diff --git a/src/libcore/slice/mod.rs b/src/libcore/slice/mod.rs
index c9c73f4d66ee9..d93e5a9ca2b63 100644
--- a/src/libcore/slice/mod.rs
+++ b/src/libcore/slice/mod.rs
@@ -2146,14 +2146,13 @@ impl<T> [T] {
     /// Copying four bytes within a slice:
     ///
     /// ```
-    /// # #![feature(copy_within)]
     /// let mut bytes = *b"Hello, World!";
     ///
     /// bytes.copy_within(1..5, 8);
     ///
     /// assert_eq!(&bytes, b"Hello, Wello!");
     /// ```
-    #[unstable(feature = "copy_within", issue = "54236")]
+    #[stable(feature = "copy_within", since = "1.37.0")]
     pub fn copy_within<R: ops::RangeBounds<usize>>(&mut self, src: R, dest: usize)
     where
         T: Copy,
@@ -2178,8 +2177,8 @@ impl<T> [T] {
         assert!(dest <= self.len() - count, "dest is out of bounds");
         unsafe {
             ptr::copy(
-                self.get_unchecked(src_start),
-                self.get_unchecked_mut(dest),
+                self.as_ptr().add(src_start),
+                self.as_mut_ptr().add(dest),
                 count,
             );
         }
diff --git a/src/libcore/tests/lib.rs b/src/libcore/tests/lib.rs
index 928bdd7a76002..bf072a9243b51 100644
--- a/src/libcore/tests/lib.rs
+++ b/src/libcore/tests/lib.rs
@@ -28,7 +28,6 @@
 #![feature(inner_deref)]
 #![feature(slice_internals)]
 #![feature(slice_partition_dedup)]
-#![feature(copy_within)]
 #![feature(int_error_matching)]
 #![feature(const_fn)]
 #![warn(rust_2018_idioms)]
diff --git a/src/libcore/tests/slice.rs b/src/libcore/tests/slice.rs
index 9710f019f4e4d..eaa799fa96ee2 100644
--- a/src/libcore/tests/slice.rs
+++ b/src/libcore/tests/slice.rs
@@ -1512,6 +1512,13 @@ fn test_copy_within() {
     let mut bytes = *b"Hello, World!";
     bytes.copy_within(.., 0);
     assert_eq!(&bytes, b"Hello, World!");
+
+    // Ensure that copying at the end of slice won't cause UB.
+    let mut bytes = *b"Hello, World!";
+    bytes.copy_within(13..13, 5);
+    assert_eq!(&bytes, b"Hello, World!");
+    bytes.copy_within(5..5, 13);
+    assert_eq!(&bytes, b"Hello, World!");
 }
 
 #[test]
@@ -1536,6 +1543,13 @@ fn test_copy_within_panics_src_inverted() {
     // 2 is greater than 1, so this range is invalid.
     bytes.copy_within(2..1, 0);
 }
+#[test]
+#[should_panic(expected = "attempted to index slice up to maximum usize")]
+fn test_copy_within_panics_src_out_of_bounds() {
+    let mut bytes = *b"Hello, World!";
+    // an inclusive range ending at usize::max_value() would make src_end overflow
+    bytes.copy_within(usize::max_value()..=usize::max_value(), 0);
+}
 
 #[test]
 fn test_is_sorted() {