commands: Teach next to create a new commit after an edit. WIP: C…

…urrently not building

This makes the commands symmetrical again.  

Fixes jj-vcs#2209

cli/src/commands/next.rs

@@ -16,6 +16,7 @@ use itertools::Itertools;
 use jj_lib::commit::Commit;
 use jj_lib::repo::Repo;
 use jj_lib::revset::{RevsetExpression, RevsetIteratorExt};
+use jj_lib::rewrite::merge_commit_trees;
 
 use crate::cli_util::{short_commit_hash, user_error, CommandError, CommandHelper};
 use crate::ui::Ui;
@@ -87,23 +88,47 @@ pub(crate) fn cmd_next(
     let target_expression = if edit {
         descendant_expression
     } else {
-        descendant_expression.minus(&RevsetExpression::commit(current_wc_id.clone()).descendants())
+        // Jujutsu will always create a new commit for next, even where Mercurial cannot and fails.
+        // The decision and all discussion around it are available here:
+        // https://github.com/martinvonz/jj/pull/1200#discussion_r1298623933
+        //
+        // If users ever request erroring out, add back `.descendants()` to the revset below.
+        descendant_expression.minus(&RevsetExpression::commit(current_wc_id.clone()))
     };
     let targets: Vec<Commit> = target_expression
         .evaluate_programmatic(workspace_command.repo().as_ref())?
         .iter()
         .commits(workspace_command.repo().store())
         .take(2)
         .try_collect()?;
+    // Do we create a commit to allow `next` onto new?
+    let mut created_commit = false;
+
+    let mut tx = workspace_command.start_transaction();
     let target = match targets.as_slice() {
         [target] => target,
-        [] => {
+        [] if edit => {
             // We found no descendant.
             return Err(user_error(format!(
                 "No descendant found {amount} commit{} forward",
                 if amount > 1 { "s" } else { "" }
             )));
         }
+        [] => {
+            // Create a new commit to get out of the forced `edit` flow.
+            let merged_tree = merge_commit_trees(tx.repo(), &[current_wc.clone()])?;
+            let new_commit = tx
+                .mut_repo()
+                .new_commit(
+                    command.settings(),
+                    vec![current_wc_id.clone()],
+                    merged_tree.id(),
+                )
+                .write()
+                .unwrap();
+            created_commit = true;
+            &new_commit
+        }
         _ => {
             // TODO(#2126) We currently cannot deal with multiple children, which result
             // from branches. Prompt the user for resolution.
@@ -124,8 +149,13 @@ pub(crate) fn cmd_next(
         return Ok(());
     }
     let mut tx = workspace_command.start_transaction();
+    let message = if !created_commit {
+        format!("next: {current_short} -> {target_short}")
+    } else {
+        format!("next: {current_short} -> new commit")
+    };
     // Move the working-copy commit to the new parent.
     tx.check_out(target)?;
-    tx.finish(ui, format!("next: {current_short} -> {target_short}"))?;
+    tx.finish(ui, &message)?;
     Ok(())
 }

cli/tests/test_next_prev_commands.rs

@@ -214,3 +214,26 @@ fn test_next_editing() {
     Parent commit      : kkmpptxz 3fa8931e (empty) third
     "###);
 }
+
+#[test]
+fn test_next_after_edit_new_commit() {
+    // Move from first => second.
+    // While creating a new commit.
+    let test_env = TestEnvironment::default();
+    test_env.jj_cmd_success(test_env.env_root(), &["init", "repo", "--git"]);
+    let repo_path = test_env.env_root().join("repo");
+    // Create a simple linear history, which we'll traverse.
+    test_env.jj_cmd_success(&repo_path, &["commit", "-m", "first"]);
+    test_env.jj_cmd_success(&repo_path, &["commit", "-m", "second"]);
+    test_env.jj_cmd_success(&repo_path, &["commit", "-m", "third"]);
+    // Move to `first`
+    test_env.jj_cmd_success(&repo_path, &["edit", "@--"]);
+    let stdout = test_env.jj_cmd_success(&repo_path, &["next"]);
+    insta::assert_snapshot!(
+        stdout,
+        @r###"
+    Working copy now at: royxmykx f039cf03 (empty) (no description set)
+    Parent commit      : kkmpptxz 3fa8931e (empty) third
+    "###
+    );
+}