| 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 |
