diff --git a/cli/src/commands/parallelize.rs b/cli/src/commands/parallelize.rs
index 33e5327957..7edd6cdc1f 100644
--- a/cli/src/commands/parallelize.rs
+++ b/cli/src/commands/parallelize.rs
@@ -113,7 +113,7 @@ pub(crate) fn cmd_parallelize(
             // Commits in the target set do not depend on each other but they still depend
             // on other parents
             if let Some(new_parents) = new_target_parents.get(rewriter.old_commit().id()) {
-                rewriter.set_new_rewritten_parents(new_parents.clone());
+                rewriter.set_new_rewritten_parents(new_parents);
             } else if rewriter
                 .old_commit()
                 .parent_ids()
@@ -128,7 +128,7 @@ pub(crate) fn cmd_parallelize(
                         new_parents.push(parent.clone());
                     }
                 }
-                rewriter.set_new_rewritten_parents(new_parents);
+                rewriter.set_new_rewritten_parents(&new_parents);
             }
             if rewriter.parents_changed() {
                 let builder = rewriter.rebase(command.settings())?;
diff --git a/lib/src/repo.rs b/lib/src/repo.rs
index 1b077cf18f..af266c0209 100644
--- a/lib/src/repo.rs
+++ b/lib/src/repo.rs
@@ -989,7 +989,7 @@ impl MutableRepo {
     ///
     /// If `parent_mapping` contains cycles, this function may either panic or
     /// drop parents that caused cycles.
-    pub fn new_parents(&self, old_ids: Vec<CommitId>) -> Vec<CommitId> {
+    pub fn new_parents(&self, old_ids: &[CommitId]) -> Vec<CommitId> {
         assert!(!old_ids.is_empty());
         let mut new_ids = Vec::with_capacity(old_ids.len());
         let mut to_visit = old_ids.iter().rev().collect_vec();
@@ -1040,7 +1040,7 @@ impl MutableRepo {
             // mappings, not transitive ones.
             // TODO: keep parent_mapping updated with transitive mappings so we don't need
             // to call `new_parents()` here.
-            let new_parent_ids = self.new_parents(rewrite.new_parent_ids().to_vec());
+            let new_parent_ids = self.new_parents(rewrite.new_parent_ids());
             self.update_references(settings, old_parent_id, new_parent_ids)?;
         }
         Ok(())
@@ -1226,7 +1226,7 @@ impl MutableRepo {
     ) -> BackendResult<()> {
         let mut to_visit = self.find_descendants_to_rebase(roots)?;
         while let Some(old_commit) = to_visit.pop() {
-            let new_parent_ids = self.new_parents(old_commit.parent_ids().to_vec());
+            let new_parent_ids = self.new_parents(old_commit.parent_ids());
             let rewriter = CommitRewriter::new(self, old_commit, new_parent_ids);
             callback(rewriter)?;
         }
diff --git a/lib/src/rewrite.rs b/lib/src/rewrite.rs
index 2a8164a3d5..b58213e7a6 100644
--- a/lib/src/rewrite.rs
+++ b/lib/src/rewrite.rs
@@ -173,7 +173,7 @@ impl<'repo> CommitRewriter<'repo> {
 
     /// Set the old commit's intended new parents to be the rewritten versions
     /// of the given parents.
-    pub fn set_new_rewritten_parents(&mut self, unrewritten_parents: Vec<CommitId>) {
+    pub fn set_new_rewritten_parents(&mut self, unrewritten_parents: &[CommitId]) {
         self.new_parents = self.mut_repo.new_parents(unrewritten_parents);
     }
 
@@ -426,7 +426,7 @@ impl<'settings, 'repo> DescendantRebaser<'settings, 'repo> {
     fn rebase_one(&mut self, old_commit: Commit) -> BackendResult<()> {
         let old_commit_id = old_commit.id().clone();
         let old_parent_ids = old_commit.parent_ids();
-        let new_parent_ids = self.mut_repo.new_parents(old_parent_ids.to_vec());
+        let new_parent_ids = self.mut_repo.new_parents(old_parent_ids);
         let rewriter = CommitRewriter::new(self.mut_repo, old_commit, new_parent_ids);
         if !rewriter.parents_changed() {
             // The commit is already in place.
@@ -806,10 +806,7 @@ pub fn move_commits(
     // tests.
     while let Some(old_commit_id) = to_visit.pop() {
         let old_commit = to_visit_commits.get(&old_commit_id).unwrap();
-        let parent_ids = to_visit_commits_new_parents
-            .get(&old_commit_id)
-            .cloned()
-            .unwrap();
+        let parent_ids = to_visit_commits_new_parents.get(&old_commit_id).unwrap();
         let new_parent_ids = mut_repo.new_parents(parent_ids);
         let rewriter = CommitRewriter::new(mut_repo, old_commit.clone(), new_parent_ids);
         if rewriter.parents_changed() {