Types are not mentioned in the enforcer of Iterators in `for` loop

``` rust
fn main() {
    for _c in 1i {}
}
```

```
<anon>:2:15: 2:17 error: `for` loop expression does not implement the `Iterator` trait
<anon>:2     for _c in 1i {}
                       ^~
error: aborting due to previous error
```

It would be nice if this mentioned the type (e.g. "`for` loop expression has type `int`, which does not implement the `Iterator` trait").
