Subject: 30.0.50; prog-fill-reindent-defun vs. fill-paragraph for strings and comments

From: Jens Schmidt <[email-masked]>
Date: Fri, 08 Dec 2023 22:53:38 +0100

While preparing a reply for bug#67462 I came accross another issue where
the new binding of M-q to `prog-fill-reindent-defun' in Emacs 30 causes
a change in behavior compared to Emacs 29.

Here is a test case starting at "emacs -Q":

- In *scratch* goto EOB, then insert the following text all on one line
  (including double quotes, without leading blanks):

  "This buffer is for text that is not saved, and for Lisp evaluation. To create a file, visit it with ‘C-x C-f’ and enter text in its buffer."

- Then C-a and M-q: No reaction

    vs.

- Then C-a and M-x fill-paragraph RET: The string gets filled to

  "This buffer is for text that is not saved, and for Lisp
   evaluation. To create a file, visit it with ‘C-x C-f’ and enter
   text in its buffer."

The problem here being that `prog-fill-reindent-defun' has a rather
narrow range where it decides to fill intead of indenting:

  (or treesit-text-node
      (nth 8 (syntax-ppss))
      (re-search-forward "\\s-*\\s<" (line-end-position) t))

That is when point is

  - (not sure about the treesit condition) OR
  - *in* a text node OR
  - *in* a comment node OR
  - *before* whitespace => <-comment syntax

I guess that heuristics needs to be improved a bit, at least if you
intend to be compatible with the Emacs 29 behavior of M-q.

Here are some more examples where there are differences or
inconsistencies, with "pipe" denoting point:

  "any" prog mode

    "after a string"|
    |
    "on an empty line before a string"

      This one could differ in a treesitter mode, I guess.

  python-mode

    |"""doc string"""

      Same on as the initial example, but particularly nasty, since
      python-mode has a whole set of rules how to fill doc strings.

  c-mode

    |/* before a non-<-comment */
    /* after a non-<-comment */|

      Well, that doesn't really count, since c-mode and derivatives
      don't use prog-fill-reindent-defun, do they?  So let's go for
      good-old:

  sql-mode

    |/* before a non-<-comment */
    /* after a non-<-comment */|
    |-- before a non-<-comment

      sql-mode doesn't even have <-comments, so you always have to move
      point *into* a comment to get it filled.  BTW, exactly like
      javascript-mode, which is a bit more important, probably:

  javascript-mode

     |// before a non-<-comment

  emacs-lisp-mode

    (foo bar baz ?\|; baz bar foo)

      So this goes the other direction: If you M-q here and expect the
      line or surrounding function to be indented, you will get
      disappointed, because the heuristics detects this as a comment
      that needs to be filled.

      This one I also find questionable:

    (foo bar baz "string|" baz bar foo)

      Why would I want to fill here?  Here I'd like to indent.

Actually, now I think the fill-vs-indent heuristics of
`prog-fill-reindent-defun' needs to be improved a *lot* to make this
feature predictable and usable!  (And yes, I'll still follow up on
bug#67462 as well.)

In GNU Emacs 30.0.50 (build 2, x86_64-pc-linux-gnu, GTK+ Version
 3.24.24, cairo version 1.16.0) of 2023-12-08 built on sappc2
Repository revision: 3b1fd42732f7ca5b2db6ad6e75af1c037e1c54e4
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12011000
System Description: Debian GNU/Linux 11 (bullseye)

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