cli: include diff summary in commit/describe editor content

src/commands.rs

@@ -1879,7 +1879,8 @@ fn cmd_describe(
     } else if let Some(message) = &args.message {
         description = message.into()
     } else {
-        description = edit_description(ui, workspace_command.repo(), commit.description())?;
+        let template = description_template_for_commit(&workspace_command, &commit)?;
+        description = edit_description(ui, workspace_command.repo(), &template)?;
     }
     if description == *commit.description() {
         ui.write("Nothing changed.\n")?;
@@ -1908,7 +1909,8 @@ fn cmd_commit(ui: &mut Ui, command: &CommandHelper, args: &CommitArgs) -> Result
     let description = if let Some(message) = &args.message {
         message.into()
     } else {
-        edit_description(ui, workspace_command.repo(), commit.description())?
+        let template = description_template_for_commit(&workspace_command, &commit)?;
+        edit_description(ui, workspace_command.repo(), &template)?
     };
     commit_builder = commit_builder.set_description(description);
     let mut tx = workspace_command.start_transaction(&format!("commit {}", commit.id().hex()));
@@ -2457,6 +2459,27 @@ don't make any changes, then the operation will be aborted.",
     Ok(())
 }
 
+fn description_template_for_commit(
+    workspace_command: &WorkspaceCommandHelper,
+    commit: &Commit,
+) -> Result<String, CommandError> {
+    let mut diff_summary_bytes = Vec::new();
+    diff_util::show_patch(
+        &mut PlainTextFormatter::new(&mut diff_summary_bytes),
+        workspace_command,
+        commit,
+        &EverythingMatcher,
+        &[DiffFormat::Summary],
+    )?;
+    if diff_summary_bytes.is_empty() {
+        Ok(commit.description().to_owned())
+    } else {
+        Ok(commit.description().to_owned()
+            + "\n"
+            + &diff_summary_to_description(&diff_summary_bytes))
+    }
+}
+
 fn description_template_for_cmd_split(
     workspace_command: &WorkspaceCommandHelper,
     intro: &str,
@@ -2473,13 +2496,17 @@ fn description_template_for_cmd_split(
         &EverythingMatcher,
         &[DiffFormat::Summary],
     )?;
-    let diff_summary = std::str::from_utf8(&diff_summary_bytes).expect(
+    Ok(format!("JJ: {intro}\n{overall_commit_description}\n")
+        + &diff_summary_to_description(&diff_summary_bytes))
+}
+
+fn diff_summary_to_description(bytes: &[u8]) -> String {
+    let text = std::str::from_utf8(bytes).expect(
         "Summary diffs and repo paths must always be valid UTF8.",
         // Double-check this assumption for diffs that include file content.
     );
-    Ok(format!("JJ: {intro}\n{overall_commit_description}\n")
-        + "JJ: This commit contains the following changes:\n"
-        + &textwrap::indent(diff_summary, "JJ:     "))
+    "JJ: This commit contains the following changes:\n".to_owned()
+        + &textwrap::indent(text, "JJ:     ")
 }
 
 fn cmd_split(ui: &mut Ui, command: &CommandHelper, args: &SplitArgs) -> Result<(), CommandError> {

tests/test_commit_command.rs

@@ -43,7 +43,7 @@ fn test_commit_with_editor() {
     // set a new one
     test_env.jj_cmd_success(&workspace_path, &["describe", "-m=initial"]);
     let edit_script = test_env.set_up_fake_editor();
-    std::fs::write(edit_script, ["dump editor0", "write\nmodified"].join("\0")).unwrap();
+    std::fs::write(&edit_script, ["dump editor0", "write\nmodified"].join("\0")).unwrap();
     test_env.jj_cmd_success(&workspace_path, &["commit"]);
     insta::assert_snapshot!(get_log_output(&test_env, &workspace_path), @r###"
     @ 3df78bc2b9b5 (no description set)
@@ -56,6 +56,23 @@ fn test_commit_with_editor() {
 
     JJ: Lines starting with "JJ: " (like this one) will be removed.
     "###);
+
+    // Check that the editor content includes diff summary
+    std::fs::write(workspace_path.join("file1"), "foo\n").unwrap();
+    std::fs::write(workspace_path.join("file2"), "foo\n").unwrap();
+    test_env.jj_cmd_success(&workspace_path, &["describe", "-m=add files"]);
+    std::fs::write(&edit_script, "dump editor1").unwrap();
+    test_env.jj_cmd_success(&workspace_path, &["commit"]);
+    insta::assert_snapshot!(
+        std::fs::read_to_string(test_env.env_root().join("editor1")).unwrap(), @r###"
+    add files
+
+    JJ: This commit contains the following changes:
+    JJ:     A file1
+    JJ:     A file2
+
+    JJ: Lines starting with "JJ: " (like this one) will be removed.
+    "###);
 }
 
 #[test]