pcase generates an unprintable expansion for a form in test erc--restore-initialize-priors

[Alan Mackenzie <acm@muc.de>]

Hello, Stefan and Emacs.

The ert test erc--restore-initialize-priors in test/lisp/erc/erc-tests.el
looks like this:

(ert-deftest erc--restore-initialize-priors ()
  (unless (>= emacs-major-version 28)
    (ert-skip "Lisp nesting exceeds `max-lisp-eval-depth'"))
  (should (pcase (macroexpand-1 '(erc--restore-initialize-priors erc-my-mode
                                   foo (ignore 1 2 3)
                                   bar #'spam
                                   baz nil))
            (`(let* ((,p (or erc--server-reconnecting erc--target-priors))
                     (,q (and ,p (alist-get 'erc-my-mode ,p))))
                (unless (local-variable-if-set-p 'erc-my-mode)
                  (error "Not a local minor mode var: %s" 'erc-my-mode))
                (setq foo (if ,q (alist-get 'foo ,p) (ignore 1 2 3))
                      bar (if ,q (alist-get 'bar ,p) #'spam)
                      baz (if ,q (alist-get 'baz ,p) nil)))
             t))))

..  Note that the pcase form is expanding a backquoted form lacking in
pcase features such as nested backquotes, pcase variable names, and the
like.

The pcase expansion thus consists of a deeply nested alternation of forms
like 
    (if (consp x1961) ...)
and
    (let* ((x1962 (car-safe x1961))) ...)
..

The level of nesting is greater than, or close enough to 200 that
printing it can lead to the detection of an apparent circular structure
in print_object in src/print.c.  There PRINT_CIRCLE is #defined as 200.

This leads the printer to printing out the error message:

    Apparently circular structure being printed

and throwing an error.  When this happened to me, the ert structure
containing this expansion was accessible from the stack, so it triggered
the error again recursively, resulting in the log file erc-tests.log
being 131 MB big.  I haven't looked to see what terminated this recursive
exception.

The situation seems to be what triggered bug#71178.

The form being compared using pcase, although not tiny, is not all that
big, and it would be easy to increase its size to cause it to violate any
reasonable value of PRINT_CIRCLE.

Would it be possible and a good idea to amend pcase such that it
generates less deeply nested expansions for forms such as we have here?
Or does anybody have any ideas how better to resolve the problem?

-- 
Alan Mackenzie (Nuremberg, Germany).