add panic and comment

actix-http/src/message.rs

@@ -343,9 +343,17 @@ impl ResponseHead {
 }
 
 pub struct Message<T: Head> {
+    // Rc here should not be cloned by anyone.
+    // It's used to reuse allocation of T and no shared ownership is allowed.
     head: Rc<T>,
 }
 
+impl<T: Head> Clone for Message<T> {
+    fn clone(&self) -> Self {
+        panic!("Message<T> type should not be Clone.")
+    }
+}
+
 impl<T: Head> Message<T> {
     /// Get new message from the pool of objects
     pub fn new() -> Self {