cmd/d2: Allow running subcommands on a bundled plugin

cmd/d2/help.go

@@ -37,7 +37,7 @@ func layoutHelp(ctx context.Context, ms *xmain.State) error {
 	} else if len(ms.FlagSet.Args()) == 2 {
 		return longLayoutHelp(ctx, ms)
 	} else {
-		return xmain.UsageErrorf("too many arguments passed")
+		return pluginSubcommand(ctx, ms)
 	}
 }
 
@@ -110,6 +110,17 @@ For more information on setup, please visit https://github.com/terrastruct/d2.`,
 		layout, strings.Join(names, ", "))
 }
 
+func pluginSubcommand(ctx context.Context, ms *xmain.State) error {
+	layout := ms.FlagSet.Arg(1)
+	plugin, _, err := d2plugin.FindPlugin(ctx, layout)
+	if errors.Is(err, exec.ErrNotFound) {
+		return layoutNotFound(ctx, layout)
+	}
+
+	ms.Args = ms.FlagSet.Args()[2:]
+	return d2plugin.Serve(plugin)(ctx, ms)
+}
+
 func humanPath(fp string) string {
 	if strings.HasPrefix(fp, os.Getenv("HOME")) {
 		return filepath.Join("~", strings.TrimPrefix(fp, os.Getenv("HOME")))

cmd/d2/main.go

@@ -40,7 +40,7 @@ func run(ctx context.Context, ms *xmain.State) (err error) {
 	err = ms.FlagSet.Parse(ms.Args)
 
 	if !errors.Is(err, pflag.ErrHelp) && err != nil {
-		return fmt.Errorf("failed to parse flags: %w", err)
+		return xmain.UsageErrorf("failed to parse flags: %v", err)
 	}
 
 	if len(ms.FlagSet.Args()) > 0 {