Add requested comment

src/liballoc/vec.rs

@@ -2694,6 +2694,9 @@ impl<'a, T, F> Iterator for DrainFilter<'a, T, F>
                     let del = self.del;
                     let src: *const T = &v[i];
                     let dst: *mut T = &mut v[i - del];
+                    // This is safe because self.vec has length 0
+                    // thus its elements will not have Drop::drop
+                    // called on them in the event of a panic.
                     ptr::copy_nonoverlapping(src, dst, 1);
                 }
             }