bug#36216: 27.0.50; Variable binding depth exceeds max-specpld-size during bootstrap

[Juanma Barranquero <lekktu@gmail.com>]

> Well, does the same command fails in the same way when invoked from
> the command line?

Once I remove the .elc files, any invocation of

./temacs --batch

(with or without explicitly loading loadup.el or passing the --temacs
arg) produces the same error, because it automatically loads
loadup.el.

> If so, just invoke it with the appropriate --eval
> argument that increases the value of max-specpdl-size.

AFAICS, setting max-specpdl-size in an --eval doesn't work, because
temacs loads loadup.el before processing the --eval. If I instrument
loadup.el to show its value, I get:

$ ./temacs --eval "(setq max-specpdl-size 1450)" --batch
Loading loadup.el (source)...
> (loadup.el) max-specpdl-size = 1300
dump mode: nil

Obviously, the problem disappears if I bind max-specpdl-size to a
bigger value around the load of cl-generic, or if I set it explicitly
in the conditional code at the beginning of loadup.el that also sets
max-lisp-eval-depth

(if (or (member dump-mode '("bootstrap" "pbootstrap"))
;; FIXME this is irritatingly fragile.
        (and (stringp (nth 4 command-line-args))
             (string-match "^unidata-gen\\(\\.elc?\\)?$"
                           (nth 4 command-line-args)))
        (member (nth 7 command-line-args) '("unidata-gen-file"
                                            "unidata-gen-charprop"))
        (null dump-mode))
    (progn
      [...etc etc...]
      (setq max-specpdl-size 1450)   ;;; <=== THIS WORKS
      ;; During bootstrapping the byte-compiler is run interpreted
      ;; when compiling itself, which uses a lot more stack
      ;; than usual.
      (setq max-lisp-eval-depth 2200)))

I wonder if it wouldn't just make sense to borrow the same trick
loadup.el uses with pcase.el to disable eager macroexpansion, i.e.,
something like

diff --git i/lisp/loadup.el w/lisp/loadup.el
index 67e8aa7d40..9f08b19043 100644
--- i/lisp/loadup.el
+++ w/lisp/loadup.el
@@ -246,7 +246,10 @@
 (load "language/cham")

 (load "indent")
-(load "emacs-lisp/cl-generic")
+(if (byte-code-function-p (symbol-function 'macroexpand-all))
+    (load "emacs-lisp/cl-generic")
+  (let ((macroexp--pending-eager-loads '(skip)))
+    (load "emacs-lisp/cl-generic")))
 (load "frame")
 (load "startup")
 (load "term/tty-colors")