Fix sending nil messages to super on GNUStep, and clean up documentation

objc2/src/message/gnustep.rs

@@ -13,7 +13,13 @@ where
     A: MessageArguments,
     R: Encode,
 {
+    // If `receiver` is NULL, objc_msg_lookup will return a standard C-method
+    // taking two arguments, the receiver and the selector. Transmuting and
+    // calling such a function with multiple parameters is UB, so instead we
+    // just return NULL directly.
     if receiver.is_null() {
+        // SAFETY: Caller guarantees that messages to NULL-receivers only
+        // return pointers, and a mem::zeroed pointer is just a NULL-pointer.
         return unsafe { mem::zeroed() };
     }
 
@@ -33,6 +39,11 @@ where
     A: MessageArguments,
     R: Encode,
 {
+    if receiver.is_null() {
+        // SAFETY: Same as in `send_unverified`.
+        return unsafe { mem::zeroed() };
+    }
+
     let sup = ffi::objc_super {
         receiver: receiver as *mut _,
         super_class: superclass as *const Class as *const _,

objc2/src/message/mod.rs

@@ -382,14 +382,26 @@ mod tests {
     #[test]
     fn test_send_message_nil() {
         let nil: *mut Object = ::core::ptr::null_mut();
-        let result: usize = unsafe { msg_send![nil, hash] };
-        assert!(result == 0);
 
         let result: *mut Object = unsafe { msg_send![nil, description] };
         assert!(result.is_null());
 
+        // This result should not be relied on
+        let result: usize = unsafe { msg_send![nil, hash] };
+        assert_eq!(result, 0);
+
+        // This result should not be relied on
+        #[cfg(target_pointer_width = "16")]
+        let result: f32 = 0.0;
+        #[cfg(target_pointer_width = "32")]
+        let result: f32 = unsafe { msg_send![nil, floatValue] };
+        #[cfg(target_pointer_width = "64")]
         let result: f64 = unsafe { msg_send![nil, doubleValue] };
-        assert!(result == 0.0);
+        assert_eq!(result, 0.0);
+
+        // This result should not be relied on
+        let result: *mut Object = unsafe { msg_send![nil, multiple: 1u32, arguments: 2i8] };
+        assert!(result.is_null());
     }
 
     #[test]