error: `Countable` can only be derived for `struct`s with zero fields
 --> tests/countable/struct_fields.rs:4:12
  |
4 | struct Tag1(u8);
  |            ^^^^

error: `Countable` can only be derived for `struct`s with zero fields
 --> tests/countable/struct_fields.rs:7:13
  |
7 |   struct Tag2 {
  |  _____________^
8 | |     field: u8,
9 | | }
  | |_^