error: #[serde(transparent)] requires at least one field that is neither skipped nor has a default
  --> tests/ui/transparent/de_at_least_one.rs:4:1
   |
4  | / #[serde(transparent)]
5  | | struct S {
6  | |     #[serde(skip)]
7  | |     a: u8,
8  | |     #[serde(default)]
9  | |     b: u8,
10 | | }
   | |_^