E0282 suggests changing desugared loop variable to `__next`

```
fn main() {
    for (ref i,) in [].iter() {
        i.clone();
    }
}
```
```
error[E0282]: type annotations needed
  --> $DIR/issue-20261.rs:14:11
   |
LL |     for (ref i,) in [].iter() {
   |         -------- consider giving `__next` a type
LL |         i.clone();
   |           ^^^^^ cannot infer type for `_`
   |
= note: type must be known at this point
```