Adjacently tagged empty tuple variant or struct variant generates broken Serialize

[dtolnay]

#[derive(Serialize)]
#[serde(tag = "t", content = "c")]
enum E {
    A {}, // same for `A()`
}

error[E0392]: parameter `'__a` is never used
  --> src/main.rs:68:10
   |
68 | #[derive(Serialize)]
   |          ^^^^^^^^^ unused type parameter
   |
   = help: consider removing `'__a` or using a marker such as `std::marker::PhantomData`