feature/featuretags: add test that all ts_omit_foo tags are declared

Updates #12614

Change-Id: I49351fe0c463af0b8d940e8088d4748906a8aec3
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

feature/buildfeatures/feature_completion_scripts_disabled.go

@@ -0,0 +1,13 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Code generated by gen.go; DO NOT EDIT.
+
+//go:build ts_omit_completion_scripts
+
+package buildfeatures
+
+// HasCompletionScripts is whether the binary was built with support for modular feature "embed CLI shell completion scripts".
+// Specifically, it's whether the binary was NOT built with the "ts_omit_completion_scripts" build tag.
+// It's a const so it can be used for dead code elimination.
+const HasCompletionScripts = false

feature/buildfeatures/feature_completion_scripts_enabled.go

@@ -0,0 +1,13 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Code generated by gen.go; DO NOT EDIT.
+
+//go:build !ts_omit_completion_scripts
+
+package buildfeatures
+
+// HasCompletionScripts is whether the binary was built with support for modular feature "embed CLI shell completion scripts".
+// Specifically, it's whether the binary was NOT built with the "ts_omit_completion_scripts" build tag.
+// It's a const so it can be used for dead code elimination.
+const HasCompletionScripts = true

feature/featuretags/featuretags.go

@@ -139,7 +139,11 @@ var Features = map[FeatureTag]FeatureMeta{
 	},
 	"completion": {Sym: "Completion", Desc: "CLI shell completion"},
 	"conn25":     {Sym: "Conn25", Desc: "Route traffic for configured domains through connector devices"},
-	"cloud":      {Sym: "Cloud", Desc: "detect cloud environment to learn instances IPs and DNS servers"},
+	"completion_scripts": {
+		Sym: "CompletionScripts", Desc: "embed CLI shell completion scripts",
+		Deps: []FeatureTag{"completion"},
+	},
+	"cloud": {Sym: "Cloud", Desc: "detect cloud environment to learn instances IPs and DNS servers"},
 	"dbus": {
 		Sym:                  "DBus",
 		Desc:                 "Linux DBus support",

feature/featuretags/featuretags_test.go

@@ -5,7 +5,12 @@ package featuretags
 
 import (
 	"maps"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"regexp"
 	"slices"
+	"strings"
 	"testing"
 
 	"tailscale.com/util/set"
@@ -83,3 +88,38 @@ func TestRequiredBy(t *testing.T) {
 		}
 	}
 }
+
+// Verify that all "ts_omit_foo" build tags are declared in featuretags.go
+func TestAllOmitBuildTagsDeclared(t *testing.T) {
+	dir, err := os.Getwd()
+	if err != nil {
+		t.Fatal(err)
+	}
+	root := filepath.Join(dir, "..", "..")
+
+	cmd := exec.Command("git", "grep", "ts_omit_")
+	cmd.Dir = root
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		if _, err := exec.LookPath("git"); err != nil {
+			t.Skipf("git not found in PATH; skipping test")
+		}
+		t.Fatalf("git grep failed: %v\nOutput:\n%s", err, out)
+	}
+	rx := regexp.MustCompile(`\bts_omit_[\w_]+\b`)
+	found := set.Set[string]{}
+	rx.ReplaceAllFunc(out, func(tag []byte) []byte {
+		tagStr := string(tag)
+		found.Add(tagStr)
+		return tag
+	})
+	for tag := range found {
+		if strings.EqualFold(tag, "ts_omit_foo") {
+			continue
+		}
+		ft := FeatureTag(strings.TrimPrefix(tag, "ts_omit_"))
+		if _, ok := Features[ft]; !ok {
+			t.Errorf("found undeclared ts_omit_* build tags: %v", tag)
+		}
+	}
+}