On Mon, 28 Apr 2025 14:01:18 -0400 Stefan Monnier <monnier@iro.umontreal.ca> wrote:

>> But actually there's a simpler and better way to do that: just duplicate
>> all the $s to $$, then substitute-in-file-name turns all the $$ back to
>> $ without expanding environment variables.  So I did that in the
>> attached updated patch.
>
> LGTM, pushed to `master`.

This patch causes two regressions wrt emacs-30.  One I pointed out
upthread (<87a58hal6w.fsf@gmx.net>,
<https://lists.gnu.org/archive/html/bug-gnu-emacs/2025-04/msg00884.html>).
For this, the following patch fixes the regression according to my
testing: