Should cargo fix --prepare-for 2018 fix unrelated warnings?

[Manishearth]

If you pass this crate through cargo fix --prepare-for 2018

fn main() {
    let x = 1;
}

it fixes to:

fn main() {
    let _x = 1;
}

Should we be fixing non-edition warnings at all? These are marked applicable so we should be able to trust them, but perhaps we should exclude them for the sake of doing the minimum necessary?

cc @killercup @alexcrichton

[alexcrichton]

I believe this is covered by #5738