error: can only derive `Countable` for `enum`s with `repr(u8)`
 --> tests/countable/enum_zero_variants.rs:4:1
  |
4 | enum Tag {}
  | ^^^^^^^^^^^

error: cannot derive `Countable` for `enum`s with zero variants
 --> tests/countable/enum_zero_variants.rs:4:1
  |
4 | enum Tag {}
  | ^^^^^^^^^^^