Implement Clone for Box<[T]> where T: Clone

Closes #25097

src/libcollectionstest/slice.rs

@@ -1270,6 +1270,59 @@ fn test_to_vec() {
     assert_eq!(ys, [1, 2, 3]);
 }
 
+#[test]
+fn test_box_slice_clone() {
+    let data = vec![vec![0, 1], vec![0], vec![1]];
+    let data2 = data.clone().into_boxed_slice().clone().to_vec();
+
+    assert_eq!(data, data2);
+}
+
+#[test]
+fn test_box_slice_clone_panics() {
+    use std::sync::Arc;
+    use std::sync::atomic::{AtomicUsize, Ordering};
+    use std::thread::spawn;
+
+    struct Canary {
+        count: Arc<AtomicUsize>,
+        panics: bool
+    }
+
+    impl Drop for Canary {
+        fn drop(&mut self) {
+            self.count.fetch_add(1, Ordering::SeqCst);
+        }
+    }
+
+    impl Clone for Canary {
+        fn clone(&self) -> Self {
+            if self.panics { panic!() }
+
+            Canary {
+                count: self.count.clone(),
+                panics: self.panics
+            }
+        }
+    }
+
+    let drop_count = Arc::new(AtomicUsize::new(0));
+    let canary = Canary { count: drop_count.clone(), panics: false };
+    let panic = Canary { count: drop_count.clone(), panics: true };
+
+    spawn(move || {
+        // When xs is dropped, +5.
+        let xs = vec![canary.clone(), canary.clone(), canary.clone(),
+                      panic, canary].into_boxed_slice();
+
+        // When panic is cloned, +3.
+        xs.clone();
+    }).join().unwrap_err();
+
+    // Total = 8
+    assert_eq!(drop_count.load(Ordering::SeqCst), 8);
+}
+
 mod bench {
     use std::iter::repeat;
     use std::{mem, ptr};