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 +2028,33 @@ impl ValueParserFactory for RevisionArg {
     }
 }
 
+fn resolve_default_command(config: &config::Config, app: &Command, string_args: &mut Vec<String>) {
+    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;
+    }
+
+    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 matches
+        .as_ref()
+        .map_or(true, |m| m.subcommand_name().is_none())
+    {
+        // Insert the default command directly after the path to the binary.
+        let default_command = config
+            .get_string("ui.default-command")
+            .unwrap_or("log".to_string());
+        string_args.insert(1, default_command);
+    }
+}
+
 fn resolve_aliases(
     config: &config::Config,
     app: &Command,
@@ -2132,6 +2159,7 @@ pub fn expand_args(
         }
     }
 
+    resolve_default_command(config, app, &mut string_args);
     resolve_aliases(config, app, &string_args)
 }
 

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 +47,17 @@ 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]