I think we need a separate enum for variant kinds and struct kinds, as unit variants may have a discriminant value, which currently we don't document but should. See https://github.com/Enselic/cargo-public-api/issues/121 and [zulip discussion](https://rust-lang.zulipchat.com/#narrow/stream/266220-rustdoc/topic/Enum.20discriminant.20values.20in.20HTML.20output)