repo: consider empty and undescribed merge commits as discardable

CHANGELOG.md

@@ -24,6 +24,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   key](https://toml.io/en/v1.0.0#keys). Quote meta characters as needed.
   Example: `jj config get "revset-aliases.'trunk()'"`
 
+* When updating the working copy away from an empty and undescribed commit, it
+  is now abandoned even if it is a merge commit.
+
 * If a new working-copy commit is created because the old one was abandoned, and
   the old commit was merge, then the new commit will now also be.
   [#2859](https://github.com/martinvonz/jj/issues/2859)

lib/src/commit.rs

@@ -153,16 +153,10 @@ impl Commit {
         &self.data.committer
     }
 
-    /// A commit is discardable if it has one parent, no change from its
-    /// parent, and an empty description.
-    pub fn is_discardable(&self) -> BackendResult<bool> {
-        if self.description().is_empty() {
-            let parents: Vec<_> = self.parents().try_collect()?;
-            if let [parent_commit] = &*parents {
-                return Ok(self.tree_id() == parent_commit.tree_id());
-            }
-        }
-        Ok(false)
+    /// A commit is discardable if it has no change from its parent, and an
+    /// empty description.
+    pub fn is_discardable(&self, repo: &dyn Repo) -> BackendResult<bool> {
+        Ok(self.description().is_empty() && self.is_empty(repo)?)
     }
 
     /// A quick way to just check if a signature is present.

lib/src/repo.rs

@@ -1355,7 +1355,7 @@ impl MutableRepo {
                 .store()
                 .get_commit(&wc_commit_id)
                 .map_err(EditCommitError::WorkingCopyCommitNotFound)?;
-            if wc_commit.is_discardable()?
+            if wc_commit.is_discardable(self)?
                 && self
                     .view
                     .with_ref(|v| local_branch_target_ids(v).all(|id| id != wc_commit.id()))

lib/tests/test_mut_repo.rs

@@ -151,8 +151,7 @@ fn test_edit_previous_empty_merge() {
     let new_wc_commit = write_random_commit(mut_repo, &settings);
     mut_repo.edit(ws_id, &new_wc_commit).unwrap();
     mut_repo.rebase_descendants(&settings).unwrap();
-    // TODO: The old commit should no longer be visible
-    assert!(mut_repo.view().heads().contains(old_wc_commit.id()));
+    assert!(!mut_repo.view().heads().contains(old_wc_commit.id()));
 }
 
 #[test]