Allow customizing comment leader in editor

CHANGELOG.md

@@ -109,6 +109,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 * templates now support additional string methods `.starts_with(x)`, `.ends_with(x)`
   `.remove_prefix(x)`, `.remove_suffix(x)`, and `.substr(start, end)`.
 
+* The comment marker in editor prompts (e.g. `jj describe`) can now be customized
+  using the `ui.editor-comment-prefix` config option. The default remains `JJ: `.
+
 ### Fixed bugs
 
 * Fix issues related to .gitignore handling of untracked directories

cli/src/commands/mod.rs

@@ -20,8 +20,8 @@ mod git;
 mod operation;
 
 use std::collections::{BTreeMap, HashSet};
-use std::fmt::Debug;
-use std::io::{BufRead, Read, Seek, SeekFrom, Write};
+use std::fmt::{Debug, Display, Write as _};
+use std::io::{BufRead, Read, Seek, SeekFrom, Write as _};
 use std::path::{Path, PathBuf};
 use std::sync::Arc;
 use std::{fs, io};
@@ -67,6 +67,29 @@ use crate::graphlog::{get_graphlog, Edge};
 use crate::text_util;
 use crate::ui::Ui;
 
+/// Helper struct for [`comment_lines()`] which avoids allocations.
+struct CommentLines<'a> {
+    prefix: &'a str,
+    content: &'a str,
+}
+
+impl<'a> Display for CommentLines<'a> {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        let prefix = self.prefix;
+        let padding = if prefix.ends_with(' ') { "" } else { " " };
+        for line in self.content.lines() {
+            writeln!(f, "{prefix}{padding}{line}")?;
+        }
+        Ok(())
+    }
+}
+
+/// Prefixes each line of the passed string with the given comment marker.
+/// Always ensures a trailing newline.
+fn comment_lines<'a>(prefix: &'a str, s: &'a str) -> impl Display + 'a {
+    CommentLines { prefix, content: s }
+}
+
 #[derive(clap::Parser, Clone, Debug)]
 enum Commands {
     Abandon(AbandonArgs),
@@ -1893,12 +1916,22 @@ fn edit_description(
     settings: &UserSettings,
 ) -> Result<String, CommandError> {
     let description_file_path = (|| -> Result<_, io::Error> {
+        let comment_prefix = settings.comment_prefix();
         let mut file = tempfile::Builder::new()
             .prefix("editor-")
             .suffix(".jjdescription")
             .tempfile_in(repo.repo_path())?;
-        file.write_all(description.as_bytes())?;
-        file.write_all(b"\nJJ: Lines starting with \"JJ: \" (like this one) will be removed.\n")?;
+        writeln!(file, "{description}")?;
+        write!(
+            file,
+            "{}",
+            comment_lines(
+                &comment_prefix,
+                &format!(
+                    "Lines starting with \"{comment_prefix}\" (like this one) will be removed."
+                )
+            )
+        )?;
         let (_, path) = file.keep().map_err(|e| e.error)?;
         Ok(path)
     })()
@@ -1923,7 +1956,7 @@ fn edit_description(
     // Normalize line ending, remove leading and trailing blank lines.
     let description = description
         .lines()
-        .filter(|line| !line.starts_with("JJ: "))
+        .filter(|line| !line.starts_with(&settings.comment_prefix()))
         .join("\n");
     Ok(text_util::complete_newline(description.trim_matches('\n')))
 }
@@ -1976,7 +2009,9 @@ fn edit_sparse(
         .lines()
         .filter(|line| {
             line.as_ref()
-                .map(|line| !line.starts_with("JJ: ") && !line.trim().is_empty())
+                .map(|line| {
+                    !line.starts_with(&settings.comment_prefix()) && !line.trim().is_empty()
+                })
                 .unwrap_or(true)
         })
         .map(|line| {
@@ -2363,11 +2398,26 @@ fn combine_messages(
         } else if destination.description().is_empty() {
             source.description().to_string()
         } else {
-            let combined = "JJ: Enter a description for the combined commit.\n".to_string()
-                + "JJ: Description from the destination commit:\n"
-                + destination.description()
-                + "\nJJ: Description from the source commit:\n"
-                + source.description();
+            let comment_prefix = settings.comment_prefix();
+            let mut combined = comment_lines(
+                &comment_prefix,
+                "Enter a description for the combined commit.",
+            )
+            .to_string();
+            writeln!(
+                combined,
+                "{}{}",
+                comment_lines(&comment_prefix, "Description from the destination commit:"),
+                destination.description()
+            )
+            .unwrap();
+            write!(
+                combined,
+                "{}{}",
+                comment_lines(&comment_prefix, "Description from the source commit:"),
+                source.description()
+            )
+            .unwrap();
             edit_description(repo, &combined, settings)?
         }
     } else {
@@ -2997,7 +3047,7 @@ fn description_template_for_commit(
     if diff_summary_bytes.is_empty() {
         Ok(description)
     } else {
-        Ok(description + "\n" + &diff_summary_to_description(&diff_summary_bytes))
+        Ok(description + "\n" + &diff_summary_to_description(settings, &diff_summary_bytes))
     }
 }
 
@@ -3025,16 +3075,26 @@ fn description_template_for_cmd_split(
     } else {
         overall_commit_description.to_owned()
     };
-    Ok(format!("JJ: {intro}\n{description}\n") + &diff_summary_to_description(&diff_summary_bytes))
+    Ok(format!(
+        "{}{description}\n",
+        comment_lines(&settings.comment_prefix(), intro)
+    ) + &diff_summary_to_description(settings, &diff_summary_bytes))
 }
 
-fn diff_summary_to_description(bytes: &[u8]) -> String {
+fn diff_summary_to_description(settings: &UserSettings, 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.
     );
-    "JJ: This commit contains the following changes:\n".to_owned()
-        + &textwrap::indent(text, "JJ:     ")
+    let comment_prefix = settings.comment_prefix();
+    format!(
+        "{}{}",
+        comment_lines(
+            &comment_prefix,
+            "This commit contains the following changes:"
+        ),
+        comment_lines(&comment_prefix, &textwrap::indent(text, "    "))
+    )
 }
 
 #[instrument(skip_all)]

cli/src/config-schema.json

@@ -124,6 +124,11 @@
                     "type": "string",
                     "description": "Editor to use for commands that involve editing text"
                 },
+                "editor-comment-prefix": {
+                    "type": "string",
+                    "description": "Prefix that marks lines in editor prompts as comments",
+                    "default": "JJ: "
+                },
                 "diff-editor": {
                     "type": "string",
                     "description": "Editor tool to use for editing diffs",

cli/tests/test_comment_prefix.rs

@@ -0,0 +1,83 @@
+// Copyright 2023 The Jujutsu Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+use crate::common::TestEnvironment;
+
+pub mod common;
+
+#[test]
+fn test_describe() {
+    let mut test_env = TestEnvironment::default();
+    test_env.jj_cmd_success(test_env.env_root(), &["init", "repo", "--git"]);
+    let repo_path = test_env.env_root().join("repo");
+
+    let edit_script = test_env.set_up_fake_editor();
+
+    // Set an initial description using `-m` flag
+    let stdout = test_env.jj_cmd_success(&repo_path, &["describe", "-m", "description from CLI"]);
+    insta::assert_snapshot!(stdout, @r###"
+    Working copy now at: qpvuntsm cf3e8673 (empty) description from CLI
+    Parent commit      : zzzzzzzz 00000000 (empty) (no description set)
+    "###);
+
+    // Check that comment lines are commented and previous description is present
+    // uncommented
+    std::fs::write(&edit_script, "dump editor0").unwrap();
+    let stdout = test_env.jj_cmd_success(&repo_path, &["describe"]);
+    insta::assert_snapshot!(stdout, @r###"
+    Nothing changed.
+    "###);
+    insta::assert_snapshot!(
+        std::fs::read_to_string(test_env.env_root().join("editor0")).unwrap(), @r###"
+    description from CLI
+
+    JJ: Lines starting with "JJ: " (like this one) will be removed.
+    "###);
+
+    // Set a custom comment prefix
+    // https://github.com/rust-lang/rust-clippy/issues/11068
+    #[allow(clippy::needless_raw_string_hashes)]
+    test_env.add_config(r##"ui.editor-comment-prefix = "#""##);
+
+    // Check that the custom prefix is being used
+    std::fs::write(&edit_script, "dump editor0").unwrap();
+    let stdout = test_env.jj_cmd_success(&repo_path, &["describe"]);
+    insta::assert_snapshot!(stdout, @r###"
+    Nothing changed.
+    "###);
+    insta::assert_snapshot!(
+        std::fs::read_to_string(test_env.env_root().join("editor0")).unwrap(), @r###"
+    description from CLI
+
+    # Lines starting with "#" (like this one) will be removed.
+    "###);
+
+    // Set an empty prefix
+    #[allow(clippy::needless_raw_string_hashes)]
+    // https://github.com/rust-lang/rust-clippy/issues/11068
+    test_env.add_config(r##"ui.editor-comment-prefix = """##);
+
+    // Check that we fall back to the default
+    std::fs::write(&edit_script, "dump editor0").unwrap();
+    let stdout = test_env.jj_cmd_success(&repo_path, &["describe"]);
+    insta::assert_snapshot!(stdout, @r###"
+    Nothing changed.
+    "###);
+    insta::assert_snapshot!(
+        std::fs::read_to_string(test_env.env_root().join("editor0")).unwrap(), @r###"
+    description from CLI
+
+    JJ: Lines starting with "JJ: " (like this one) will be removed.
+    "###);
+}

lib/src/settings.rs

@@ -205,6 +205,14 @@ impl UserSettings {
             e @ Err(_) => e,
         }
     }
+
+    pub fn comment_prefix(&self) -> String {
+        self.config
+            .get_string("ui.editor-comment-prefix")
+            .ok()
+            .filter(|p| !p.is_empty())
+            .unwrap_or_else(|| "JJ: ".to_owned())
+    }
 }
 
 /// This Rng uses interior mutability to allow generating random values using an