Warn about clone impls for Copy types

[oli-obk]

The following is legal and totally weird/useless

#[derive(Copy)]
struct A;

impl Clone for A {
    fn clone(&self) -> A {
        panic!("bla");
    }
}

fn main() {
    let a = A;
    let b = a;
    let c = a.clone();
}

[llogiq]

It depends. Copy is usually shallow, while clone may copy data behind pointers – it's a deep copy.

To have both may be useful in some situations. Implementing clone by accident isn't terribly common, so I doubt the value proposition of such a lint.

[oli-obk]

It's rather rare, I agree.