Test ManuallyDrop::clone_from.

library/core/tests/manually_drop.rs

@@ -2,6 +2,7 @@ use core::mem::ManuallyDrop;
 
 #[test]
 fn smoke() {
+    #[derive(Clone)]
     struct TypeWithDrop;
     impl Drop for TypeWithDrop {
         fn drop(&mut self) {
@@ -16,4 +17,11 @@ fn smoke() {
     let x: Box<ManuallyDrop<[TypeWithDrop]>> =
         Box::new(ManuallyDrop::new([TypeWithDrop, TypeWithDrop]));
     drop(x);
+
+    // test clone and clone_from implementations
+    let mut x = ManuallyDrop::new(TypeWithDrop);
+    let y = x.clone();
+    x.clone_from(&y);
+    drop(x);
+    drop(y);
 }