Why looking-at-p works?

[Marcin Borkowski]

Hi all,

(defsubst looking-at-p (regexp)
  "\
Same as `looking-at' except this function does not change the match data."
  (let ((inhibit-changing-match-data t))
    (looking-at regexp)))

What happens is that if I make looking-at in the above code fail (e.g.,
by saying (looking-at-p 123)), inhibit-changing-match-data remains nil,
even though there is no unwind-protect here.  Why does it work like
this?

TIA,

-- 
Marcin Borkowski
http://mbork.pl