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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;
}
```
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