Add #[must_use] message to Iterator and Future

Author: taiki-e

src/libcore/future/future.rs

@@ -23,7 +23,7 @@ use task::{Poll, LocalWaker};
 ///
 /// When using a future, you generally won't call `poll` directly, but instead
 /// `await!` the value.
-#[must_use]
+#[must_use = "futures do nothing unless polled"]
 pub trait Future {
     /// The result of the `Future`.
     type Output;

src/libcore/iter/iterator.rs

@@ -88,7 +88,7 @@ fn _assert_is_object_safe(_: &dyn Iterator<Item=()>) {}
     message="`{Self}` is not an iterator"
 )]
 #[doc(spotlight)]
-#[must_use]
+#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
 pub trait Iterator {
     /// The type of the elements being iterated over.
     #[stable(feature = "rust1", since = "1.0.0")]