`impl Trait` is only allowed as a function return and argument type.

Erroneous code example:

```compile_fail,E0562
fn main() {
    let count_to_ten: impl Iterator<Item=usize> = 0..10;
    // error: `impl Trait` not allowed outside of function and inherent method
    //        return types
    for i in count_to_ten {
        println!("{}", i);
    }
}
```

Make sure `impl Trait` appears in a function signature.

```
fn count_to_n(n: usize) -> impl Iterator<Item=usize> {
    0..n
}

fn main() {
    for i in count_to_n(10) {  // ok!
        println!("{}", i);
    }
}
```

See the [reference] for more details on `impl Trait`.

[reference]: https://doc.rust-lang.org/stable/reference/types/impl-trait.html