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.

Since clap does not natively support setting a default subcommand [1],
it will only parse global options when invoking `jj` in this way. This
limitation is also why we have to create the LogArgs struct literal
(with derived default values) to pass through to the cmd_log function.

Note that this behavior (and limitation) 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 invokes `jj log` with the default options, instead
+  of showing help.
+
 ### Fixed bugs
 
 * Modify/delete conflicts now include context lines

src/commands/mod.rs

@@ -325,7 +325,7 @@ struct ShowArgs {
 struct StatusArgs {}
 
 /// Show commit history
-#[derive(clap::Args, Clone, Debug)]
+#[derive(clap::Args, Clone, Debug, Default)]
 struct LogArgs {
     /// Which revisions to show. Defaults to the `ui.default-revset` setting,
     /// or `@ | (remote_branches() | tags()).. | ((remote_branches() |
@@ -3474,18 +3474,28 @@ 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(
     ui: &mut Ui,
     command_helper: &CommandHelper,
     matches: &ArgMatches,
 ) -> Result<(), CommandError> {
-    let derived_subcommands: Commands = Commands::from_arg_matches(matches).unwrap();
-    match &derived_subcommands {
+    let derived_subcommands: Option<Commands> = Commands::from_arg_matches(matches).ok();
+
+    let subcommand = match derived_subcommands {
+        Some(command) => command,
+        None => {
+            // TODO: Make the default subcommand configurable.
+            let default_sub_args = LogArgs {
+                ..Default::default()
+            };
+            Commands::Log(default_sub_args)
+        }
+    };
+
+    match &subcommand {
         Commands::Version(sub_args) => cmd_version(ui, command_helper, sub_args),
         Commands::Init(sub_args) => cmd_init(ui, command_helper, sub_args),
         Commands::Config(sub_args) => cmd_config(ui, command_helper, sub_args),

src/diff_util.rs

@@ -32,7 +32,7 @@ use jujutsu_lib::{conflicts, diff, files, rewrite, tree};
 use crate::cli_util::{CommandError, WorkspaceCommandHelper};
 use crate::formatter::Formatter;
 
-#[derive(clap::Args, Clone, Debug)]
+#[derive(clap::Args, Clone, Debug, Default)]
 #[command(group(clap::ArgGroup::new("short-format").args(&["summary", "types"])))]
 #[command(group(clap::ArgGroup::new("long-format").args(&["git", "color_words"])))]
 pub struct DiffFormatArgs {

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 +47,17 @@ 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###"
+    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");
+    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 +67,12 @@ 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"]));
 }
 
 #[test]