Rework mutual dependency between erc and erc-backend

* lisp/erc/erc.el: Declare needed variables exported by erc-backend.el
as special near the top of the file, and only require `erc-backend'
after providing `erc' as a feature at the very end.

* lisp/erc/erc-backend.el: Don't preemptively provide `erc-backend'.

* test/lisp/erc/erc-tests.el (erc--meta--backend-dependencies): Add
utility test to scrape for unused vars that may accumulate over time.
(Bug#54825)

lisp/erc/erc-backend.el

@@ -102,7 +102,6 @@
 ;; There's a fairly strong mutual dependency between erc.el and erc-backend.el.
 ;; Luckily, erc.el does not need erc-backend.el for macroexpansion whereas the
 ;; reverse is true:
-(provide 'erc-backend)
 (require 'erc)
 
 ;;;; Variables and options

lisp/erc/erc.el

@@ -130,7 +130,26 @@
   "Running scripts at startup and with /LOAD."
   :group 'erc)
 
-(require 'erc-backend)
+;; Defined in erc-backend
+(defvar erc--server-reconnecting)
+(defvar erc-channel-members-changed-hook)
+(defvar erc-server-367-functions)
+(defvar erc-server-announced-name)
+(defvar erc-server-connect-function)
+(defvar erc-server-connected)
+(defvar erc-server-current-nick)
+(defvar erc-server-filter-data)
+(defvar erc-server-lag)
+(defvar erc-server-last-sent-time)
+(defvar erc-server-parameters)
+(defvar erc-server-process)
+(defvar erc-server-quitting)
+(defvar erc-server-reconnect-count)
+(defvar erc-server-reconnecting)
+(defvar erc-session-client-certificate)
+(defvar erc-session-connector)
+(defvar erc-session-port)
+(defvar erc-session-server)
 
 ;; tunable connection and authentication parameters
 
@@ -7023,6 +7042,8 @@ Otherwise, connect to HOST:PORT as USER and /join CHANNEL."
 
 (provide 'erc)
 
+(require 'erc-backend)
+
 ;; Deprecated. We might eventually stop requiring the goodies automatically.
 ;; IMPORTANT: This require must appear _after_ the above (provide 'erc) to
 ;; avoid a recursive require error when byte-compiling the entire package.

test/lisp/erc/erc-tests.el

@@ -48,6 +48,27 @@
   (cl-letf (((symbol-function 'read-string) (lambda (&rest _) "1d")))
     (should (equal (erc--read-time-period "foo: ") 86400))))
 
+(ert-deftest erc--meta--backend-dependencies ()
+  (with-temp-buffer
+    (insert-file-contents-literally
+     (concat (file-name-sans-extension (symbol-file 'erc)) ".el"))
+    (let ((beg (search-forward ";; Defined in erc-backend"))
+          (end (search-forward "\n\n"))
+          vars)
+      (save-excursion
+        (save-restriction
+          (narrow-to-region beg end)
+          (goto-char (point-min))
+          (with-syntax-table lisp-data-mode-syntax-table
+            (condition-case _
+                (while (push (cadr (read (current-buffer))) vars))
+              (end-of-file)))))
+      (should (= (point) end))
+      (dolist (var vars)
+        (setq var (concat "\\_<" (symbol-name var) "\\_>"))
+        (ert-info (var)
+          (should (save-excursion (search-forward-regexp var nil t))))))))
+
 (ert-deftest erc-with-all-buffers-of-server ()
   (let (proc-exnet
         proc-onet