cli: default to log when no subcommand is provided

This is a convenience optimization to improve the default user
experience, since `jj log` is a frequently run command. Accessing the
help information explicitly still follows normal CLI conventions, and
instructions are displayed appropriately if the user happens to make a
mistake. Discoverability should not be adversely harmed.

Note that this behavior mirrors what Sapling does [2], where `sl` will
display the smartlog by default.

[1] clap-rs/clap#975
[2] https://sapling-scm.com/docs/overview/smartlog

CHANGELOG.md

@@ -79,6 +79,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 * `jj git fetch` and `jj git push` will now use the single defined remote even if it is not named "origin".
 
+* `jj` with no subcommand now defaults to `jj log` instead of showing help. This
+  command can be overridden by setting `ui.default-command`.
+
 ### Fixed bugs
 
 * Modify/delete conflicts now include context lines

docs/config.md

@@ -124,6 +124,16 @@ Which elements can be colored is not yet documented, but see
 the [default color configuration](https://github.com/martinvonz/jj/blob/main/src/config/colors.toml)
 for some examples of what's possible.
 
+### Default command
+
+When `jj` is run with no explicit subcommand, the value of the
+`ui.default-command` setting will used instead. Possible values are any valid
+subcommand name, subcommand alias, or user-defined alias (defaults to `"log"`).
+
+```toml
+ui.default-command = "log"
+```
+
 ### Diff format
 
 ```toml

src/cli_util.rs

@@ -2028,6 +2028,49 @@ impl ValueParserFactory for RevisionArg {
     }
 }
 
+fn resolve_default_command(
+    ui: &mut Ui,
+    config: &config::Config,
+    app: &Command,
+    string_args: &mut Vec<String>,
+) -> Result<(), CommandError> {
+    const PRIORITY_FLAGS: &[&str] = &["help", "--help", "-h", "--version", "-V"];
+
+    let has_priority_flag = string_args
+        .iter()
+        .any(|arg| PRIORITY_FLAGS.contains(&arg.as_str()));
+    if has_priority_flag {
+        return Ok(());
+    }
+
+    let app_clone = app
+        .clone()
+        .allow_external_subcommands(true)
+        .ignore_errors(true);
+    let matches = app_clone.try_get_matches_from(string_args.clone()).ok();
+
+    if let Some(matches) = matches {
+        if matches.subcommand_name().is_none() {
+            if config.get_string("ui.default-command").is_err() {
+                writeln!(
+                    ui.hint(),
+                    "Hint: Use `jj -h` for a list of available commands."
+                )?;
+                writeln!(
+                    ui.hint(),
+                    "Set the config `ui.default-command = \"log\"` to disable this message."
+                )?;
+            }
+            let default_command = config
+                .get_string("ui.default-command")
+                .unwrap_or("log".to_string());
+            // Insert the default command directly after the path to the binary.
+            string_args.insert(1, default_command);
+        }
+    }
+    Ok(())
+}
+
 fn resolve_aliases(
     config: &config::Config,
     app: &Command,
@@ -2119,6 +2162,7 @@ fn handle_early_args(
 }
 
 pub fn expand_args(
+    ui: &mut Ui,
     app: &Command,
     args_os: ArgsOs,
     config: &config::Config,
@@ -2132,6 +2176,7 @@ pub fn expand_args(
         }
     }
 
+    resolve_default_command(ui, config, app, &mut string_args)?;
     resolve_aliases(config, app, &string_args)
 }
 
@@ -2299,7 +2344,7 @@ impl CliRunner {
         layered_configs.read_user_config()?;
         let config = layered_configs.merge();
         ui.reset(&config)?;
-        let string_args = expand_args(&self.app, std::env::args_os(), &config)?;
+        let string_args = expand_args(ui, &self.app, std::env::args_os(), &config)?;
         let (matches, args) = parse_args(
             ui,
             &self.app,

src/commands/mod.rs

@@ -3474,9 +3474,7 @@ fn cmd_sparse(ui: &mut Ui, command: &CommandHelper, args: &SparseArgs) -> Result
 
 pub fn default_app() -> Command {
     let app: Command = Commands::augment_subcommands(Args::command());
-    app.arg_required_else_help(true)
-        .subcommand_required(true)
-        .version(env!("JJ_VERSION"))
+    app.version(env!("JJ_VERSION"))
 }
 
 pub fn run_command(

src/config-schema.json

@@ -51,6 +51,11 @@
                     "description": "Whether to allow initializing a repo with the native backend",
                     "default": false
                 },
+                "default-command": {
+                    "type": "string",
+                    "description": "Default command to run when no explicit command is given",
+                    "default": "log"
+                },
                 "default-revset": {
                     "type": "string",
                     "description": "Default set of revisions to show when no explicit revset is given for jj log and similar commands",

tests/test_alias.rs

@@ -69,7 +69,7 @@ fn test_alias_bad_name() {
     insta::assert_snapshot!(stderr, @r###"
     error: unrecognized subcommand 'foo.'
 
-    Usage: jj [OPTIONS] <COMMAND>
+    Usage: jj [OPTIONS] [COMMAND]
 
     For more information, try '--help'.
     "###);
@@ -86,7 +86,7 @@ fn test_alias_calls_unknown_command() {
     insta::assert_snapshot!(stderr, @r###"
     error: unrecognized subcommand 'nonexistent'
 
-    Usage: jj [OPTIONS] <COMMAND>
+    Usage: jj [OPTIONS] [COMMAND]
 
     For more information, try '--help'.
     "###);
@@ -123,7 +123,7 @@ fn test_alias_calls_help() {
 
     To get started, see the tutorial at https://github.com/martinvonz/jj/blob/main/docs/tutorial.md.
 
-    Usage: jj [OPTIONS] <COMMAND>
+    Usage: jj [OPTIONS] [COMMAND]
     "###);
 }
 

tests/test_global_opts.rs

@@ -47,12 +47,25 @@ fn test_non_utf8_arg() {
 #[test]
 fn test_no_subcommand() {
     let 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 stderr = test_env.jj_cmd_cli_error(test_env.env_root(), &[]);
-    insta::assert_snapshot!(stderr.lines().next().unwrap(), @"Jujutsu (An experimental VCS)");
+    // Outside of a repo.
+    let stderr = test_env.jj_cmd_failure(test_env.env_root(), &[]);
+    insta::assert_snapshot!(stderr, @r###"
+    Hint: Use `jj -h` for a list of available commands.
+    Set the config `ui.default-command = "log"` to disable this message.
+    Error: There is no jj repo in "."
+    "###);
 
-    let stderr = test_env.jj_cmd_cli_error(test_env.env_root(), &["-R."]);
-    insta::assert_snapshot!(stderr.lines().next().unwrap(), @"error: 'jj' requires a subcommand but one was not provided");
+    test_env.add_config(r#"ui.default-command="log""#);
+    let stderr = test_env.jj_cmd_failure(test_env.env_root(), &[]);
+    insta::assert_snapshot!(stderr, @r###"
+    Error: There is no jj repo in "."
+    "###);
+
+    let stdout = test_env.jj_cmd_success(test_env.env_root(), &["--help"]);
+    insta::assert_snapshot!(stdout.lines().next().unwrap(), @"Jujutsu (An experimental VCS)");
 
     let stdout = test_env.jj_cmd_success(test_env.env_root(), &["--version"]);
     let sanitized = stdout.replace(|c: char| c.is_ascii_hexdigit(), "?");
@@ -62,8 +75,28 @@ fn test_no_subcommand() {
         "{sanitized}"
     );
 
-    let stdout = test_env.jj_cmd_success(test_env.env_root(), &["--help"]);
-    insta::assert_snapshot!(stdout.lines().next().unwrap(), @"Jujutsu (An experimental VCS)");
+    let stdout = test_env.jj_cmd_success(test_env.env_root(), &["-R", "repo"]);
+    assert_eq!(stdout, test_env.jj_cmd_success(&repo_path, &["log"]));
+
+    // Inside of a repo.
+    let stdout = test_env.jj_cmd_success(&repo_path, &[]);
+    assert_eq!(stdout, test_env.jj_cmd_success(&repo_path, &["log"]));
+
+    let stdout = test_env.jj_cmd_success(&repo_path, &["-T", "show"]);
+    let stdout = stdout.lines().skip(2).join("\n");
+    insta::assert_snapshot!(stdout, @r###"
+    │  Author: Test User <test.user@example.com> (2001-02-03 04:05:07.000 +07:00)
+    │  Committer: Test User <test.user@example.com> (2001-02-03 04:05:07.000 +07:00)
+    │
+    │      (no description set)
+    │
+    ◉  Commit ID: 0000000000000000000000000000000000000000
+       Change ID: zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz
+       Author:  <> (1970-01-01 00:00:00.000 +00:00)
+       Committer:  <> (1970-01-01 00:00:00.000 +00:00)
+
+           (no description set)
+    "###);
 }
 
 #[test]