store: add a empty_merged_tree_id() helper

Many (most?) callers of `Store::empty_tree_id()` really want a
`MergedTreeId`, so let's create a helper for that. It returns the
`Legacy` variant, which is what all current callers used. That should
be all we need since the two variants compare equal these days, and
since trees built based on the legacy variant can get promoted to the
new variant on write if the config is enabled.

lib/src/rewrite.rs

@@ -20,7 +20,7 @@ use std::sync::Arc;
 use itertools::{process_results, Itertools};
 use tracing::instrument;
 
-use crate::backend::{BackendError, CommitId, MergedTreeId, ObjectId};
+use crate::backend::{BackendError, CommitId, ObjectId};
 use crate::commit::Commit;
 use crate::dag_walk;
 use crate::index::Index;
@@ -48,7 +48,7 @@ pub fn merge_commit_trees_without_repo(
     commits: &[Commit],
 ) -> Result<MergedTree, TreeMergeError> {
     if commits.is_empty() {
-        Ok(store.get_root_tree(&MergedTreeId::Legacy(store.empty_tree_id().clone()))?)
+        Ok(store.get_root_tree(&store.empty_merged_tree_id())?)
     } else {
         let mut new_tree = commits[0].tree()?;
         let commit_ids = commits

lib/src/store.rs

@@ -72,6 +72,10 @@ impl Store {
         self.backend.empty_tree_id()
     }
 
+    pub fn empty_merged_tree_id(&self) -> MergedTreeId {
+        MergedTreeId::Legacy(self.backend.empty_tree_id().clone())
+    }
+
     pub fn root_commit_id(&self) -> &CommitId {
         self.backend.root_commit_id()
     }

lib/src/working_copy.rs

@@ -455,14 +455,14 @@ impl TreeState {
     }
 
     fn empty(store: Arc<Store>, working_copy_path: PathBuf, state_path: PathBuf) -> TreeState {
-        let tree_id = store.empty_tree_id().clone();
+        let tree_id = store.empty_merged_tree_id();
         // Canonicalize the working copy path because "repo/." makes libgit2 think that
         // everything should be ignored
         TreeState {
             store,
             working_copy_path: working_copy_path.canonicalize().unwrap(),
             state_path,
-            tree_id: MergedTreeId::Legacy(tree_id.clone()),
+            tree_id,
             file_states: BTreeMap::new(),
             sparse_patterns: vec![RepoPath::root()],
             own_mtime: MillisSinceEpoch(0),

lib/tests/test_commit_builder.rs

@@ -12,7 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-use jj_lib::backend::{ChangeId, MergedTreeId, MillisSinceEpoch, ObjectId, Signature, Timestamp};
+use jj_lib::backend::{ChangeId, MillisSinceEpoch, ObjectId, Signature, Timestamp};
 use jj_lib::matchers::EverythingMatcher;
 use jj_lib::merged_tree::DiffSummary;
 use jj_lib::repo::Repo;
@@ -202,7 +202,7 @@ fn test_rewrite_update_missing_user(use_git: bool) {
         .new_commit(
             &missing_user_settings,
             vec![repo.store().root_commit_id().clone()],
-            MergedTreeId::Legacy(repo.store().empty_tree_id().clone()),
+            repo.store().empty_merged_tree_id(),
         )
         .write()
         .unwrap();
@@ -258,7 +258,7 @@ fn test_commit_builder_descendants(use_git: bool) {
         .new_commit(
             &settings,
             vec![store.root_commit_id().clone()],
-            MergedTreeId::Legacy(store.empty_tree_id().clone()),
+            store.empty_merged_tree_id(),
         )
         .write()
         .unwrap();

lib/tests/test_git.rs

@@ -21,8 +21,7 @@ use assert_matches::assert_matches;
 use git2::Oid;
 use itertools::Itertools;
 use jj_lib::backend::{
-    BackendError, ChangeId, CommitId, MergedTreeId, MillisSinceEpoch, ObjectId, Signature,
-    Timestamp,
+    BackendError, ChangeId, CommitId, MillisSinceEpoch, ObjectId, Signature, Timestamp,
 };
 use jj_lib::commit::Commit;
 use jj_lib::commit_builder::CommitBuilder;
@@ -2304,7 +2303,7 @@ fn create_rooted_commit<'repo>(
         .new_commit(
             settings,
             vec![mut_repo.store().root_commit_id().clone()],
-            MergedTreeId::Legacy(mut_repo.store().empty_tree_id().clone()),
+            mut_repo.store().empty_merged_tree_id(),
         )
         .set_author(signature.clone())
         .set_committer(signature)

lib/tests/test_id_prefix.rs

@@ -13,7 +13,7 @@
 // limitations under the License.
 
 use itertools::Itertools;
-use jj_lib::backend::{CommitId, MergedTreeId, MillisSinceEpoch, ObjectId, Signature, Timestamp};
+use jj_lib::backend::{CommitId, MillisSinceEpoch, ObjectId, Signature, Timestamp};
 use jj_lib::id_prefix::IdPrefixContext;
 use jj_lib::index::HexPrefix;
 use jj_lib::index::PrefixResolution::{AmbiguousMatch, NoMatch, SingleMatch};
@@ -43,7 +43,7 @@ fn test_id_prefix() {
             .new_commit(
                 &settings,
                 vec![parent_id.clone()],
-                MergedTreeId::Legacy(repo.store().empty_tree_id().clone()),
+                repo.store().empty_merged_tree_id(),
             )
             .set_author(signature.clone())
             .set_committer(signature)

lib/tests/test_init.rs

@@ -14,7 +14,6 @@
 
 use std::path::{Path, PathBuf};
 
-use jj_lib::backend::MergedTreeId;
 use jj_lib::git_backend::GitBackend;
 use jj_lib::op_store::WorkspaceId;
 use jj_lib::repo::Repo;
@@ -124,10 +123,7 @@ fn test_init_checkout(use_git: bool) {
         .get_wc_commit_id(&WorkspaceId::default())
         .unwrap();
     let wc_commit = repo.store().get_commit(wc_commit_id).unwrap();
-    assert_eq!(
-        *wc_commit.tree_id(),
-        MergedTreeId::Legacy(repo.store().empty_tree_id().clone())
-    );
+    assert_eq!(*wc_commit.tree_id(), repo.store().empty_merged_tree_id());
     assert_eq!(
         wc_commit.store_commit().parents,
         vec![repo.store().root_commit_id().clone()]

lib/tests/test_merged_tree.rs

@@ -184,8 +184,7 @@ fn test_from_legacy_tree() {
 
     // Also test that MergedTreeBuilder can create the same tree by starting from an
     // empty legacy tree.
-    let mut tree_builder =
-        MergedTreeBuilder::new(MergedTreeId::Legacy(store.empty_tree_id().clone()));
+    let mut tree_builder = MergedTreeBuilder::new(store.empty_merged_tree_id());
     for (path, value) in tree.entries() {
         tree_builder.set_or_remove(path, Merge::normal(value));
     }

lib/tests/test_mut_repo.rs

@@ -12,7 +12,6 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-use jj_lib::backend::MergedTreeId;
 use jj_lib::op_store::{RefTarget, WorkspaceId};
 use jj_lib::repo::Repo;
 use maplit::hashset;
@@ -104,7 +103,7 @@ fn test_checkout_previous_empty(use_git: bool) {
         .new_commit(
             &settings,
             vec![repo.store().root_commit_id().clone()],
-            MergedTreeId::Legacy(repo.store().empty_tree_id().clone()),
+            repo.store().empty_merged_tree_id(),
         )
         .write()
         .unwrap();
@@ -135,7 +134,7 @@ fn test_checkout_previous_empty_with_description(use_git: bool) {
         .new_commit(
             &settings,
             vec![repo.store().root_commit_id().clone()],
-            MergedTreeId::Legacy(repo.store().empty_tree_id().clone()),
+            repo.store().empty_merged_tree_id(),
         )
         .set_description("not empty")
         .write()
@@ -167,7 +166,7 @@ fn test_checkout_previous_empty_with_local_branch(use_git: bool) {
         .new_commit(
             &settings,
             vec![repo.store().root_commit_id().clone()],
-            MergedTreeId::Legacy(repo.store().empty_tree_id().clone()),
+            repo.store().empty_merged_tree_id(),
         )
         .write()
         .unwrap();
@@ -199,7 +198,7 @@ fn test_checkout_previous_empty_non_head(use_git: bool) {
         .new_commit(
             &settings,
             vec![repo.store().root_commit_id().clone()],
-            MergedTreeId::Legacy(repo.store().empty_tree_id().clone()),
+            repo.store().empty_merged_tree_id(),
         )
         .write()
         .unwrap();

lib/tests/test_revset.rs

@@ -21,9 +21,7 @@ use std::path::Path;
 
 use assert_matches::assert_matches;
 use itertools::Itertools;
-use jj_lib::backend::{
-    ChangeId, CommitId, MergedTreeId, MillisSinceEpoch, ObjectId, Signature, Timestamp,
-};
+use jj_lib::backend::{ChangeId, CommitId, MillisSinceEpoch, ObjectId, Signature, Timestamp};
 use jj_lib::commit::Commit;
 use jj_lib::git;
 use jj_lib::git_backend::GitBackend;
@@ -119,7 +117,7 @@ fn test_resolve_symbol_commit_id() {
             .new_commit(
                 &settings,
                 vec![repo.store().root_commit_id().clone()],
-                MergedTreeId::Legacy(repo.store().empty_tree_id().clone()),
+                repo.store().empty_merged_tree_id(),
             )
             .set_description(format!("test {i}"))
             .set_author(signature.clone())

lib/tests/test_working_copy.rs

@@ -54,10 +54,7 @@ fn test_root(use_git: bool) {
         .unwrap();
     let wc_commit = repo.store().get_commit(wc_commit_id).unwrap();
     assert_eq!(new_tree.id(), *wc_commit.tree_id());
-    assert_eq!(
-        new_tree.id(),
-        MergedTreeId::Legacy(repo.store().empty_tree_id().clone())
-    );
+    assert_eq!(new_tree.id(), repo.store().empty_merged_tree_id());
 }
 
 #[test_case(false ; "local backend")]
@@ -176,10 +173,8 @@ fn test_checkout_file_transitions(use_git: bool) {
     if use_git {
         kinds.push(Kind::GitSubmodule);
     }
-    let mut left_tree_builder =
-        MergedTreeBuilder::new(MergedTreeId::Legacy(store.empty_tree_id().clone()));
-    let mut right_tree_builder =
-        MergedTreeBuilder::new(MergedTreeId::Legacy(store.empty_tree_id().clone()));
+    let mut left_tree_builder = MergedTreeBuilder::new(store.empty_merged_tree_id());
+    let mut right_tree_builder = MergedTreeBuilder::new(store.empty_merged_tree_id());
     let mut files = vec![];
     for left_kind in &kinds {
         for right_kind in &kinds {
@@ -435,7 +430,7 @@ fn test_snapshot_racy_timestamps(use_git: bool) {
     let workspace_root = test_workspace.workspace.workspace_root().clone();
 
     let file_path = workspace_root.join("file");
-    let mut previous_tree_id = MergedTreeId::Legacy(repo.store().empty_tree_id().clone());
+    let mut previous_tree_id = repo.store().empty_merged_tree_id();
     let wc = test_workspace.workspace.working_copy_mut();
     for i in 0..100 {
         {
@@ -724,7 +719,7 @@ fn test_dotgit_ignored(use_git: bool) {
         "contents",
     );
     let new_tree = test_workspace.snapshot().unwrap();
-    let empty_tree_id = MergedTreeId::Legacy(store.empty_tree_id().clone());
+    let empty_tree_id = store.empty_merged_tree_id();
     assert_eq!(new_tree.id(), empty_tree_id);
     std::fs::remove_dir_all(&dotgit_path).unwrap();
 
@@ -870,10 +865,7 @@ fn test_fsmonitor() {
     {
         let mut locked_wc = wc.start_mutation().unwrap();
         let tree_id = snapshot(&mut locked_wc, &[]);
-        assert_eq!(
-            tree_id,
-            MergedTreeId::Legacy(repo.store().empty_tree_id().clone())
-        );
+        assert_eq!(tree_id, repo.store().empty_merged_tree_id());
         locked_wc.discard();
     }