bug#31656: 26.1; `fill-paragraph' malformats in emacs-lisp-mode

[Noam Postavsky <npostavs@gmail.com>]

Eli Zaretskii <eliz@gnu.org> writes:

>> From: Stefan Guath <stefan@automata.se>
>> Date: Wed, 30 May 2018 14:50:01 +0200
>> 
>> `emacs-lisp-docstring-fill-column' shadows `fill-column' in too many cases in emacs-lisp-mode. The
>> documentation of `emacs-lisp-docstring-fill-column' states: "Value of ‘fill-column’ to use when filling a
>> docstring". But it incorrectly seems to be used in a lot more cases than just in a docstring (the only case that
>> I've found where ‘fill-column’ is actually respected is within comments). A work-around is to set
>> `emacs-lisp-docstring-fill-column' to nil, but it would be nice to have it working properly instead.
>> 
>> I might be missing something, but think the incorrect behavior is to be found in `lisp-fill-paragraph' that is
>> invoked by `fill-paragraph' through `fill-paragraph-function'. It seems like `lisp-fill-paragraph' unconditionally
>> sets `fill-column' to `emacs-lisp-docstring-fill-column' without checking whether point is within a doc string
>> first. The only requirements for enable shadowing currently seems to be "(and (integerp
>> emacs-lisp-docstring-fill-column) (derived-mode-p 'emacs-lisp-mode))", which doesn't seems sufficient.
>
> AFAICT, this behavior was in Emacs since about forever (since 1995, t
> be precise).  So maybe we just need to adjust the doc string to
> reflect the reality?
>
> Or are there real-life use cases where this behavior is grossly
> inappropriate?

I don't think it makes sense to apply normal plain text filling rules to
code.  Maybe it doesn't come up much because people don't usually call
M-q on code, and usually lines of code are kept short enough that they
wouldn't get filled anyway.  But picking a random example from rgrep
^.\{100,\}$ on the Emacs code base:

(defun feedmail-default-date-generator (maybe-file)
  "Default function for generating Date: header contents."
  (feedmail-say-debug ">in-> feedmail-default-date-generator")
  (when maybe-file
    (feedmail-say-debug (concat "4 cre " (feedmail-rfc822-date (nth 4 (file-attributes maybe-file)))))
    (feedmail-say-debug (concat "5 mod " (feedmail-rfc822-date (nth 5 (file-attributes maybe-file)))))
    (feedmail-say-debug (concat "6 sta " (feedmail-rfc822-date (nth 6 (file-attributes maybe-file))))))
  (let ((date-time))
    (if (and (not feedmail-queue-use-send-time-for-date) maybe-file)
	(setq date-time (nth 5 (file-attributes maybe-file))))
    (feedmail-rfc822-date date-time))
  )

Running M-q on every line turns it into this nonsense:

(defun feedmail-default-date-generator (maybe-file)
  "Default function for generating Date: header contents."
  (feedmail-say-debug ">in-> feedmail-default-date-generator")
  (when maybe-file
    (feedmail-say-debug (concat "4
    cre " (feedmail-rfc822-date (nth 4 (file-attributes
    maybe-file)))))
    (feedmail-say-debug (concat "5
    mod " (feedmail-rfc822-date (nth 5 (file-attributes
    maybe-file)))))
    (feedmail-say-debug (concat "6
    sta " (feedmail-rfc822-date (nth 6 (file-attributes
    maybe-file))))))
  (let ((date-time))
    (if (and (not feedmail-queue-use-send-time-for-date)
    maybe-file)
	(setq date-time (nth 5 (file-attributes maybe-file))))
    (feedmail-rfc822-date date-time)) )