cmd/tailscale: print a helpful error for Taildrive CLI on macOS GUI

Rather than printing `unknown subcommand: drive` for any Taildrive
commands run in the macOS GUI, print an error message directing the user
to the GUI client and the docs page.

Updates #17210
Fixes #18823

Change-Id: I6435007b5911baee79274b56e3ee101e6bb6d809
Signed-off-by: Alex Chan <alexc@tailscale.com>

cmd/tailscale/cli/drive_macgui.go

@@ -0,0 +1,33 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build !ts_omit_drive && ts_mac_gui
+
+package cli
+
+import (
+	"context"
+	"errors"
+
+	"github.com/peterbourgon/ff/v3/ffcli"
+)
+
+func init() {
+	maybeDriveCmd = driveCmdStub
+}
+
+func driveCmdStub() *ffcli.Command {
+	return &ffcli.Command{
+		Name:       "drive",
+		ShortHelp:  "Share a directory with your tailnet",
+		ShortUsage: "tailscale drive [...any]",
+		LongHelp:   hidden + "Taildrive allows you to share directories with other machines on your tailnet.",
+		Exec: func(_ context.Context, args []string) error {
+			return errors.New(
+				"Taildrive CLI commands are not supported when using the macOS GUI app. " +
+					"Please use the Tailscale menu bar icon to configure Taildrive in Settings.\n\n" +
+					"See https://tailscale.com/docs/features/taildrive",
+			)
+		},
+	}
+}