bug#39190: 28.0.50; two buffers with same buffer-file-name (diff-syntax-fontify-props)

[Juri Linkov]

>> That's the problem with the current approach: we end up making decisions
>> which can't be justified.  Whether mode hooks should be run or not
>> depends on the hook and various other things.  What we *do* know, OTOH
>> is that `vc-find-revision-no-save`s buffer is not associated with
>> a file, so setting `buffer-file-name` can result in bugs.
>
> delay-mode-hooks was intended as the safest workaround to install in emacs-27.

delay-mode-hooks is now in emacs-27.

> Using alternatives of buffer-file-name are not straightforward
> and need more testing in emacs-28.

I don't know how to handle this more properly because some hooks
should be run with buffer-file-name, e.g. generic-mode-find-file-hook
that uses buffer-file-name to add more font-lock highlighting.
But some hooks like in Eglot assume that buffer-file-name associates the
buffer with a file.

Also I tried to use indirect buffers:

(let ((buffer (make-indirect-buffer
               (current-buffer)
               (generate-new-buffer-name (buffer-name)))))
  (funcall
   (unwind-protect
       (with-current-buffer buffer
         (let ((buffer-file-name "/tmp/foo.el"))
           (set-auto-mode)
           major-mode))
     (kill-buffer buffer))))

but not sure if it's enough to just set its deduced mode.