if code == "package main" && pkg != "cmd/go" && pkg != "cmd/cgo" {
			return false
		}
		if !strings.HasPrefix(p, "//") {
			break
		}
		if strings.HasPrefix(p, "//go:build ") {
			matched, err := matchexpr(p[len("//go:build "):])
			if err != nil {
				errprintf("%s: %v", file, err)
			}
			return matched
		}
	}

	return true
}