cli: let backend decide whether merge with root can be performed

One less CLI revset helper. It might look odd that "jj rebase" says "Merge
failed" whereas "jj new" doesn't, but that depends on where the BackendError
is detected.

cli/src/cli_util.rs

@@ -1651,21 +1651,6 @@ pub fn print_trackable_remote_branches(ui: &Ui, view: &View) -> io::Result<()> {
     Ok(())
 }
 
-/// Resolves revsets into revisions for use; useful for rebases or operations
-/// that take multiple parents.
-pub fn resolve_all_revs(
-    workspace_command: &WorkspaceCommandHelper,
-    revisions: &[RevisionArg],
-) -> Result<IndexSet<Commit>, CommandError> {
-    let commits = resolve_multiple_nonempty_revsets_default_single(workspace_command, revisions)?;
-    let root_commit_id = workspace_command.repo().store().root_commit_id();
-    if commits.len() >= 2 && commits.iter().any(|c| c.id() == root_commit_id) {
-        Err(user_error("Cannot merge with root revision"))
-    } else {
-        Ok(commits)
-    }
-}
-
 pub fn resolve_multiple_nonempty_revsets_default_single(
     workspace_command: &WorkspaceCommandHelper,
     revisions: &[RevisionArg],

cli/src/commands/new.rs

@@ -22,7 +22,9 @@ use jj_lib::revset::{RevsetExpression, RevsetIteratorExt};
 use jj_lib::rewrite::{merge_commit_trees, rebase_commit};
 use tracing::instrument;
 
-use crate::cli_util::{self, short_commit_hash, CommandHelper, RevisionArg};
+use crate::cli_util::{
+    resolve_multiple_nonempty_revsets_default_single, short_commit_hash, CommandHelper, RevisionArg,
+};
 use crate::command_error::{user_error, CommandError};
 use crate::description_util::join_message_paragraphs;
 use crate::ui::Ui;
@@ -98,9 +100,10 @@ Please use `jj new 'all:x|y'` instead of `jj new --allow-large-revsets x y`.",
         !args.revisions.is_empty(),
         "expected a non-empty list from clap"
     );
-    let target_commits = cli_util::resolve_all_revs(&workspace_command, &args.revisions)?
-        .into_iter()
-        .collect_vec();
+    let target_commits =
+        resolve_multiple_nonempty_revsets_default_single(&workspace_command, &args.revisions)?
+            .into_iter()
+            .collect_vec();
     let target_ids = target_commits.iter().map(|c| c.id().clone()).collect_vec();
     let mut tx = workspace_command.start_transaction();
     let mut num_rebased;

cli/src/commands/rebase.rs

@@ -29,7 +29,7 @@ use jj_lib::settings::UserSettings;
 use tracing::instrument;
 
 use crate::cli_util::{
-    self, resolve_multiple_nonempty_revsets_default_single, short_commit_hash, CommandHelper,
+    resolve_multiple_nonempty_revsets_default_single, short_commit_hash, CommandHelper,
     RevisionArg, WorkspaceCommandHelper,
 };
 use crate::command_error::{user_error, CommandError};
@@ -191,9 +191,10 @@ Please use `jj rebase -d 'all:x|y'` instead of `jj rebase --allow-large-revsets
         simplify_ancestor_merge: false,
     };
     let mut workspace_command = command.workspace_helper(ui)?;
-    let new_parents = cli_util::resolve_all_revs(&workspace_command, &args.destination)?
-        .into_iter()
-        .collect_vec();
+    let new_parents =
+        resolve_multiple_nonempty_revsets_default_single(&workspace_command, &args.destination)?
+            .into_iter()
+            .collect_vec();
     if let Some(rev_str) = &args.revision {
         assert_eq!(
             // In principle, `-r --skip-empty` could mean to abandon the `-r`

cli/src/commands/workspace.rs

@@ -29,7 +29,8 @@ use jj_lib::workspace::Workspace;
 use tracing::instrument;
 
 use crate::cli_util::{
-    self, check_stale_working_copy, print_checkout_stats, short_commit_hash, CommandHelper,
+    check_stale_working_copy, print_checkout_stats,
+    resolve_multiple_nonempty_revsets_default_single, short_commit_hash, CommandHelper,
     RevisionArg, WorkingCopyFreshness, WorkspaceCommandHelper,
 };
 use crate::command_error::{internal_error_with_message, user_error, CommandError};
@@ -202,7 +203,7 @@ fn cmd_workspace_add(
             vec![tx.repo().store().root_commit()]
         }
     } else {
-        cli_util::resolve_all_revs(&old_workspace_command, &args.revision)?
+        resolve_multiple_nonempty_revsets_default_single(&old_workspace_command, &args.revision)?
             .into_iter()
             .collect_vec()
     };

cli/tests/test_new_command.rs

@@ -141,7 +141,7 @@ fn test_new_merge() {
     // merge with root
     let stderr = test_env.jj_cmd_failure(&repo_path, &["new", "@", "root()"]);
     insta::assert_snapshot!(stderr, @r###"
-    Error: Cannot merge with root revision
+    Error: The Git backend does not support creating merge commits with the root commit as one of the parents.
     "###);
 }
 

cli/tests/test_rebase_command.rs

@@ -515,7 +515,8 @@ fn test_rebase_multiple_destinations() {
         &["rebase", "-r", "a", "-d", "b", "-d", "root()"],
     );
     insta::assert_snapshot!(stderr, @r###"
-    Error: Cannot merge with root revision
+    Error: Merge failed
+    Caused by: The Git backend does not support creating merge commits with the root commit as one of the parents.
     "###);
 }