Rollup merge of #21976 - mzabaluev:fix-copy-mut-lifetime, r=alexcrichton

Part of #21923

src/libcore/mem.rs

@@ -331,8 +331,9 @@ pub unsafe fn copy_lifetime<'a, S: ?Sized, T: ?Sized + 'a>(_ptr: &'a S,
 #[unstable(feature = "core",
            reason = "this function may be removed in the future due to its \
                      questionable utility")]
-pub unsafe fn copy_mut_lifetime<'a, S: ?Sized, T: ?Sized + 'a>(_ptr: &'a mut S,
+pub unsafe fn copy_mut_lifetime<'a, S: ?Sized, T: ?Sized + 'a>(_ptr: &'a S,
                                                                ptr: &mut T)
-                                                            -> &'a mut T {
+                                                              -> &'a mut T
+{
     transmute(ptr)
 }