Generate subcommand error during help

Author: ammario

README.md

@@ -66,6 +66,22 @@ Serpent is designed for high-configurability. To us, that means providing
 many ways to configure the same value (env, YAML, flags, etc.) and keeping
 the code clean and testable as you scale the number of options.
 
+Serpent's [Option](https://pkg.go.dev/github.com/coder/serpent#Option) type looks like:
+
+```go
+type Option {
+	Name string
+	Flag string
+	Env string
+	Default string
+	Value pflag.Value
+	// ...
+}
+```
+
+And is used by each [Cmd](https://pkg.go.dev/github.com/coder/serpent#Cmd) when
+passed as an array to the `Options` field.
+
 ## More coming...
 This README is a stub for now. We'll better explain the design and usage
 of `serpent` in the future.

cmd_test.go

@@ -157,6 +157,17 @@ func TestCommand(t *testing.T) {
 		require.Error(t, err)
 	})
 
+	t.Run("NoSubcommand", func(t *testing.T) {
+		t.Parallel()
+		i := cmd().Invoke(
+			"na",
+		)
+		io := fakeIO(i)
+		err := i.Run()
+		require.Error(t, err)
+		require.Contains(t, io.Stderr.String(), "unknown subcommand")
+	})
+
 	t.Run("UnknownFlags", func(t *testing.T) {
 		t.Parallel()
 		i := cmd().Invoke(

help.go

@@ -324,6 +324,12 @@ var usageWantsArgRe = regexp.MustCompile(`<.*>`)
 // output for a given command.
 func defaultHelpFn() HandlerFunc {
 	return func(inv *Invocation) error {
+		var unknownSubcommandErr error
+		if len(inv.Args) > 0 {
+			// Detected unknown subcommand.
+			unknownSubcommandErr = fmt.Errorf("error: unknown subcommand %q", strings.Join(inv.Args, " "))
+			_, _ = fmt.Fprintf(inv.Stderr, "%v\n\n", unknownSubcommandErr)
+		}
 		// We use stdout for help and not stderr since there's no straightforward
 		// way to distinguish between a user error and a help request.
 		//
@@ -347,6 +353,7 @@ func defaultHelpFn() HandlerFunc {
 		if len(inv.Args) > 0 && !usageWantsArgRe.MatchString(inv.Command.Use) {
 			_, _ = fmt.Fprintf(inv.Stderr, "---\nerror: unknown subcommand %q\n", inv.Args[0])
 		}
-		return nil
+		// Return an error so that exit status is non-zero.
+		return unknownSubcommandErr
 	}
 }