derive(Clone, Debug) should not prevent "field is never read" from firing

```rust
struct A { f: () }
#[derive(Clone)]
struct B { f: () }

fn main() {
    let _ = A { f: () };
    let _ = B { f: () };
}
```

https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=ddbb661ad3a117642c4f750b31da4b18

Only `A`'s field triggers the warning. It's clear why `B` doesn't do this, but it clearly should. Doesn't seem easy to fix though :(