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:

#![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;
}