rebase: skip rebasing if commit's parents are identical to new parents

cli/src/commands/rebase.rs

@@ -290,13 +290,27 @@ fn rebase_descendants(
     rebase_options: RebaseOptions,
 ) -> Result<(), CommandError> {
     workspace_command.check_rewritable(old_commits)?;
+    let (skipped_commits, old_commits) = old_commits
+        .iter()
+        .partition::<Vec<_>, _>(|commit| commit.parents().eq(new_parents));
+    for commit in skipped_commits.iter() {
+        writeln!(
+            ui.stderr(),
+            "Skipping rebase of commit {}",
+            short_commit_hash(commit.id()),
+        )?;
+        continue;
+    }
+    if old_commits.is_empty() {
+        return Ok(());
+    }
     for old_commit in old_commits.iter() {
         check_rebase_destinations(workspace_command.repo(), new_parents, old_commit)?;
     }
     let mut tx = workspace_command.start_transaction();
     // `rebase_descendants` takes care of sorting in reverse topological order, so
     // no need to do it here.
-    for old_commit in old_commits {
+    for old_commit in old_commits.iter() {
         rebase_commit_with_options(
             settings,
             tx.mut_repo(),
@@ -330,6 +344,14 @@ fn rebase_revision(
 ) -> Result<(), CommandError> {
     let old_commit = workspace_command.resolve_single_rev(rev_str)?;
     workspace_command.check_rewritable([&old_commit])?;
+    if old_commit.parents().eq(new_parents) {
+        writeln!(
+            ui.stderr(),
+            "Skipping rebase of commit {}",
+            short_commit_hash(old_commit.id()),
+        )?;
+        return Ok(());
+    }
     if new_parents.contains(&old_commit) {
         return Err(user_error(format!(
             "Cannot rebase {} onto itself",

cli/tests/test_duplicate_command.rs

@@ -323,9 +323,7 @@ fn test_rebase_duplicates() {
     let (stdout, stderr) = test_env.jj_cmd_ok(&repo_path, &["rebase", "-s", "a", "-d", "a-"]);
     insta::assert_snapshot!(stdout, @"");
     insta::assert_snapshot!(stderr, @r###"
-    Rebased 4 commits
-    Working copy now at: zsuskuln 29bd36b6 b | b
-    Parent commit      : rlvkpnrz 2f6dc5a1 a | a
+    Skipping rebase of commit 2443ea76b0b1
     "###);
     // Some of the duplicate commits' timestamps were changed a little to make them
     // have distinct commit ids.

cli/tests/test_rebase_command.rs

@@ -145,7 +145,8 @@ fn test_rebase_branch() {
     let (stdout, stderr) = test_env.jj_cmd_ok(&repo_path, &["rebase", "-b=e", "-b=d", "-d=b"]);
     insta::assert_snapshot!(stdout, @"");
     insta::assert_snapshot!(stderr, @r###"
-    Rebased 2 commits
+    Skipping rebase of commit 514fa6b265d4
+    Rebased 1 commits
     Working copy now at: znkkpsqq 9ca2a154 e | e
     Parent commit      : zsuskuln 1394f625 b | b
     Added 1 files, modified 0 files, removed 0 files
@@ -692,9 +693,7 @@ fn test_rebase_with_child_and_descendant_bug_2600() {
         test_env.jj_cmd_ok(&repo_path, &["rebase", "-s", "base", "-d", "root()"]);
     insta::assert_snapshot!(stdout, @"");
     insta::assert_snapshot!(stderr, @r###"
-    Rebased 4 commits
-    Working copy now at: vruxwmqv 13b23fa1 c | c
-    Parent commit      : royxmykx 15092f8a b | b
+    Skipping rebase of commit 0c61db1be8c8
     "###);
     // This should be a no-op
     insta::assert_snapshot!(get_log_output(&test_env, &repo_path), @r###"