Give a "modifying moved binding" or "unread value" warning in move-closures

[critiqjo]

Take a look at this logically wrong example:

fn main() {
    let mut x = 1;
    let y = (1..10).fold(0, move|acc, e| {
        x *= e; // should warn: new value of `x` never read
        acc + e
    });
    println!("{} {}", x, y); // outputs: `1 45`
}

It would be best if the compiler points out that "updating x inside the closure will not have an external effect." Though, I would be happy if the compiler at least pointed out that "the modified value in the new binding x is never read."

[KalitaAlexey]

I like it. Having such a lint will remove another kind of bugs.

[steveklabnik]

Triage: this code still does not warn