working copy: add tree_id() to backend trait

Looks like I missed this earlier. I think it makes sense to have on
all working copy implementations.

cli/src/commands/debug.rs

@@ -105,7 +105,7 @@ pub fn cmd_debug(
             let workspace_command = command.workspace_helper(ui)?;
             let wc = workspace_command.working_copy();
             writeln!(ui.stdout(), "Current operation: {:?}", wc.operation_id())?;
-            writeln!(ui.stdout(), "Current tree: {:?}", wc.current_tree_id()?)?;
+            writeln!(ui.stdout(), "Current tree: {:?}", wc.tree_id()?)?;
             for (file, state) in wc.file_states()? {
                 writeln!(
                     ui.stdout(),

lib/src/local_working_copy.rs

@@ -1304,6 +1304,10 @@ impl WorkingCopy for LocalWorkingCopy {
         &self.checkout_state().operation_id
     }
 
+    fn tree_id(&self) -> Result<&MergedTreeId, WorkingCopyStateError> {
+        Ok(self.tree_state()?.current_tree_id())
+    }
+
     fn sparse_patterns(&self) -> Result<&[RepoPath], WorkingCopyStateError> {
         Ok(self.tree_state()?.sparse_patterns())
     }
@@ -1323,7 +1327,7 @@ impl WorkingCopy for LocalWorkingCopy {
             tree_state: OnceCell::new(),
         };
         let old_operation_id = wc.operation_id().clone();
-        let old_tree_id = wc.current_tree_id()?.clone();
+        let old_tree_id = wc.tree_id()?.clone();
         Ok(LockedLocalWorkingCopy {
             wc,
             lock,
@@ -1444,10 +1448,6 @@ impl LocalWorkingCopy {
         Ok(self.tree_state.get_mut().unwrap())
     }
 
-    pub fn current_tree_id(&self) -> Result<&MergedTreeId, WorkingCopyStateError> {
-        Ok(self.tree_state()?.current_tree_id())
-    }
-
     pub fn file_states(&self) -> Result<&BTreeMap<RepoPath, FileState>, WorkingCopyStateError> {
         Ok(self.tree_state()?.file_states())
     }
@@ -1564,7 +1564,7 @@ impl LockedWorkingCopy for LockedLocalWorkingCopy {
         mut self,
         operation_id: OperationId,
     ) -> Result<LocalWorkingCopy, WorkingCopyStateError> {
-        assert!(self.tree_state_dirty || &self.old_tree_id == self.wc.current_tree_id()?);
+        assert!(self.tree_state_dirty || &self.old_tree_id == self.wc.tree_id()?);
         if self.tree_state_dirty {
             self.wc
                 .tree_state_mut()?

lib/src/working_copy.rs

@@ -49,6 +49,9 @@ pub trait WorkingCopy {
     /// The operation this working copy was most recently updated to.
     fn operation_id(&self) -> &OperationId;
 
+    /// The ID of the tree this working copy was most recently updated to.
+    fn tree_id(&self) -> Result<&MergedTreeId, WorkingCopyStateError>;
+
     /// Patterns that decide which paths from the current tree should be checked
     /// out in the working copy. An empty list means that no paths should be
     /// checked out in the working copy. A single `RepoPath::root()` entry means

lib/tests/test_local_working_copy_concurrent.rs

@@ -18,7 +18,7 @@ use std::thread;
 use assert_matches::assert_matches;
 use jj_lib::repo::Repo;
 use jj_lib::repo_path::RepoPath;
-use jj_lib::working_copy::{CheckoutError, LockedWorkingCopy, SnapshotOptions};
+use jj_lib::working_copy::{CheckoutError, LockedWorkingCopy, SnapshotOptions, WorkingCopy};
 use jj_lib::workspace::Workspace;
 use testutils::{create_tree, write_working_copy_file, TestRepo, TestWorkspace};
 
@@ -67,7 +67,7 @@ fn test_concurrent_checkout() {
         &TestRepo::default_store_factories(),
     )
     .unwrap();
-    assert_eq!(*ws3.working_copy().current_tree_id().unwrap(), tree_id2);
+    assert_eq!(*ws3.working_copy().tree_id().unwrap(), tree_id2);
 }
 
 #[test]