mu-labels: improve command-line handling

Esp., better handle the 'no subcommand' case: give the user some more useful
help.

mu/mu-cmd-labels.cc

@@ -168,7 +168,10 @@ label_import(Mu::Store& store, const Options& opts)
 Result<void>
 Mu::mu_cmd_labels(Mu::Store &store, const Options &opts)
 {
-	switch (opts.labels.sub) {
+	if (!opts.labels.sub)
+		return Ok(); // nothing to do.
+
+	switch (*opts.labels.sub) {
 	case Options::Labels::Sub::List:
 		return label_list(store, opts);
 	case Options::Labels::Sub::RestoreList:
@@ -181,7 +184,6 @@ Mu::mu_cmd_labels(Mu::Store &store, const Options &opts)
 		return label_export(store, opts);
 	case Options::Labels::Sub::Import:
 		return label_import(store, opts);
-
 	default:
 		return Err(Error{Error::Code::Internal,
 				"invalid sub-command"});
@@ -191,7 +193,7 @@ Mu::mu_cmd_labels(Mu::Store &store, const Options &opts)
 #ifdef BUILD_TESTS
 
 /*
- * Tests.
+ * Tests...
  *
  */
 #include <config.h>

mu/mu-options.cc

@@ -496,9 +496,8 @@ sub_init(CLI::App& sub, Options& opts)
 static void
 sub_labels(CLI::App& sub, Options& opts)
 {
-	sub.require_subcommand(0);
+	sub.require_subcommand(0, 1);
 
-	// update
 	auto update{sub.add_subcommand("update", "update labels")};
 	update->add_option("--labels", opts.labels.delta_labels,
 		       "One or more comma-separated +label,-label")
@@ -506,12 +505,11 @@ sub_labels(CLI::App& sub, Options& opts)
 		->type_name("<delta-label>")
 		->required();
 	update->add_flag("-n,--dry-run", opts.labels.dry_run,
-		     "Output what would change without changing anything");
+			 "Output what would change without changing anything");
 	update->add_option("query", opts.labels.query, "Query for messages to update")
 		->required();
 	add_muhome_option(*update, opts);
 
-	// clear
 	auto clear = sub.add_subcommand("clear", "clear all labels from matched messages");
 	clear ->add_option("query", opts.labels.query, "Query for messages to clear of labels")
 		->required();
@@ -519,22 +517,18 @@ sub_labels(CLI::App& sub, Options& opts)
 		     "Output what would change without changing anything");
 	add_muhome_option(*clear, opts);
 
-	// list
 	[[maybe_unused]] auto list = sub.add_subcommand("list", "list labels in the store");
 	add_muhome_option(*list, opts);
 
-	// restore-list
 	[[maybe_unused]] auto restore_list = sub.add_subcommand(
 		"restore-list", "restore the labels cache");
 	add_muhome_option(*restore_list, opts);
 
-	// export
 	[[maybe_unused]] auto exportsub = sub.add_subcommand("export", "export labels to a file");
 	add_muhome_option(*exportsub, opts);
 	exportsub->add_option("output", opts.labels.file, "File to export labels to")
 		->type_name("<file>");
 
-	// import
 	auto importsub = sub.add_subcommand("import", "import labels from a file");
 	importsub->add_flag("-n,--dry-run", opts.labels.dry_run,
 			    "Output what would change without changing anything");
@@ -543,12 +537,6 @@ sub_labels(CLI::App& sub, Options& opts)
 		->type_name("<file>");
 	add_muhome_option(*importsub, opts);
 
-	// XXX: it'd be nice to make "update" the default command, such that
-	//   mu label foo --labels +a,-b
-	// would be interpreted as
-	//   mu label update foo --labels +a,-b
-	// but no succeeded yet; CLI11 treats 'foo' as unknown sub-command
-
 	sub.final_callback([&](){
 		if (sub.got_subcommand("list")) {
 			opts.labels.sub = Options::Labels::Sub::List;
@@ -568,6 +556,9 @@ sub_labels(CLI::App& sub, Options& opts)
 		} else if (sub.got_subcommand("import")){
 			opts.labels.sub = Options::Labels::Sub::Import;
 			opts.labels.read_only = opts.labels.dry_run;
+		} else {
+			mu_println("{}", sub.help());
+			opts.labels.read_only = true;
 		}
 	});
 }
@@ -905,8 +896,8 @@ Copyright (C) 2008-2025 Dirk-Jan C. Binnema
 
 License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>.
 This is free software: you are free to change and redistribute it.
-There is NO WARRANTY, to the extent permitted by law.
+There is NO WARRANTY, to the extent permitted by law.)");
-)");
+
 	app.set_version_flag("-V,--version", PACKAGE_VERSION);
 	app.set_help_flag("-h,--help", "Show help information");
 	app.set_help_all_flag("--help-all");

mu/mu-options.hh

@@ -220,7 +220,7 @@ struct Options {
 			Export, // export labels
 			Import, // import labels
 		};
-		Sub sub;
+		Option<Sub> sub; // the chosen sub-command
 	} labels;
 
 	/*