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