Support Option and similar enums as type of static variable with linkage attribute.

Compiler MCP:
https://github.com/rust-lang/compiler-team/issues/565

compiler/rustc_error_codes/src/error_codes.rs

@@ -494,6 +494,7 @@ E0786: include_str!("./error_codes/E0786.md"),
 E0787: include_str!("./error_codes/E0787.md"),
 E0788: include_str!("./error_codes/E0788.md"),
 E0790: include_str!("./error_codes/E0790.md"),
+E0791: include_str!("./error_codes/E0791.md"),
 ;
 //  E0006, // merged with E0005
 //  E0008, // cannot bind by-move into a pattern guard

compiler/rustc_error_codes/src/error_codes/E0791.md

@@ -0,0 +1,41 @@
+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;
+}
+```