/invalid_base_ty.rs:4:29
  |
4 |     struct NegativeBaseTy : isize {}
  |                             ^^^^^

error: unsupported base type for `bitfields!`: only primitive unsigned types are supported
 --> $DIR/invalid_base_ty.rs:8:26
  |
8 |     struct OtherBaseTy : Option<String> {}
  |                          ^^^^^^^^^^^^^^