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Abstract return types (written `impl Trait` for some trait `Trait`) are only
allowed as function and inherent impl return types.
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` only appears in return-type position.
```
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 [RFC 1522] for more details.
[RFC 1522]: https://github.com/rust-lang/rfcs/blob/master/text/1522-conservative-impl-trait.md
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