cli: backout: add templates.backout_description configuration

Closes #5676.

CHANGELOG.md

@@ -51,6 +51,9 @@ to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
   useful for making machine-readable templates by escaping problematic
   characters like `\n`.
 
+* The description of commits backed out by `jj backout` can now be configured
+  using `templates.backout_description`.
+
 ### Fixed bugs
 
 * `jj status` now shows untracked files under untracked directories.

cli/src/commands/backout.rs

@@ -12,6 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+use bstr::ByteVec as _;
 use clap_complete::ArgValueCandidates;
 use itertools::Itertools as _;
 use jj_lib::object_id::ObjectId;
@@ -22,9 +23,13 @@ use crate::cli_util::CommandHelper;
 use crate::cli_util::RevisionArg;
 use crate::command_error::CommandError;
 use crate::complete;
+use crate::formatter::PlainTextFormatter;
 use crate::ui::Ui;
 
-/// Apply the reverse of a revision on top of another revision
+/// Apply the reverse of given revisions on top of another revision
+///
+/// The description of the new revisions can be customized with the
+/// `templates.backout_description` config variable.
 #[derive(clap::Args, Clone, Debug)]
 pub(crate) struct BackoutArgs {
     /// The revision(s) to apply the reverse of
@@ -67,7 +72,6 @@ pub(crate) fn cmd_backout(
         let destination = workspace_command.resolve_single_rev(ui, revision_str)?;
         parents.push(destination);
     }
-    let mut tx = workspace_command.start_transaction();
     let transaction_description = if to_back_out.len() == 1 {
         format!("back out commit {}", to_back_out[0].id().hex())
     } else {
@@ -77,18 +81,31 @@ pub(crate) fn cmd_backout(
             to_back_out.len() - 1
         )
     };
+    let commits_to_back_out_with_new_commit_descriptions = {
+        let template_text = command
+            .settings()
+            .get_string("templates.backout_description")?;
+        let template = workspace_command.parse_commit_template(ui, &template_text)?;
+
+        to_back_out
+            .into_iter()
+            .map(|commit| {
+                let mut output = Vec::new();
+                template
+                    .format(&commit, &mut PlainTextFormatter::new(&mut output))
+                    .expect("write() to vec backed formatter should never fail");
+                // Template output is usually UTF-8, but it can contain file content.
+                let commit_description = output.into_string_lossy();
+                (commit, commit_description)
+            })
+            .collect_vec()
+    };
+    let mut tx = workspace_command.start_transaction();
     let mut new_base_tree = merge_commit_trees(tx.repo(), &parents)?;
-    for commit_to_back_out in to_back_out {
-        let commit_to_back_out_subject = commit_to_back_out
-            .description()
-            .lines()
-            .next()
-            .unwrap_or_default();
-        let new_commit_description = format!(
-            "Back out \"{}\"\n\nThis backs out commit {}.\n",
-            commit_to_back_out_subject,
-            &commit_to_back_out.id().hex()
-        );
+
+    for (commit_to_back_out, new_commit_description) in
+        commits_to_back_out_with_new_commit_descriptions
+    {
         let old_base_tree = commit_to_back_out.parent_tree(tx.repo())?;
         let old_tree = commit_to_back_out.tree()?;
         let new_tree = new_base_tree.merge(&old_tree, &old_base_tree)?;

cli/src/config/templates.toml

@@ -1,4 +1,12 @@
 [templates]
+backout_description = '''
+concat(
+  'Back out "' ++ description.first_line() ++ '"' ++ "\n",
+  "\n",
+  "This backs out commit " ++ commit_id ++ ".\n",
+)
+'''
+
 bookmark_list = '''
 if(remote,
   if(tracked,

cli/tests/cli-reference@.md.snap

@@ -122,7 +122,7 @@ To get started, see the tutorial at https://jj-vcs.github.io/jj/latest/tutorial/
 
 * `abandon` — Abandon a revision
 * `absorb` — Move changes from a revision into the stack of mutable revisions
-* `backout` — Apply the reverse of a revision on top of another revision
+* `backout` — Apply the reverse of given revisions on top of another revision
 * `bookmark` — Manage bookmarks [default alias: b]
 * `commit` — Update the description and create a new change on top
 * `config` — Manage config options
@@ -259,7 +259,9 @@ The modification made by `jj absorb` can be reviewed by `jj op show -p`.
 
 ## `jj backout`
 
-Apply the reverse of a revision on top of another revision
+Apply the reverse of given revisions on top of another revision
+
+The description of the new revisions can be customized with the `templates.backout_description` config variable.
 
 **Usage:** `jj backout [OPTIONS]`
 

cli/tests/test_backout_command.rs

@@ -188,6 +188,46 @@ fn test_backout_multiple() {
     "#);
 }
 
+#[test]
+fn test_backout_description_template() {
+    let test_env = TestEnvironment::default();
+    test_env.jj_cmd_ok(test_env.env_root(), &["git", "init", "repo"]);
+    test_env.add_config(
+        r#"
+        [templates]
+        backout_description = '''
+        separate(" ",
+          "Revert commit",
+          commit_id.short(),
+          '"' ++ description.first_line() ++ '"',
+        )
+        '''
+        "#,
+    );
+    let repo_path = test_env.env_root().join("repo");
+    create_commit(&test_env, &repo_path, "a", &[], &[("a", "a\n")]);
+
+    // Test the setup
+    insta::assert_snapshot!(get_log_output(&test_env, &repo_path), @r#"
+    @  2443ea76b0b1 a
+    ◆  000000000000
+    "#);
+    let stdout = test_env.jj_cmd_success(&repo_path, &["diff", "-s"]);
+    insta::assert_snapshot!(stdout, @r###"
+    A a
+    "###);
+
+    // Verify that message of backed out commit follows the template
+    let (stdout, stderr) = test_env.jj_cmd_ok(&repo_path, &["backout", "-r", "a"]);
+    insta::assert_snapshot!(stdout, @"");
+    insta::assert_snapshot!(stderr, @"");
+    insta::assert_snapshot!(get_log_output(&test_env, &repo_path), @r#"
+    ○  1db880a5204e Revert commit 2443ea76b0b1 "a"
+    @  2443ea76b0b1 a
+    ◆  000000000000
+    "#);
+}
+
 fn get_log_output(test_env: &TestEnvironment, cwd: &Path) -> String {
     let template = r#"commit_id.short() ++ " " ++ description"#;
     test_env.jj_cmd_success(cwd, &["log", "-T", template])