Mark into_iter as #[must_use]

`into_iter` serves no function if not iterated over.

If it's being used just to consume `self`, that would be better served
by `drop`.

Author: joshtriplett

library/core/src/iter/traits/collect.rs

@@ -306,6 +306,7 @@ pub trait IntoIterator {
     /// assert_eq!(None, iter.next());
     /// ```
     #[lang = "into_iter"]
+    #[must_use = "Iterators do nothing unless iterated over"]
     #[stable(feature = "rust1", since = "1.0.0")]
     fn into_iter(self) -> Self::IntoIter;
 }

library/coretests/tests/array.rs

@@ -193,7 +193,7 @@ fn iterator_drops() {
     // Simple: drop new iterator.
     let i = Cell::new(0);
     {
-        IntoIterator::into_iter(five(&i));
+        drop(IntoIterator::into_iter(five(&i)));
     }
     assert_eq!(i.get(), 5);