in src/CodegenFunctionish.hack [229:241]
public function addHHFixMe(int $code, string $why): this {
$max_length = $this->getMaxCodeLength() - 6;
$str = \sprintf('HH_FIXME[%d] %s', $code, $why);
invariant(
\strlen($str) <= $max_length,
'ERROR: Your fixme has to fit on one line, with indentation '.
'and comments. So you need to shorten your message by %d '.
'characters.',
\strlen($str) - $max_length,
);
$this->fixme = $str;
return $this;
}