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`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
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