cli: add better error message when immutable_heads() cannot be resolved

cli/src/cli_util.rs

@@ -1012,7 +1012,12 @@ impl WorkspaceCommandHelper {
         let immutable = revset_util::parse_immutable_expression(&self.revset_parse_context())?;
         let mut expression = self.attach_revset_evaluator(immutable)?;
         expression.intersect_with(&to_rewrite_revset);
-        if let Some(commit_id) = expression.evaluate_to_commit_ids()?.next() {
+
+        let mut commit_id_iter = expression.evaluate_to_commit_ids().map_err(|e| {
+            config_error_with_message("Invalid `revset-aliases.immutable_heads()`", e)
+        })?;
+
+        if let Some(commit_id) = commit_id_iter.next() {
             let error = if &commit_id == self.repo().store().root_commit_id() {
                 user_error(format!(
                     "The root commit {} is immutable",

cli/tests/test_immutable_commits.rs

@@ -56,6 +56,29 @@ fn test_rewrite_immutable_generic() {
     insta::assert_snapshot!(stderr, @r###"
     Error: The root commit 000000000000 is immutable
     "###);
+
+    // Error mutating the repo if immutable_heads() uses a ref that can't be resolved
+    test_env.add_config(r#"revset-aliases."immutable_heads()" = "branch_that_does_not_exist""#);
+    let stderr = test_env.jj_cmd_failure(&repo_path, &["new", "main"]);
+    insta::assert_snapshot!(stderr, @r###"
+    Config error: Invalid `revset-aliases.immutable_heads()`
+    Caused by: Revision "branch_that_does_not_exist" doesn't exist
+    For help, see https://github.com/martinvonz/jj/blob/main/docs/config.md.
+    "###);
+
+    // Mutating the repo works if ref is wrapped in present()
+    test_env.add_config(
+        r#"revset-aliases."immutable_heads()" = "present(branch_that_does_not_exist)""#,
+    );
+    let (stdout, stderr) = test_env.jj_cmd_ok(&repo_path, &["new", "main"]);
+    insta::assert_snapshot!(stdout, @r###"
+    "###);
+    insta::assert_snapshot!(stderr, @r###"
+    Working copy now at: kpqxywon dbce15b4 (empty) (no description set)
+    Parent commit      : kkmpptxz c8d4c7ca main | b
+    Added 0 files, modified 1 files, removed 0 files
+    "###);
+
     // Error if we redefine immutable_heads() with an argument
     test_env.add_config(r#"revset-aliases."immutable_heads(foo)" = "none()""#);
     let stderr = test_env.jj_cmd_failure(&repo_path, &["edit", "root()"]);

docs/contributing.md

@@ -114,7 +114,7 @@ You will probably also want to make the `gh-pages` branch immutable (and thereby
 hidden from the default `jj log` output) by running the following in your repo:
 
 ```shell
-jj config set --repo "revset-aliases.immutable_heads()" "main@origin | gh-pages@origin"
+jj config set --repo "revset-aliases.immutable_heads()" 'remote_branches(exact:"main") | remote_branches(exact:"gh-pages")'
 ```
 
 ### Summary