Rollup merge of rust-lang#48065 - Xaeroxe:patch-1, r=alexcrichton

Apply optimization from rust-lang#44355 to retain

As discussed in rust-lang#44355 this PR applies a similar optimization to `Vec::retain`.  For `drain_filter`, a very similar function, this improved performance by up to 20%.

src/liballoc/vec.rs

@@ -805,22 +805,7 @@ impl<T> Vec<T> {
     pub fn retain<F>(&mut self, mut f: F)
         where F: FnMut(&T) -> bool
     {
-        let len = self.len();
-        let mut del = 0;
-        {
-            let v = &mut **self;
-
-            for i in 0..len {
-                if !f(&v[i]) {
-                    del += 1;
-                } else if del > 0 {
-                    v.swap(i - del, i);
-                }
-            }
-        }
-        if del > 0 {
-            self.truncate(len - del);
-        }
+        self.drain_filter(|x| !f(x));
     }
 
     /// Removes all but the first of consecutive elements in the vector that resolve to the same