undo: show hint when workspace is removed by operation

When an operation like `jj undo` or `jj op revert` removes the current
workspace, show a helpful hint explaining what happened and how to recover.

Author: murataslan1

cli/src/cli_util.rs

@@ -2168,9 +2168,18 @@ See https://docs.jj-vcs.dev/latest/working-copy/#stale-working-copy \
         if self.may_update_working_copy {
             if let Some(new_commit) = &maybe_new_wc_commit {
                 self.update_working_copy(ui, maybe_old_wc_commit.as_ref(), new_commit)?;
-            } else {
-                // It seems the workspace was deleted, so we shouldn't try to
-                // update it.
+            } else if maybe_old_wc_commit.is_some() {
+                // Workspace was deleted by this operation
+                writeln!(
+                    ui.hint_default(),
+                    "Workspace '{name}' was removed by this operation.",
+                    name = self.workspace_name().as_symbol()
+                )?;
+                writeln!(
+                    ui.hint_no_heading(),
+                    "Use `jj workspace add` to create a new one, or `jj op revert` to restore \
+                     the previous state."
+                )?;
             }
         }
 

cli/tests/test_undo_redo_commands.rs

@@ -20,10 +20,14 @@ fn test_undo_root_operation() {
     test_env.run_jj_in(".", ["git", "init", "repo"]).success();
     let work_dir = test_env.work_dir("repo");
 
+    // Undoing the first operation after init restores to root operation,
+    // which removes the workspace. Show a helpful hint.
     let output = work_dir.run_jj(["undo"]);
     insta::assert_snapshot!(output, @r"
     ------- stderr -------
     Restored to operation: 000000000000 root()
+    Hint: Workspace 'default' was removed by this operation.
+    Use `jj workspace add` to create a new one, or `jj op revert` to restore the previous state.
     [EOF]
     ");
 
@@ -152,6 +156,8 @@ fn test_undo_with_rev_arg_falls_back_to_revert() {
     Warning: `jj undo <operation>` is deprecated; use `jj op revert <operation>` instead
     Reverted operation: 8f47435a3990 (2001-02-03 08:05:07) add workspace 'default'
     Rebased 1 descendant commits
+    Hint: Workspace 'default' was removed by this operation.
+    Use `jj workspace add` to create a new one, or `jj op revert` to restore the previous state.
     [EOF]
     ");
 

cli/tests/test_workspaces.rs

@@ -992,7 +992,12 @@ fn test_workspaces_forget() {
         .run_jj(["workspace", "add", "../secondary"])
         .success();
     let output = main_dir.run_jj(["workspace", "forget"]);
-    insta::assert_snapshot!(output, @"");
+    insta::assert_snapshot!(output, @r"
+    ------- stderr -------
+    Hint: Workspace 'default' was removed by this operation.
+    Use `jj workspace add` to create a new one, or `jj op revert` to restore the previous state.
+    [EOF]
+    ");
 
     // When listing workspaces, only the secondary workspace shows up
     let output = main_dir.run_jj(["workspace", "list"]);