[rustc.git] / src / librustc_error_codes / error_codes / E0321.md

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

Erroneous code example:

```compile_fail,E0321
#![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.
Commit	Line	Data
60c5eb7d	1	A cross-crate opt-out trait was implemented on something which wasn't a struct
74b04a01 XL	2	or enum type.
	3
	4	Erroneous code example:
60c5eb7d XL	5
	6	```compile_fail,E0321
	7	#![feature(optin_builtin_traits)]
	8
	9	struct Foo;
	10
	11	impl !Sync for Foo {}
	12
	13	unsafe impl Send for &'static Foo {}
	14	// error: cross-crate traits with a default impl, like `core::marker::Send`,
	15	// can only be implemented for a struct/enum type, not
	16	// `&'static Foo`
	17	```
	18
	19	Only structs and enums are permitted to impl Send, Sync, and other opt-out
	20	trait, and the struct or enum must be local to the current crate. So, for
	21	example, `unsafe impl Send for Rc<Foo>` is not allowed.