Correct the use of must_use on btree::IterMut

Signed-off-by: Jonathan Brouwer <jonathantbrouwer@gmail.com>

library/alloc/src/collections/btree/map.rs

@@ -382,6 +382,7 @@ fn default() -> Self {
 /// documentation for more.
 ///
 /// [`iter_mut`]: BTreeMap::iter_mut
+#[must_use = "iterators are lazy and do nothing unless consumed"]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct IterMut<'a, K: 'a, V: 'a> {
     range: LazyLeafRange<marker::ValMut<'a>, K, V>,
@@ -391,7 +392,6 @@ pub struct IterMut<'a, K: 'a, V: 'a> {
     _marker: PhantomData<&'a mut (K, V)>,
 }
 
-#[must_use = "iterators are lazy and do nothing unless consumed"]
 #[stable(feature = "collection_debug", since = "1.17.0")]
 impl<K: fmt::Debug, V: fmt::Debug> fmt::Debug for IterMut<'_, K, V> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {