[rustc.git] / compiler / rustc_error_codes / src / error_codes / E0791.md

Static variables with the `#[linkage]` attribute within external blocks
must have one of the following types, which are equivalent to a nullable
pointer in C:

* `*mut T` or `*const T`, where `T` may be any type.

* An enumerator type with no `#[repr]` attribute and with two variants, where
  one of the variants has no fields, and the other has a single field of one of
  the following non-nullable types:
  * Reference type
  * Function pointer type

  The variants can appear in either order.

For example, the following declaration is invalid:

```compile_fail,E0791
#![feature(linkage)]

extern "C" {
    #[linkage = "extern_weak"]
    static foo: i8;
}
```

The following declarations are valid:

```
#![feature(linkage)]

extern "C" {
    #[linkage = "extern_weak"]
    static foo: Option<unsafe extern "C" fn()>;

    #[linkage = "extern_weak"]
    static bar: Option<&'static i8>;

    #[linkage = "extern_weak"]
    static baz: *mut i8;
}
```
Commit	Line	Data
487cf647 FG	1	Static variables with the `#[linkage]` attribute within external blocks
	2	must have one of the following types, which are equivalent to a nullable
	3	pointer in C:
	4
	5	* `mut T` or `const T`, where `T` may be any type.
	6
	7	* An enumerator type with no `#[repr]` attribute and with two variants, where
	8	one of the variants has no fields, and the other has a single field of one of
	9	the following non-nullable types:
	10	* Reference type
	11	* Function pointer type
	12
	13	The variants can appear in either order.
	14
	15	For example, the following declaration is invalid:
	16
	17	```compile_fail,E0791
	18	#![feature(linkage)]
	19
	20	extern "C" {
	21	#[linkage = "extern_weak"]
	22	static foo: i8;
	23	}
	24	```
	25
	26	The following declarations are valid:
	27
	28	```
	29	#![feature(linkage)]
	30
	31	extern "C" {
	32	#[linkage = "extern_weak"]
	33	static foo: Option<unsafe extern "C" fn()>;
	34
	35	#[linkage = "extern_weak"]
	36	static bar: Option<&'static i8>;
	37
	38	#[linkage = "extern_weak"]
	39	static baz: *mut i8;
	40	}
	41	```