A cross-crate opt-out trait was implemented on something which wasn't a struct or enum type. Erroneous code example:
#![feature(optin_builtin_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.