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# Error code E0321

A cross-crate opt-out trait was implemented on something which wasn’t a struct or enum type.

Erroneous code example:

    #![allow(unused)]
    #![feature(auto_traits)]

    fn main() {
    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.