diff --git a/futures-util/src/stream/mapped_futures/iter.rs b/futures-util/src/stream/mapped_futures/iter.rs
index ed7d3758e..0ec0ff489 100644
--- a/futures-util/src/stream/mapped_futures/iter.rs
+++ b/futures-util/src/stream/mapped_futures/iter.rs
@@ -38,6 +38,14 @@ pub struct IntoIter<K: Hash + Eq, Fut: Unpin> {
     pub(super) inner: MappedFutures<K, Fut>,
 }
 
+/// Immutable iterator over all keys in the mapping.
+pub struct Keys<'a, K: Hash + Eq, Fut> {
+    pub(super) inner: std::iter::Map<
+        std::collections::hash_set::Iter<'a, HashTask<K, Fut>>,
+        Box<dyn FnMut(&'a HashTask<K, Fut>) -> &'a K>,
+    >,
+}
+
 impl<K: Hash + Eq, Fut: Unpin> Iterator for IntoIter<K, Fut> {
     type Item = Fut;
 
@@ -161,12 +169,6 @@ impl<'a, K: Hash + Eq, Fut: Unpin> Iterator for Iter<'a, K, Fut> {
 
 impl<K: Hash + Eq, Fut: Unpin> ExactSizeIterator for Iter<'_, K, Fut> {}
 
-pub struct Keys<'a, K: Hash + Eq, Fut> {
-    pub(super) inner: std::iter::Map<
-        std::collections::hash_set::Iter<'a, HashTask<K, Fut>>,
-        Box<dyn FnMut(&'a HashTask<K, Fut>) -> &'a K>,
-    >,
-}
 impl<K: Hash + Eq, Fut: Unpin> ExactSizeIterator for Keys<'_, K, Fut> {}
 
 impl<'a, K: Hash + Eq, Fut> Iterator for Keys<'a, K, Fut> {
diff --git a/futures-util/src/stream/mapped_futures/mod.rs b/futures-util/src/stream/mapped_futures/mod.rs
index bd7cc4ba3..f70d98ad0 100644
--- a/futures-util/src/stream/mapped_futures/mod.rs
+++ b/futures-util/src/stream/mapped_futures/mod.rs
@@ -309,6 +309,7 @@ impl<K: Hash + Eq, Fut> MappedFutures<K, Fut> {
         None
     }
 
+    /// Returns an iterator of keys in the mapping.
     pub fn keys<'a>(&'a self) -> Keys<'a, K, Fut> {
         Keys {
             inner: self.hash_set.iter().map(Box::new(|hash_task| HashTask::key_unwrap(hash_task))),