bug#66187: read-file-name unexpected behavior when MUSTMATCH is a function

[Eli Zaretskii <eliz@gnu.org>]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: joseph@breatheoutbreathe.in,  michael_heerdegen@web.de,
>   philipk@posteo.net,  66187@debbugs.gnu.org
> Date: Fri, 06 Oct 2023 12:43:51 -0400
> 
> >> Code which wants the 29.1 behavior can easily get it by modifying their
> >> `require-match` function accordingly (and such code then works
> >> correctly both with Emacs-29.1 and with the new behavior).
> > Do we have any users of this feature in the tree?
> 
> We do, and it turns out to be a good argument in favor of the change.
> 
> The use of a function for `completing-read`s REQUIRE-MATCH and
> `read-file-name`s MUSTMATCH was added in
> 
>     commit 49e06183f5972817d93dad6acf5351c204e61cc5
>     Author: Lars Ingebrigtsen <larsi@gnus.org>
>     Date:   Fri Jun 10 10:16:57 2022 +0200
>     
>         Allow REQUIRE-MATCH to be a function
>         
>         * doc/lispref/minibuf.texi (Minibuffer Completion): Document it.
>         
>         * lisp/minibuffer.el (completion--complete-and-exit): Allow
>         REQUIRE-MATCH to be a function.
>         (read-file-name): Mention it.
>         
>         * src/minibuf.c (Fcompleting_read): Mention it.
> 
> and the surrounding commits point to its motivation in:
> 
>     commit 7ee736a884766f2017a934d936bfbfa4c70b5099
>     Author: Lars Ingebrigtsen <larsi@gnus.org>
>     Date:   Fri Jun 10 10:19:15 2022 +0200
>     
>         Allow specifying a wildcard argument to list-directory again
>         
>         * lisp/files.el (list-directory): Allow specifying a wildcard
>         argument interactively again (bug#55877).
> 
> But the code in that patch uses MUSTMATCH to *restrict* the possible inputs,
> whereas Joseph's point is that the code currently does not allow using
> MUSTMATCH to restrict the possible inputs.
> 
> And indeed we can currently do `M-x list-directory RET /etc/passwd RET`
> and Emacs happily runs `list-directory` on that non-directory, contrary
> to the intention of the code (IIUC):
> 
>      [...]
>      (list (read-file-name
>             (if pfx "List directory (verbose): "
> 	      "List directory (brief): ")
> 	    nil default-directory
>             (lambda (file)
>               (or (file-directory-p file)
>                   (insert-directory-wildcard-in-dir-p
>                    (file-name-as-directory (expand-file-name file))))))
>      [...]
> 

So you are saying that the current uses of MUSTMATCH simply don't do
what the change was supposed to allow?