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