Emacs master at 1b33c00f4c28d730b17293f94ce5325f7e916bc8 fails to build
for me as follows:

Loading /var/tmp/portage/app-editors/emacs-31.0.9999/work/emacs/lisp/ldefs-boot.el (source)...

Error: void-function (rx)
  (rx (or (seq "." (or "py" "pth" "pyi" "pyw")) (seq "/" (or "SConstruct" "SConscript"))) eos)
  (defconst python--auto-mode-alist-regexp (rx (or (seq "." (or "py" "pth" "pyi" "pyw")) (seq "/" (or "SConstruct" "SConscript"))) eos))
  eval-buffer(#<buffer  *load*> nil "/var/tmp/portage/app-editors/emacs-31.0.9999/work/emacs/lisp/ldefs-boot.el" nil t)
  (if eval-function (funcall eval-function buffer (if dump-mode file fullname)) (eval-buffer buffer nil (if dump-mode file fullname) nil t))
  (let ((read-symbol-shorthands shorthands)) (if eval-function (funcall eval-function buffer (if dump-mode file fullname)) (eval-buffer buffer nil (if dump-mode file fullname) nil t)))
  (let ((load-true-file-name fullname) (load-file-name fullname) (set-auto-coding-for-load t) (inhibit-file-name-operation nil) shorthands) (with-current-buffer buffer (set-buffer-multib
yte t) (let (deactivate-mark) (insert-file-contents fullname)) (setq shorthands (and hack-read-symbol-shorthands-function (funcall hack-read-symbol-shorthands-function))) (if (and enable
-multibyte-characters (or (eq (coding-system-type last-coding-system-used) 'raw-text))) (set-buffer-multibyte nil)) (set-buffer-modified-p nil)) (let ((read-symbol-shorthands shorthands)
) (if eval-function (funcall eval-function buffer (if dump-mode file fullname)) (eval-buffer buffer nil (if dump-mode file fullname) nil t))))
  (unwind-protect (let ((load-true-file-name fullname) (load-file-name fullname) (set-auto-coding-for-load t) (inhibit-file-name-operation nil) shorthands) (with-current-buffer buffer (s
et-buffer-multibyte t) (let (deactivate-mark) (insert-file-contents fullname)) (setq shorthands (and hack-read-symbol-shorthands-function (funcall hack-read-symbol-shorthands-function)))
 (if (and enable-multibyte-characters (or (eq (coding-system-type last-coding-system-used) 'raw-text))) (set-buffer-multibyte nil)) (set-buffer-modified-p nil)) (let ((read-symbol-shorth
ands shorthands)) (if eval-function (funcall eval-function buffer (if dump-mode file fullname)) (eval-buffer buffer nil (if dump-mode file fullname) nil t)))) (let (kill-buffer-hook kill
-buffer-query-functions) (kill-buffer buffer)))
  (let ((buffer (generate-new-buffer " *load*")) (load-in-progress t) (source (string-suffix-p ".el" fullname))) (unless nomessage (if source (message "Loading %s (source)..." file) (message "Loading %s..." file))) (when purify-flag (push (purecopy file) preloaded-file-list)) (unwind-protect (let ((load-true-file-name fullname) (load-file-name fullname) (set-auto-coding-for-load t) (inhibit-file-name-operation nil) shorthands) (with-current-buffer buffer (set-buffer-multibyte t) (let (deactivate-mark) (insert-file-contents fullname)) (setq shorthands (and hack-read-symbol-shorthands-function (funcall hack-read-symbol-shorthands-function))) (if (and enable-multibyte-characters (or (eq (coding-system-type last-coding-system-used) 'raw-text))) (set-buffer-multibyte nil)) (set-buffer-modified-p nil)) (let ((read-symbol-shorthands shorthands)) (if eval-function (funcall eval-function buffer (if dump-mode file fullname)) (eval-buffer buffer nil (if dump-mode file fullname) nil t)))) (let (kill-buffer-hook kill-buffer-query-functions) (kill-buffer buffer))) (do-after-load-evaluation fullname) (unless (or nomessage noninteractive) (if source (message "Loading %s (source)...done" file) (message "Loading %s...done" file))) t)
  (if (null (file-readable-p fullname)) (and (null noerror) (signal 'file-error (list "Cannot open load file" file))) (let ((buffer (generate-new-buffer " *load*")) (load-in-progress t) (source (string-suffix-p ".el" fullname))) (unless nomessage (if source (message "Loading %s (source)..." file) (message "Loading %s..." file))) (when purify-flag (push (purecopy file) preloaded-file-list)) (unwind-protect (let ((load-true-file-name fullname) (load-file-name fullname) (set-auto-coding-for-load t) (inhibit-file-name-operation nil) shorthands) (with-current-buffer buffer (set-buffer-multibyte t) (let (deactivate-mark) (insert-file-contents fullname)) (setq shorthands (and hack-read-symbol-shorthands-function (funcall hack-read-symbol-shorthands-function))) (if (and enable-multibyte-characters (or (eq (coding-system-type last-coding-system-used) 'raw-text))) (set-buffer-multibyte nil)) (set-buffer-modified-p nil)) (let ((read-symbol-shorthands shorthands)) (if eval-function (funcall eval-function buffer (if dump-mode file fullname)) (eval-buffer buffer nil (if dump-mode file fullname) nil t)))) (let (kill-buffer-hook kill-buffer-query-functions) (kill-buffer buffer))) (do-after-load-evaluation fullname) (unless (or nomessage noninteractive) (if source (message "Loading %s (source)...done" file) (message "Loading %s...done" file))) t))
  load-with-code-conversion("/var/tmp/portage/app-editors/emacs-31.0.9999/work/emacs/lisp/ldefs-boot.el" "/var/tmp/portage/app-editors/emacs-31.0.9999/work/emacs/lisp/ldefs-boot.el" nil nil)
  load("ldefs-boot.el")
  (condition-case nil (load "loaddefs") (file-error (load "ldefs-boot.el")))
  load("loadup.el")
Symbol's function definition is void: rx
make[2]: *** [Makefile:1016: bootstrap-emacs.pdmp] Error 255
make[2]: Leaving directory '/var/tmp/portage/app-editors/emacs-31.0.9999/work/emacs/src'
make[1]: *** [Makefile:554: src] Error 2
make[1]: Leaving directory '/var/tmp/portage/app-editors/emacs-31.0.9999/work/emacs'
make[1]: Entering directory
  '/var/tmp/portage/app-editors/emacs-31.0.9999/work/emacs'

I've attached the full build log. It is a clean build. Let me know if
more information is required.