Add runtime warnings

route/rule_user.go

@@ -4,9 +4,22 @@ import (
 	"strings"
 
 	"github.com/sagernet/sing-box/adapter"
+	"github.com/sagernet/sing-box/common/warning"
+	C "github.com/sagernet/sing-box/constant"
 	F "github.com/sagernet/sing/common/format"
 )
 
+var (
+	warnUserOnNonLinux = warning.New(
+		func() bool { return !C.IsLinux },
+		"rule item `user` is only supported on Linux",
+	)
+	warnUserOnCGODisabled = warning.New(
+		func() bool { return !C.CGO_ENABLED },
+		"rule item `user` is only supported with CGO enabled, rebuild with CGO_ENABLED=1",
+	)
+)
+
 var _ RuleItem = (*UserItem)(nil)
 
 type UserItem struct {
@@ -15,6 +28,8 @@ type UserItem struct {
 }
 
 func NewUserItem(users []string) *UserItem {
+	warnUserOnNonLinux.Check()
+	warnUserOnCGODisabled.Check()
 	userMap := make(map[string]bool)
 	for _, protocol := range users {
 		userMap[protocol] = true