cli: hint for same change ids

cli/src/cli_util.rs

@@ -1017,9 +1017,20 @@ impl WorkspaceCommandHelper {
                 let mut iter = [commit0, commit1].into_iter().chain(iter);
                 let commits: Vec<_> = iter.by_ref().take(5).try_collect()?;
                 let elided = iter.next().is_some();
-                let hint = if let RevsetExpression::CommitRef(RevsetCommitRef::Symbol(
-                    branch_name,
-                )) = revset_expression.as_ref()
+                let hint = if commits[0].change_id() == commits[1].change_id() {
+                    // Separate hint if there's commits with same change id
+                    format!(
+                        r#"The revset "{revision_str}" resolved to these revisions:{eol}{commits}{ellipsis}
+These commits have the same change id. Abandon one of them with "jj abandon -r <REVISION>"."#,
+                        eol = "\n",
+                        commits = commits
+                            .iter()
+                            .map(|c| self.format_commit_summary(c))
+                            .join("\n"),
+                        ellipsis = elided.then_some("\n...").unwrap_or_default()
+                    )
+                } else if let RevsetExpression::CommitRef(RevsetCommitRef::Symbol(branch_name)) =
+                    revset_expression.as_ref()
                 {
                     // Separate hint if there's a conflicted branch
                     format!(

cli/tests/test_checkout.rs

@@ -12,7 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-use std::path::Path;
+use std::path::{Path, PathBuf};
 
 use crate::common::TestEnvironment;
 
@@ -100,6 +100,38 @@ fn test_checkout_not_single_rev() {
     "###);
 }
 
+#[test]
+fn test_checkout_conflicting_change_ids() {
+    let (test_env, repo_path) = set_up();
+
+    test_env.jj_cmd_success(&repo_path, &["branch", "create", "conflict"]);
+    test_env.jj_cmd_success(&repo_path, &["describe", "--message", "foo"]);
+    test_env.jj_cmd_success(&repo_path, &["git", "push", "--branch", "conflict"]);
+    test_env.jj_cmd_success(&repo_path, &["describe", "--message", "bar"]);
+    test_env.jj_cmd_success(&repo_path, &["git", "fetch"]);
+    test_env.jj_cmd_success(&repo_path, &["checkout", "conflict@origin"]);
+
+    let stdout = test_env.jj_cmd_success(&repo_path, &["log"]);
+    insta::assert_snapshot!(stdout, @r###"
+    @  wqnwkozp [email protected] 2001-02-03 04:05:19.000 +07:00 434ac141
+    │  (empty) (no description set)
+    ◉  yqosqzyt?? [email protected] 2001-02-03 04:05:15.000 +07:00 conflict@origin 6553d57b
+    │  (empty) foo
+    │ ◉  yqosqzyt?? [email protected] 2001-02-03 04:05:17.000 +07:00 conflict* 8d2be885
+    ├─╯  (empty) bar
+    ◉  zzzzzzzz root() 00000000
+    "###);
+
+    let stderr = test_env.jj_cmd_failure(&repo_path, &["checkout", "yqosqzyt"]);
+    insta::assert_snapshot!(stderr, @r###"
+    Error: Revset "yqosqzyt" resolved to more than one revision
+    Hint: The revset "yqosqzyt" resolved to these revisions:
+    yqosqzyt 8d2be885 conflict* | (empty) bar
+    yqosqzyt 6553d57b conflict@origin | (empty) foo
+    These commits have the same change id. Abandon one of them with "jj abandon -r <REVISION>".
+    "###);
+}
+
 #[test]
 fn test_checkout_conflicting_branches() {
     // Much of this test is borrowed from `test_git_fetch_conflicting_branches` in
@@ -138,3 +170,32 @@ fn get_log_output(test_env: &TestEnvironment, cwd: &Path) -> String {
     let template = r#"commit_id ++ " " ++ description"#;
     test_env.jj_cmd_success(cwd, &["log", "-T", template])
 }
+
+fn set_up() -> (TestEnvironment, PathBuf) {
+    let test_env = TestEnvironment::default();
+    test_env.jj_cmd_success(test_env.env_root(), &["init", "--git", "origin"]);
+    let origin_path = test_env.env_root().join("origin");
+    let origin_git_repo_path = origin_path
+        .join(".jj")
+        .join("repo")
+        .join("store")
+        .join("git");
+
+    test_env.jj_cmd_success(&origin_path, &["describe", "-m=description 1"]);
+    test_env.jj_cmd_success(&origin_path, &["branch", "create", "branch1"]);
+    test_env.jj_cmd_success(&origin_path, &["new", "root()", "-m=description 2"]);
+    test_env.jj_cmd_success(&origin_path, &["branch", "create", "branch2"]);
+    test_env.jj_cmd_success(&origin_path, &["git", "export"]);
+
+    test_env.jj_cmd_success(
+        test_env.env_root(),
+        &[
+            "git",
+            "clone",
+            origin_git_repo_path.to_str().unwrap(),
+            "local",
+        ],
+    );
+    let workspace_root = test_env.env_root().join("local");
+    (test_env, workspace_root)
+}