windows: Mark notify_completion() as unsafe

The soundness of this method relies on a valid object being passed in.

Author: antrik

src/platform/windows/mod.rs

@@ -449,7 +449,11 @@ impl MessageReader {
     }
 
     /// Called when we receive an IO Completion Packet for this handle.
-    fn notify_completion(&mut self, err: u32) -> Result<(),WinError> {
+    ///
+    /// Unsafe, since as far as I can tell, the soundness of this method
+    /// relies on the validity of the `self` object (specifically, `self.ov`)
+    /// passed in by the caller.
+    unsafe fn notify_completion(&mut self, err: u32) -> Result<(),WinError> {
         win32_trace!("[$ {:?}] notify_completion", self.handle);
 
         // mark a read as no longer in progress even before we check errors