message: test BTreeMap::drain_filter for leaks
diff: +#![feature(btree_drain_filter)]
  use std::collections::{BTreeMap, BTreeSet};
  +use std::mem;
  #[derive(PartialEq, Eq, PartialOrd, Ord)]
  pub enum Foo {
  @@ -43,4 +45,9 @@ pub fn main() {
  b.insert(format!("key{}", i), i);
  }
  test_all_refs(&mut 13, b.values_mut());
  +
  + // Test forgetting the drain.
  + let mut d = b.drain_filter(|_, i| *i < 30);
  + d.next().unwrap();
  + mem::forget(d);
  }
  