// Technically, it is required for uname and gname,
		// but neither GNU nor BSD tar checks for it.
		tooLong := len(s) > size
		allowLongGNU := paxKey == paxPath || paxKey == paxLinkpath
		if hasNUL(s) || (tooLong && !allowLongGNU) {
			whyNoGNU = fmt.Sprintf("GNU cannot encode %s=%q", name, s)
			format.mustNotBe(FormatGNU)
		}
		if !isASCII(s) || tooLong {
			canSplitUSTAR := paxKey == paxPath