diff --git a/lib/src/index.rs b/lib/src/index.rs
index 4609e1ece..5ec307383 100644
--- a/lib/src/index.rs
+++ b/lib/src/index.rs
@@ -141,10 +141,7 @@ impl<'a> IndexRef<'a> {
         }
     }
 
-    pub fn heads<'candidates>(
-        &self,
-        candidates: impl IntoIterator<Item = &'candidates CommitId>,
-    ) -> Vec<CommitId> {
+    pub fn heads(&self, candidates: &mut dyn Iterator<Item = &CommitId>) -> Vec<CommitId> {
         match self {
             IndexRef::Readonly(index) => index.heads(candidates),
             IndexRef::Mutable(index) => index.heads(candidates),
@@ -677,17 +674,11 @@ impl MutableIndex {
         CompositeIndex(self).walk_revs(wanted, unwanted)
     }
 
-    pub fn heads<'candidates>(
-        &self,
-        candidates: impl IntoIterator<Item = &'candidates CommitId>,
-    ) -> Vec<CommitId> {
+    pub fn heads(&self, candidates: &mut dyn Iterator<Item = &CommitId>) -> Vec<CommitId> {
         CompositeIndex(self).heads(candidates)
     }
 
-    pub fn topo_order<'candidates>(
-        &self,
-        input: impl IntoIterator<Item = &'candidates CommitId>,
-    ) -> Vec<IndexEntry> {
+    pub fn topo_order(&self, input: &mut dyn Iterator<Item = &CommitId>) -> Vec<IndexEntry> {
         CompositeIndex(self).topo_order(input)
     }
 }
@@ -961,12 +952,8 @@ impl<'a> CompositeIndex<'a> {
         rev_walk
     }
 
-    pub fn heads<'candidates>(
-        &self,
-        candidate_ids: impl IntoIterator<Item = &'candidates CommitId>,
-    ) -> Vec<CommitId> {
+    pub fn heads(&self, candidate_ids: &mut dyn Iterator<Item = &CommitId>) -> Vec<CommitId> {
         let candidate_positions: BTreeSet<_> = candidate_ids
-            .into_iter()
             .map(|id| self.commit_id_to_pos(id).unwrap())
             .collect();
 
@@ -1012,10 +999,7 @@ impl<'a> CompositeIndex<'a> {
         candidate_positions
     }
 
-    pub fn topo_order<'input>(
-        &self,
-        input: impl IntoIterator<Item = &'input CommitId>,
-    ) -> Vec<IndexEntry<'a>> {
+    pub fn topo_order(&self, input: &mut dyn Iterator<Item = &CommitId>) -> Vec<IndexEntry<'a>> {
         let mut entries_by_generation = input
             .into_iter()
             .map(|id| IndexEntryByPosition(self.entry_by_id(id).unwrap()))
@@ -1662,17 +1646,11 @@ impl ReadonlyIndex {
         CompositeIndex(self).walk_revs(wanted, unwanted)
     }
 
-    pub fn heads<'candidates>(
-        &self,
-        candidates: impl IntoIterator<Item = &'candidates CommitId>,
-    ) -> Vec<CommitId> {
+    pub fn heads(&self, candidates: &mut dyn Iterator<Item = &CommitId>) -> Vec<CommitId> {
         CompositeIndex(self).heads(candidates)
     }
 
-    pub fn topo_order<'candidates>(
-        &self,
-        input: impl IntoIterator<Item = &'candidates CommitId>,
-    ) -> Vec<IndexEntry> {
+    pub fn topo_order(&self, input: &mut dyn Iterator<Item = &CommitId>) -> Vec<IndexEntry> {
         CompositeIndex(self).topo_order(input)
     }
 
@@ -2586,22 +2564,34 @@ mod tests {
         index.add_commit_data(id_5.clone(), new_change_id(), &[id_4.clone(), id_2.clone()]);
 
         // Empty input
-        assert!(index.heads(&[]).is_empty());
+        assert!(index.heads(&mut [].iter()).is_empty());
         // Single head
-        assert_eq!(index.heads(&[id_4.clone()]), vec![id_4.clone()]);
+        assert_eq!(index.heads(&mut [id_4.clone()].iter()), vec![id_4.clone()]);
         // Single head and parent
-        assert_eq!(index.heads(&[id_4.clone(), id_1]), vec![id_4.clone()]);
+        assert_eq!(
+            index.heads(&mut [id_4.clone(), id_1].iter()),
+            vec![id_4.clone()]
+        );
         // Single head and grand-parent
-        assert_eq!(index.heads(&[id_4.clone(), id_0]), vec![id_4.clone()]);
+        assert_eq!(
+            index.heads(&mut [id_4.clone(), id_0].iter()),
+            vec![id_4.clone()]
+        );
         // Multiple heads
         assert_eq!(
-            index.heads(&[id_4.clone(), id_3.clone()]),
+            index.heads(&mut [id_4.clone(), id_3.clone()].iter()),
             vec![id_3.clone(), id_4]
         );
         // Merge commit and ancestors
-        assert_eq!(index.heads(&[id_5.clone(), id_2]), vec![id_5.clone()]);
+        assert_eq!(
+            index.heads(&mut [id_5.clone(), id_2].iter()),
+            vec![id_5.clone()]
+        );
         // Merge commit and other commit
-        assert_eq!(index.heads(&[id_5.clone(), id_3.clone()]), vec![id_3, id_5]);
+        assert_eq!(
+            index.heads(&mut [id_5.clone(), id_3.clone()].iter()),
+            vec![id_3, id_5]
+        );
     }
 
     #[test]
diff --git a/lib/src/repo.rs b/lib/src/repo.rs
index 4db653688..ff534edf1 100644
--- a/lib/src/repo.rs
+++ b/lib/src/repo.rs
@@ -829,14 +829,14 @@ impl MutableRepo {
         let view = view.store_view_mut();
         view.public_head_ids = self
             .index
-            .heads(view.public_head_ids.iter())
+            .heads(&mut view.public_head_ids.iter())
             .iter()
             .cloned()
             .collect();
         view.head_ids.extend(view.public_head_ids.iter().cloned());
         view.head_ids = self
             .index
-            .heads(view.head_ids.iter())
+            .heads(&mut view.head_ids.iter())
             .iter()
             .cloned()
             .collect();
diff --git a/lib/src/revset.rs b/lib/src/revset.rs
index 6c4ea0cf1..bfcc41be3 100644
--- a/lib/src/revset.rs
+++ b/lib/src/revset.rs
@@ -2006,7 +2006,7 @@ pub fn evaluate_expression<'repo>(
             let candidate_ids = candidate_set.iter().commit_ids().collect_vec();
             Ok(revset_for_commit_ids(
                 repo,
-                &repo.index().heads(&candidate_ids),
+                &repo.index().heads(&mut candidate_ids.iter()),
             ))
         }
         RevsetExpression::Roots(candidates) => {
diff --git a/lib/src/rewrite.rs b/lib/src/rewrite.rs
index b30809f97..90a484eb3 100644
--- a/lib/src/rewrite.rs
+++ b/lib/src/rewrite.rs
@@ -209,7 +209,7 @@ impl<'settings, 'repo> DescendantRebaser<'settings, 'repo> {
                 new_parents.insert(old_commit, vec![new_commits.iter().next().unwrap().clone()]);
             } else {
                 // The call to index.heads() is mostly to get a predictable order
-                let new_commits = mut_repo.index().heads(&new_commits);
+                let new_commits = mut_repo.index().heads(&mut new_commits.iter());
                 divergent.insert(old_commit, new_commits);
             }
         }
@@ -402,7 +402,7 @@ impl<'settings, 'repo> DescendantRebaser<'settings, 'repo> {
             let head_set: HashSet<_> = self
                 .mut_repo
                 .index()
-                .heads(&new_parent_ids)
+                .heads(&mut new_parent_ids.iter())
                 .iter()
                 .cloned()
                 .collect();
diff --git a/src/commands/mod.rs b/src/commands/mod.rs
index 87cea3ecb..e602c9c8b 100644
--- a/src/commands/mod.rs
+++ b/src/commands/mod.rs
@@ -1892,7 +1892,7 @@ fn cmd_duplicate(
     let mut_repo = tx.mut_repo();
 
     for original_commit_id in index
-        .topo_order(to_duplicate.iter().map(|c| c.id()))
+        .topo_order(&mut to_duplicate.iter().map(|c| c.id()))
         .into_iter()
         .map(|index_entry| index_entry.commit_id())
     {