small test for extern_type

Author: RalfJung

tests/run-pass/extern_types.rs

@@ -0,0 +1,10 @@
+#![feature(extern_types)]
+
+extern {
+    type Foo;
+}
+
+fn main() {
+    let x: &Foo = unsafe { &*(16 as *const Foo) };
+    let y: &Foo = &*x;
+}