diff --git a/src/liballoc/boxed.rs b/src/liballoc/boxed.rs
index 89de2c953e4c2..7965c00a36510 100644
--- a/src/liballoc/boxed.rs
+++ b/src/liballoc/boxed.rs
@@ -194,6 +194,10 @@ impl<'a, T> Iterator for Box<Iterator<Item=T> + 'a> {
     fn next(&mut self) -> Option<T> {
         (**self).next()
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (**self).size_hint()
+    }
 }
 
 #[cfg(test)]
diff --git a/src/libcore/iter.rs b/src/libcore/iter.rs
index 3d51f75d9b309..2a21ceef7a152 100644
--- a/src/libcore/iter.rs
+++ b/src/libcore/iter.rs
@@ -107,6 +107,10 @@ impl<'a, T> Iterator for &'a mut (Iterator<Item=T> + 'a) {
     fn next(&mut self) -> Option<T> {
         (**self).next()
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (**self).size_hint()
+    }
 }
 
 /// Conversion from an `Iterator`