bug#74964: 31.0.50 master; .dir-locals.el vars does not work with changing major mode

[Stefan Monnier via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

>>>>>> On Thu, 19 Dec 2024 15:53:58 +0800, Yikai Zhao <yikai@z1k.dev> said:
>
>     Yikai> How to reproduce:
>     Yikai> With following content in /tmp/test/.dir-locals.el:
>
>     Yikai> ((nil . ((my/test . "test")))
>     Yikai>  (c-mode . ((eval . (when (eq major-mode 'c-mode)
>     Yikai>                       (c++-mode))))))
>     Yikai> ;; I use this code because I want my projects' .h files always opened
>     Yikai> in c++ mode

We need to offer our users a better solution than such a hack.
E.g. a dir-local way to set `auto-mode-alist` and/or `major-mode-remap-alist`.

> Stefan, reverting f713258416f (Bug#74349) fixes this

How 'bout a patch like the one below?


        Stefan


diff --git a/lisp/files.el b/lisp/files.el
index c92fc0608dd..594ee83efbe 100644
--- a/lisp/files.el
+++ b/lisp/files.el
@@ -4488,11 +4485,15 @@ hack-one-local-variable--obsolete
                      (substitute-command-keys instead)
                    (format-message "use `%s' instead" instead)))))))
 
+(defvar hack-local-variables--inhibit nil
+  "List of file/dir local variables to ignore.")
+
 (defun hack-one-local-variable (var val)
   "Set local variable VAR with value VAL.
 If VAR is `mode', call `VAL-mode' as a function unless it's
 already the major mode."
   (pcase var
+    ((guard (memq var hack-local-variables--inhibit)) nil)
     ('mode
      (let ((mode (intern (concat (downcase (symbol-name val))
                                  "-mode"))))
@@ -4500,7 +4501,8 @@ hack-one-local-variable
     ('eval
      (pcase val
        (`(add-hook ',hook . ,_) (hack-one-local-variable--obsolete hook)))
-     (let ((enable-local-variables nil)) ;FIXME: Should be buffer-local!
+     (let ((hack-local-variables--inhibit ;; FIXME: Should be buffer-local!
+            (cons 'eval hack-local-variables--inhibit)))
        (save-excursion (eval val t))))
     (_
      (hack-one-local-variable--obsolete var)