cli: port description template to templater

This implements a building block of "signed-off-by line" #1399 and "commit
--verbose" #1946. We'll probably need an easy way to customize the diff part,
but I'm not sure if it can be as simple as a template alias function. User
might want to embed diffs without "JJ: " prefixes?

Perhaps, we can deprecate "ui.default-description", but it's not addressed in
this patch. It could be replaced with "default_description" template alias,
but we might want to configure default per command. Suppose we add a default
"backout_description" template, it would have to be rendered against the
source commit, not the newly-created backout commit.

The template key is named as "draft_commit_description" because it is the
template to generate an editor template. "templates.commit_description_template"
sounds a bit odd.

There's one minor behavior change: the default description is now terminated
by "\n".

Closes #1354

CHANGELOG.md

@@ -82,6 +82,9 @@ to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 * New config setting `git.private-commits` to prevent commits from being pushed.
 
+* [The default commit description template](docs/config.md#default-description)
+  can now be configured by `templates.draft_commit_description`.
+
 ### Fixed bugs
 
 * `jj diff --git` no longer shows the contents of binary files.

cli/src/cli_util.rs

@@ -1638,6 +1638,10 @@ impl WorkspaceCommandTransaction<'_> {
         self.helper
     }
 
+    pub fn settings(&self) -> &UserSettings {
+        &self.helper.settings
+    }
+
     pub fn base_repo(&self) -> &Arc<ReadonlyRepo> {
         self.tx.base_repo()
     }

cli/src/commands/commit.rs

@@ -111,7 +111,7 @@ new working-copy commit.
             commit_builder.set_description(command.settings().default_description());
         }
         let temp_commit = commit_builder.write_hidden()?;
-        let template = description_template(ui, tx.base_workspace_helper(), "", &temp_commit)?;
+        let template = description_template(&tx, "", &temp_commit)?;
         edit_description(tx.base_repo(), &template, command.settings())?
     };
     commit_builder.set_description(description);

cli/src/commands/describe.rs

@@ -90,7 +90,7 @@ pub(crate) fn cmd_describe(
             commit_builder.set_description(command.settings().default_description());
         }
         let temp_commit = commit_builder.write_hidden()?;
-        let template = description_template(ui, tx.base_workspace_helper(), "", &temp_commit)?;
+        let template = description_template(&tx, "", &temp_commit)?;
         edit_description(tx.base_repo(), &template, command.settings())?
     };
     commit_builder.set_description(description);

cli/src/commands/split.rs

@@ -133,8 +133,7 @@ the operation will be aborted.
         }
         let temp_commit = commit_builder.write_hidden()?;
         let template = description_template(
-            ui,
-            tx.base_workspace_helper(),
+            &tx,
             "Enter a description for the first commit.",
             &temp_commit,
         )?;
@@ -176,8 +175,7 @@ the operation will be aborted.
         } else {
             let temp_commit = commit_builder.write_hidden()?;
             let template = description_template(
-                ui,
-                tx.base_workspace_helper(),
+                &tx,
                 "Enter a description for the second commit.",
                 &temp_commit,
             )?;

cli/src/config/templates.toml

@@ -21,6 +21,18 @@ if(overridden,
 ) ++ "\n"
 '''
 
+# TODO: Provide hook point for diff customization (#1946)? We might want a
+# syntax to comment out full text diffs without using the "JJ: " prefix.
+draft_commit_description = '''
+concat(
+  description,
+  surround(
+    "\nJJ: This commit contains the following changes:\n", "",
+    indent("JJ:     ", diff.summary()),
+  ),
+)
+'''
+
 log = 'builtin_log_compact'
 op_log = 'builtin_op_log_compact'
 show = 'builtin_log_detailed'

cli/src/description_util.rs

@@ -1,15 +1,15 @@
+use std::io::Write as _;
+
+use bstr::ByteVec as _;
 use itertools::Itertools;
 use jj_lib::commit::Commit;
-use jj_lib::matchers::EverythingMatcher;
 use jj_lib::repo::ReadonlyRepo;
 use jj_lib::settings::UserSettings;
 
-use crate::cli_util::{edit_temp_file, WorkspaceCommandHelper};
+use crate::cli_util::{edit_temp_file, WorkspaceCommandTransaction};
 use crate::command_error::CommandError;
-use crate::diff_util::DiffFormat;
 use crate::formatter::PlainTextFormatter;
 use crate::text_util;
-use crate::ui::Ui;
 
 pub fn edit_description(
     repo: &ReadonlyRepo,
@@ -89,37 +89,30 @@ pub fn join_message_paragraphs(paragraphs: &[String]) -> String {
         .join("\n")
 }
 
+/// Renders commit description template, which will be edited by user.
 pub fn description_template(
-    ui: &Ui,
-    workspace_command: &WorkspaceCommandHelper,
+    tx: &WorkspaceCommandTransaction,
     intro: &str,
     commit: &Commit,
 ) -> Result<String, CommandError> {
-    let mut diff_summary_bytes = Vec::new();
-    let diff_renderer = workspace_command.diff_renderer(vec![DiffFormat::Summary]);
-    diff_renderer.show_patch(
-        ui,
-        &mut PlainTextFormatter::new(&mut diff_summary_bytes),
-        commit,
-        &EverythingMatcher,
-    )?;
-    let mut template_chunks = Vec::new();
+    // TODO: Should "ui.default-description" be deprecated?
+    // We might want default description templates per command instead. For
+    // example, "backout_description" template will be rendered against the
+    // commit to be backed out, and the generated description could be set
+    // without spawning editor.
+
+    // Named as "draft" because the output can contain "JJ: " comment lines.
+    let template_key = "templates.draft_commit_description";
+    let template_text = tx.settings().config().get_string(template_key)?;
+    let template = tx.parse_commit_template(&template_text)?;
+
+    let mut output = Vec::new();
     if !intro.is_empty() {
-        template_chunks.push(format!("JJ: {intro}\n"));
+        writeln!(output, "JJ: {intro}").unwrap();
     }
-    template_chunks.push(commit.description().to_owned());
-    if !diff_summary_bytes.is_empty() {
-        template_chunks.push("\n".to_owned());
-        template_chunks.push(diff_summary_to_description(&diff_summary_bytes));
-    }
-    Ok(template_chunks.concat())
-}
-
-pub 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.
-    );
-    "JJ: This commit contains the following changes:\n".to_owned()
-        + &textwrap::indent(text, "JJ:     ")
+    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.
+    Ok(output.into_string_lossy())
 }

cli/tests/test_commit_command.rs

@@ -174,6 +174,7 @@ fn test_commit_with_default_description() {
 
 
     TESTED=TODO
+
     JJ: This commit contains the following changes:
     JJ:     A file1
     JJ:     A file2
@@ -182,6 +183,67 @@ fn test_commit_with_default_description() {
     "###);
 }
 
+#[test]
+fn test_commit_with_description_template() {
+    let mut test_env = TestEnvironment::default();
+    test_env.jj_cmd_ok(test_env.env_root(), &["git", "init", "repo"]);
+    test_env.add_config(
+        r#"
+        [templates]
+        draft_commit_description = '''
+        concat(
+          description,
+          "\n",
+          indent(
+            "JJ: ",
+            concat(
+              "Author: " ++ format_detailed_signature(author) ++ "\n",
+              "Committer: " ++ format_detailed_signature(committer)  ++ "\n",
+              "\n",
+              diff.stat(76),
+            ),
+          ),
+        )
+        '''
+        "#,
+    );
+    let workspace_path = test_env.env_root().join("repo");
+
+    let edit_script = test_env.set_up_fake_editor();
+    std::fs::write(edit_script, ["dump editor"].join("\0")).unwrap();
+
+    std::fs::write(workspace_path.join("file1"), "foo\n").unwrap();
+    std::fs::write(workspace_path.join("file2"), "bar\n").unwrap();
+
+    // Only file1 should be included in the diff
+    test_env.jj_cmd_ok(&workspace_path, &["commit", "file1"]);
+    insta::assert_snapshot!(
+        std::fs::read_to_string(test_env.env_root().join("editor")).unwrap(), @r###"
+
+    JJ: Author: Test User <[email protected]> (2001-02-03 08:05:08)
+    JJ: Committer: Test User <[email protected]> (2001-02-03 08:05:08)
+
+    JJ: file1 | 1 +
+    JJ: 1 file changed, 1 insertion(+), 0 deletions(-)
+
+    JJ: Lines starting with "JJ: " (like this one) will be removed.
+    "###);
+
+    // Timestamp after the reset should be available to the template
+    test_env.jj_cmd_ok(&workspace_path, &["commit", "--reset-author"]);
+    insta::assert_snapshot!(
+        std::fs::read_to_string(test_env.env_root().join("editor")).unwrap(), @r###"
+
+    JJ: Author: Test User <[email protected]> (2001-02-03 08:05:09)
+    JJ: Committer: Test User <[email protected]> (2001-02-03 08:05:09)
+
+    JJ: file2 | 1 +
+    JJ: 1 file changed, 1 insertion(+), 0 deletions(-)
+
+    JJ: Lines starting with "JJ: " (like this one) will be removed.
+    "###);
+}
+
 #[test]
 fn test_commit_without_working_copy() {
     let test_env = TestEnvironment::default();

cli/tests/test_describe_command.rs

@@ -271,6 +271,7 @@ fn test_describe_default_description() {
 
 
     TESTED=TODO
+
     JJ: This commit contains the following changes:
     JJ:     A file1
     JJ:     A file2

cli/tests/test_split_command.rs

@@ -248,6 +248,7 @@ fn test_split_with_default_description() {
 
 
     TESTED=TODO
+
     JJ: This commit contains the following changes:
     JJ:     A file1
 
@@ -366,6 +367,7 @@ fn test_split_siblings_no_descendants() {
 
 
     TESTED=TODO
+
     JJ: This commit contains the following changes:
     JJ:     A file1
 

docs/config.md

@@ -158,9 +158,24 @@ ui.default-command = ["log", "--reversed"]
 
 ### Default description
 
-The value of the `ui.default-description` setting will be used to prepopulate
-the editor when describing changes with an empty description. This could be a
-useful reminder to fill in things like BUG=, TESTED= etc.
+The editor content of a commit description can be populated by the
+`draft_commit_description` template.
+
+```toml
+[templates]
+draft_commit_description = '''
+concat(
+  description,
+  surround(
+    "\nJJ: This commit contains the following changes:\n", "",
+    indent("JJ:     ", diff.stat(72)),
+  ),
+)
+'''
+```
+
+The value of the `ui.default-description` setting can also be used in order to
+fill in things like BUG=, TESTED= etc.
 
 ```toml
 ui.default-description = "\n\nTESTED=TODO"