It is the only instance in src/*.c of that literal value being
assigned to that struct member.  A truly singular hint?  :-D

I imagine that aside from ‘raw-text’, decoding is multi-layered
due to the presence of ‘undecided’ (which requires guesswork,
heuristics, and (maybe) backtracking) and that the design tries
to move data from bytes to characters as soon as possible, to
reduce downstream complexity and for cohesion w/ the rest of
Emacs.  But, that's merely ignorant speculation...

-- 
Thien-Thi Nguyen
   GPG key: 4C807502
   (if you're human and you know it)
      read my lisp: (responsep (questions 'technical)
                               (not (via 'mailing-list)))
                     => nil