error: #[serde(default = "...")] can only be used on structs
 --> tests/ui/default-attribute/union_path.rs:4:9
  |
4 | #[serde(default = "default_u")]
  |         ^^^^^^^^^^^^^^^^^^^^^

error: Serde does not support derive for unions
 --> tests/ui/default-attribute/union_path.rs:4:1
  |
4 | / #[serde(default = "default_u")]
5 | | union Union {
6 | |     f: u8,
7 | | }
  | |_^