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