Better FFI discipline

Author: tamird

src/libstd/rand/os.rs

@@ -205,9 +205,7 @@ mod imp {
         _dummy: (),
     }
 
-    // Fake definition; this is actually a struct, but we don't use the
-    // contents here.
-    type SecRandom = c_void;
+    enum SecRandom {}
 
     #[allow(non_upper_case_globals)]
     const kSecRandomDefault: *const SecRandom = 0 as *const SecRandom;

src/libstd/rt/unwind/seh64_gnu.rs

@@ -60,8 +60,8 @@ pub struct EXCEPTION_RECORD {
     ExceptionInformation: [ULONG_PTR; 15],
 }
 
-pub type CONTEXT = c_void;
-pub type UNWIND_HISTORY_TABLE = c_void;
+pub enum CONTEXT {}
+pub enum UNWIND_HISTORY_TABLE {}
 
 #[repr(C)]
 pub struct RUNTIME_FUNCTION {