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# Error code E0791

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:

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

    fn main() {
    extern "C" {
        #[linkage = "extern_weak"]
        static foo: i8;
    }
    }

The following declarations are valid:

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

    fn main() {
    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;
    }
    }