Properly implement Clone for SharedObserver

When using an auto derive to implement a trait such as Clone on a
generic object, the code emitted will contain a constraint like: T:
Clone.
That is a problem for our SharedObserver, because our inner Observer
does not implement Clone, and that is fine and desired. In order to work
around this problem (known upstream as [issue 64417][]) we need to
implement Clone for SharedObserver manually.

[issue 64417]: rust-lang/rust#64417

rust/template/distributed_datalog/observe/src/observer.rs

@@ -31,7 +31,7 @@ where
 
 /// Wrapper around an `Observer` that allows for it to be shared by
 /// wrapping it into a combination of `Arc` & `Mutex`.
-#[derive(Debug, Clone)]
+#[derive(Debug)]
 pub struct SharedObserver<O>(pub Arc<Mutex<O>>);
 
 impl<O> SharedObserver<O> {
@@ -42,6 +42,12 @@ impl<O> SharedObserver<O> {
     }
 }
 
+impl<O> Clone for SharedObserver<O> {
+    fn clone(&self) -> Self {
+        SharedObserver(self.0.clone())
+    }
+}
+
 impl<O, T, E> Observer<T, E> for SharedObserver<O>
 where
     O: Observer<T, E>,