b := []byte(s)
	for i := 0; i < len(s); i++ {
		j := trimUTF8(b[:i])
		u := string([]rune(s[:j])) + string([]rune(s[j:]))
		if u != s {
			t.Errorf("trimUTF8(%q) = %d (-%d), not at boundary (split: %q %q)", s[:i], j, i-j, s[:j], s[j:])
		}
		if utf8.FullRune(b[j:i]) {
			t.Errorf("trimUTF8(%q) = %d (-%d), too early (missed: %q)", s[:j], j, i-j, s[j:i])
		}
	}

	return !unicode.IsLower(r)
}

// trimUTF8 returns a length t as close to len(b) as possible such that b[:t]
// does not end in the middle of a possibly-valid UTF-8 sequence.
//
// If a large text buffer must be split before position i at the latest,
// splitting at position trimUTF(b[:i]) avoids splitting a UTF-8 sequence.
func trimUTF8(b []byte) int {
	// Scan backward to find non-continuation byte.