error: unused implementer of `Iterator` that must be used
  --> $DIR/must_use-in-stdlib-traits.rs:42:4
   |
LL |    iterator();
   |    ^^^^^^^^^^^
   |
note: the lint level is defined here
  --> $DIR/must_use-in-stdlib-traits.rs:1:9
   |
LL | #![deny(unused_must_use)]
   |         ^^^^^^^^^^^^^^^
   = note: iterators are lazy and do nothing unless consumed

error: unused implementer of `Future` that must be used
  --> $DIR/must_use-in-stdlib-traits.rs:43:4
   |
LL |    future();
   |    ^^^^^^^^^
   |
   = note: futures do nothing unless you `.await` or poll them

error: unused implementer of `FnOnce` that must be used
  --> $DIR/must_use-in-stdlib-traits.rs:44:4
   |
LL |    square_fn_once();
   |    ^^^^^^^^^^^^^^^^^
   |
   = note: closures are lazy and do nothing unless called

error: unused implementer of `FnMut` that must be used
  --> $DIR/must_use-in-stdlib-traits.rs:45:4
   |
LL |    square_fn_mut();
   |    ^^^^^^^^^^^^^^^^
   |
   = note: closures are lazy and do nothing unless called

error: unused implementer of `Fn` that must be used
  --> $DIR/must_use-in-stdlib-traits.rs:46:4
   |
LL |    square_fn();
   |    ^^^^^^^^^^^^
   |
   = note: closures are lazy and do nothing unless called

error: aborting due to 5 previous errors