Error code E0321
A cross-crate opt-out trait was implemented on something which wasn't a struct or enum type.
Erroneous code example:
ⓘ
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.