bug#58506: Use ".dir-locals.eld" and ".dir-locals-2.eld" when they exist

[Stefan Kangas]

Severity: wishlist

[Spun out from Bug#58486.]

Juri Linkov <juri@linkov.net> writes:

> It's already handled by auto-mode-alist:
>
>   (defvar auto-mode-alist
>        ...
>        ;; .dir-locals.el is not really Elisp.  Could use the
>        ;; `dir-locals-file' constant if it weren't defined below.
>        ("\\.dir-locals\\(?:-2\\)?\\.el\\'" . lisp-data-mode)

It's nice if we can use the "*.eld" extension for files that are not
supposed to be executed.  But it's hard to just change that outright, as
a file ".dir-locals.eld" won't be used on old versions of Emacs.

So how about doing something like the below, as a future compatibility
patch?  Then, in some undefined future version, we can consider creating
".dir-locals.eld" files by default, instead.

diff --git a/lisp/files.el b/lisp/files.el
index 94d110f0b7..bfd1e5e8e1 100644
--- a/lisp/files.el
+++ b/lisp/files.el
@@ -4403,11 +4403,14 @@ dir-locals--all-files
                                         (dosified-file-name dir-locals-file)
                                       dir-locals-file)
                                     directory))
-           (file-2 (when (string-match "\\.el\\'" file-1)
+           (is-el (string-match (rx ".el" eos) file-1))
+           (file-2 (when is-el
                      (replace-match "-2.el" t nil file-1)))
           (out nil))
       ;; The order here is important.
-      (dolist (f (list file-2 file-1))
+      ;; Support *.eld files, too.
+      (dolist (f (list (and is-el (concat file-2 "d")) file-2
+                       (and is-el (concat file-1 "d")) file-1))
         (when (and f
                    (file-readable-p f)
                    ;; FIXME: Aren't file-regular-p and