dead_code lint considers use in derive impls

[jethrogb]

This code:

struct S {
    f: i32
}

impl S {
    fn u(self) {}
}

fn main() {
    let a = S { f: 3 };
    a.u();
}

results in this lint:

warning: field is never used: `f`
 --> test.rs:2:5
  |
2 |     f: i32
  |     ^^^^^^
  |
  = note: #[warn(dead_code)] on by default

But this code:

#[derive(Debug)] // added line compared to above
struct S {
    f: i32
}

impl S {
    fn u(self) {}
}

fn main() {
    let a = S { f: 3 };
    a.u();
}

doesn't.

[nagisa]

Related #47851

[Mark-Simulacrum]

I feel that this isn't just related but a duplicate of #47851, so closing in favor of it.