Diagnostics should mention loop iteration

[aochagavia]

The code:

fn main() {
    let v: Vec<i32> = Vec::new();
    loop {
        drop(v);
    }
}

The error message:

<anon>:4:14: 4:15 error: use of moved value: `v`
<anon>:4         drop(v);
                      ^
<anon>:4:14: 4:15 note: `v` moved here because it has type `collections::vec::Vec<i32>`, which is non-copyable
<anon>:4         drop(v);
                      ^

In this simple example, it is pretty easy to notice that the error happens in the next iteration. However, usually the code is more complicated than this and the error is very confusing (see this reddit thread). Maybe we could add something like move happens in the next iteration of the loop.

[sinistersnare]

I dont know, I feel like the error message makes perfect sense. This is just a case of learning the language and not knowing all (or a good amount of) error cases, in my opinion.

If we had error messages for everything like this, then compiler output would be inundated with crap that no one reads anyways.

[kmcallister]

Seems worth looking into improving these messages.

[birkenfeld]

Duplicate of #10876.