error: Unions can not #[derive(CustomResource)]
 --> tests/ui/union_fails.rs:6:7
  |
6 | union FooSpec {
  |       ^^^^^^^

error: Serde does not support derive for unions
 --> tests/ui/union_fails.rs:6:1
  |
6 | / union FooSpec {
7 | |     int: u32,
8 | | }
  | |_^