static LineAndColumn charIndexToLineAndColumn(S text, int charIndex) {
  int line = 1, col = 1;
  charIndex = min(charIndex, l(text));
  for i to charIndex: {
    if (text.charAt(i) == '\n') {
      ++line;
      col = 1;
    } else
      ++col;
  }
  ret LineAndColumn(line, col);
}

Began life as a copy of #1033994