checkout: don't abandon old commit if it has non-empty description

If the user entered a description, we shouldn't abandon it even if it
has no changes to the content.

CHANGELOG.md

@@ -137,6 +137,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   checkout will be checked out. That was always the intent, but the root commit
   was accidentally checked out instead.
 
+* When checking out a commit, the previous commit is no longer abandoned if it
+  has a non-empty description.
+
 ## [0.4.0] - 2022-04-02
 
 ### Breaking changes

lib/src/repo.rs

@@ -588,7 +588,10 @@ impl MutableRepo {
         if let Some(current_checkout_id) = maybe_current_checkout_id {
             let current_checkout = self.store().get_commit(&current_checkout_id).unwrap();
             assert!(current_checkout.is_open(), "current checkout is closed");
-            if current_checkout.is_empty() && self.view().heads().contains(current_checkout.id()) {
+            if current_checkout.is_empty()
+                && current_checkout.description().is_empty()
+                && self.view().heads().contains(current_checkout.id())
+            {
                 // Abandon the checkout we're leaving if it's empty and a head commit
                 self.record_abandoned_commit(current_checkout_id);
             }

lib/tests/test_mut_repo.rs

@@ -133,6 +133,39 @@ fn test_checkout_previous_empty(use_git: bool) {
     assert!(!mut_repo.view().heads().contains(old_checkout.id()));
 }
 
+#[test_case(false ; "local backend")]
+// #[test_case(true ; "git backend")]
+fn test_checkout_previous_empty_with_description(use_git: bool) {
+    // Test that MutableRepo::check_out() does not abandon the previous commit if it
+    // has a non-empty description.
+    let settings = testutils::user_settings();
+    let test_repo = testutils::init_repo(&settings, use_git);
+    let repo = &test_repo.repo;
+
+    let mut tx = repo.start_transaction("test");
+    let mut_repo = tx.mut_repo();
+    let old_checkout = CommitBuilder::for_open_commit(
+        &settings,
+        repo.store(),
+        repo.store().root_commit_id().clone(),
+        repo.store().empty_tree_id().clone(),
+    )
+    .set_description("not empty".to_string())
+    .write_to_repo(mut_repo);
+    let ws_id = WorkspaceId::default();
+    mut_repo.check_out(ws_id.clone(), &settings, &old_checkout);
+    let repo = tx.commit();
+
+    let mut tx = repo.start_transaction("test");
+    let mut_repo = tx.mut_repo();
+    let new_checkout = testutils::create_random_commit(&settings, &repo)
+        .set_open(true)
+        .write_to_repo(mut_repo);
+    mut_repo.check_out(ws_id, &settings, &new_checkout);
+    mut_repo.rebase_descendants(&settings).unwrap();
+    assert!(mut_repo.view().heads().contains(old_checkout.id()));
+}
+
 #[test_case(false ; "local backend")]
 // #[test_case(true ; "git backend")]
 fn test_checkout_previous_empty_non_head(use_git: bool) {

tests/test_new.rs

@@ -37,6 +37,7 @@ fn test_new() {
     let stdout = test_env.jj_cmd_success(&repo_path, &["log", "-T", "commit_id \" \" description"]);
     insta::assert_snapshot!(stdout, @r###"
     @ d8c0a3e1570f1f5b08113a3427b3160900c3d48e off of root
+    | o 88436dbcdbedc2b8a6ebd0687981906d09ccc68f a new commit
     | o 51e9c5819117991e4a6dc5a4a744283fc74f0746 add a file
     |/  
     o 0000000000000000000000000000000000000000 (no description set)