Auto merge of rust-lang#44355 - Xaeroxe:optimize_drain_filter, r=alex…

…crichton

Optimize drain_filter

This PR cuts out two copies from each iteration of `drain_filter` by exchanging the swap operation for a copy_nonoverlapping function call instead.  Since the data being swapped is not needed anymore we can just overwrite it instead.

src/liballoc/vec.rs

@@ -2694,7 +2694,13 @@ impl<'a, T, F> Iterator for DrainFilter<'a, T, F>
                     self.del += 1;
                     return Some(ptr::read(&v[i]));
                 } else if self.del > 0 {
-                    v.swap(i - self.del, i);
+                    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);
                 }
             }
             None