2 files changed, 2 insertions, 0 deletions
diff --git a/src/libcore/clone.rs b/src/libcore/clone.rs
index d25f498b99e..c175ae15d28 100644
--- a/src/libcore/clone.rs
+++ b/src/libcore/clone.rs
@@ -105,6 +105,7 @@ pub trait Clone : Sized {
     /// assert_eq!("Hello", hello.clone());
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[must_use = "cloning is often expensive and is not expected to have side effects"]
     fn clone(&self) -> Self;
 
     /// Performs copy-assignment from `source`.
diff --git a/src/libcore/iter/iterator.rs b/src/libcore/iter/iterator.rs
index f039d1730eb..75138f31a9d 100644
--- a/src/libcore/iter/iterator.rs
+++ b/src/libcore/iter/iterator.rs
@@ -1368,6 +1368,7 @@ pub trait Iterator {
     /// [`Result`]: ../../std/result/enum.Result.html
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[must_use = "if you really need to exhaust the iterator, consider `.for_each(drop)` instead"]
     fn collect<B: FromIterator<Self::Item>>(self) -> B where Self: Sized {
         FromIterator::from_iter(self)
     }