| A cross-crate opt-out trait was implemented on something which wasn't a struct | |
| or enum type. | |
| Erroneous code example: | |
| ```compile_fail,E0321 | |
| #![feature(auto_traits)] | |
| struct Foo; | |
| impl !Sync for Foo {} | |
| unsafe impl Send for &'static Foo {} | |
| // error: cross-crate traits with a default impl, like `core::marker::Send`, | |
| // can only be implemented for a struct/enum type, not | |
| // `&'static Foo` | |
| ``` | |
| Only structs and enums are permitted to impl Send, Sync, and other opt-out | |
| trait, and the struct or enum must be local to the current crate. So, for | |
| example, `unsafe impl Send for Rc<Foo>` is not allowed. |