cmd: Allow multiple -s for jj rebase

This addresses a portion of #1158

One application (not the prettiest, but useful until we have `jj sync`):

    jj rebase -s oldmain+~:main -d main -L

CHANGELOG.md

@@ -16,6 +16,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 * `jj restore` without `--from` works correctly even if `@` is a merge
   commit.
 
+* `jj rebase` now accepts multiple `-s` arguments. Revsets with multiple commits
+  are allowed with `--allow-large-revsets`.
+
 ### Fixed bugs
 
 * Modify/delete conflicts now include context lines

src/commands/mod.rs

@@ -733,9 +733,15 @@ struct RebaseArgs {
     /// Rebase the whole branch (relative to destination's ancestors)
     #[arg(long, short)]
     branch: Option<RevisionArg>,
-    /// Rebase this revision and its descendants
+
+    /// Rebase specified revision(s) together their tree of descendants (can be
+    /// repeated)
+    ///
+    /// Each specified revision will become a direct child of the destination
+    /// revision(s), even if some of the source revisions are descendants
+    /// of others.
     #[arg(long, short)]
-    source: Option<RevisionArg>,
+    source: Vec<RevisionArg>,
     /// Rebase only this revision, rebasing descendants onto this revision's
     /// parent(s)
     #[arg(long, short)]
@@ -2762,13 +2768,18 @@ fn cmd_rebase(ui: &mut Ui, command: &CommandHelper, args: &RebaseArgs) -> Result
     .collect_vec();
     if let Some(rev_str) = &args.revision {
         rebase_revision(ui, command, &mut workspace_command, &new_parents, rev_str)?;
-    } else if let Some(source_str) = &args.source {
+    } else if !args.source.is_empty() {
+        let source_commits = resolve_mutliple_nonempty_revsets_flag_guarded(
+            &workspace_command,
+            &args.source,
+            args.allow_large_revsets,
+        )?;
         rebase_descendants(
             ui,
             command,
             &mut workspace_command,
             &new_parents,
-            source_str,
+            &source_commits,
         )?;
     } else {
         let branch_str = args.branch.as_deref().unwrap_or("@");
@@ -2825,17 +2836,27 @@ fn rebase_descendants(
     command: &CommandHelper,
     workspace_command: &mut WorkspaceCommandHelper,
     new_parents: &[Commit],
-    source_str: &str,
+    old_commits: &IndexSet<Commit>,
 ) -> Result<(), CommandError> {
-    let old_commit = workspace_command.resolve_single_rev(source_str)?;
+    for old_commit in old_commits.iter() {
-    workspace_command.check_rewritable(&old_commit)?;
+        workspace_command.check_rewritable(old_commit)?;
-    check_rebase_destinations(workspace_command.repo(), new_parents, &old_commit)?;
+        check_rebase_destinations(workspace_command.repo(), new_parents, old_commit)?;
-    let mut tx = workspace_command.start_transaction(&format!(
+    }
+    let tx_message = if old_commits.len() == 1 {
+        format!(
             "rebase commit {} and descendants",
-        old_commit.id().hex()
+            old_commits.first().unwrap().id().hex()
-    ));
+        )
-    rebase_commit(command.settings(), tx.mut_repo(), &old_commit, new_parents)?;
+    } else {
-    let num_rebased = tx.mut_repo().rebase_descendants(command.settings())? + 1;
+        format!("rebase {} commits and their descendants", old_commits.len())
+    };
+    let mut tx = workspace_command.start_transaction(&tx_message);
+    // `rebase_descendants` takes care of sorting in reverse topological order, so
+    // no need to do it here.
+    for old_commit in old_commits {
+        rebase_commit(command.settings(), tx.mut_repo(), old_commit, new_parents)?;
+    }
+    let num_rebased = old_commits.len() + tx.mut_repo().rebase_descendants(command.settings())?;
     writeln!(ui, "Rebased {num_rebased} commits")?;
     tx.finish(ui)?;
     Ok(())

tests/test_rebase_command.rs

@@ -426,6 +426,82 @@ fn test_rebase_with_descendants() {
     o  a
     o
     "###);
+
+    // Rebase several subtrees at once.
+    test_env.jj_cmd_success(&repo_path, &["undo"]);
+    let stdout = test_env.jj_cmd_success(&repo_path, &["rebase", "-s=c", "-s=d", "-d=a"]);
+    insta::assert_snapshot!(stdout, @r###"
+    Rebased 2 commits
+    Working copy now at: 92c2bc9a8623 d
+    Added 0 files, modified 0 files, removed 2 files
+    "###);
+    insta::assert_snapshot!(get_log_output(&test_env, &repo_path), @r###"
+    @  d
+    │ o  c
+    ├─╯
+    │ o  b
+    o │  a
+    ├─╯
+    o
+    "###);
+
+    test_env.jj_cmd_success(&repo_path, &["undo"]);
+    // Reminder of the setup
+    insta::assert_snapshot!(get_log_output(&test_env, &repo_path), @r###"
+    @  d
+    o    c
+    ├─╮
+    o │  b
+    │ o  a
+    ├─╯
+    o
+    "###);
+
+    // `d` was a descendant of `b`, and both are moved to be direct descendants of
+    // `a`. `c` remains a descendant of `b`.
+    let stdout = test_env.jj_cmd_success(&repo_path, &["rebase", "-s=b", "-s=d", "-d=a"]);
+    insta::assert_snapshot!(stdout, @r###"
+    Rebased 3 commits
+    Working copy now at: f1e71cb78a06 d
+    Added 0 files, modified 0 files, removed 2 files
+    "###);
+    insta::assert_snapshot!(get_log_output(&test_env, &repo_path), @r###"
+    o  c
+    │ @  d
+    o │  b
+    ├─╯
+    o  a
+    o
+    "###);
+
+    // Same test as above, but with duplicate commits and multiple commits per
+    // argument
+    test_env.jj_cmd_success(&repo_path, &["undo"]);
+    let stderr = test_env.jj_cmd_failure(&repo_path, &["rebase", "-s=b|d", "-s=d", "-d=a"]);
+    insta::assert_snapshot!(stderr, @r###"
+    Error: Revset "b|d" resolved to more than one revision
+    Hint: The revset "b|d" resolved to these revisions:
+    df54a9fd85ae d
+    d370aee184ba b
+    If this was intentional, specify the `--allow-large-revsets` argument
+    "###);
+    let stdout = test_env.jj_cmd_success(
+        &repo_path,
+        &["rebase", "-s=b|d", "-s=d", "-d=a", "--allow-large-revsets"],
+    );
+    insta::assert_snapshot!(stdout, @r###"
+    Rebased 3 commits
+    Working copy now at: d17539f7ea7c d
+    Added 0 files, modified 0 files, removed 2 files
+    "###);
+    insta::assert_snapshot!(get_log_output(&test_env, &repo_path), @r###"
+    o  c
+    o  b
+    │ @  d
+    ├─╯
+    o  a
+    o
+    "###);
 }
 
 fn get_log_output(test_env: &TestEnvironment, repo_path: &Path) -> String {