Leverage loaddefs for migrating ERC modules

* lisp/erc/erc-common.el (erc--features-to-modules,
erc--modules-to-features, erc--module-name-migrations): Remove unused
internal functions.
(erc--normalize-module-symbol): Make aware of new migration scheme
based on symbol properties.
* lisp/erc/erc-page.el: Add autoload cookie for module migration.
* lisp/erc/erc-pcomplete.el: Add autoload cookies for module
migration.
* lisp/erc/erc-services.el: Add autoload cookie for module migration.
* lisp/erc/erc-sound.el: Add autoload cookie for module migration.
* lisp/erc/erc-stamp.el: Add autoload cookie for module migration.
* lisp/erc/erc.el (erc-modules): Reorder default value, sorted by
`string<' so that Customize does not consider the value to have been
edited.  Remove non-existent module `hecomplete' from lineup and swap
a couple more to maintain sorted order.  Change `:initialize' function
to tag all symbols for built-in modules with an `erc--module'
property.  In the `:set' function, ensure third-party modules appear
after the sorted and normalized built-ins, but in user-defined order.
Do this to prevent all modules, built-ins included, from ending up as
populated form fields for the "other" checkbox in the Customize
interface.
(erc--find-mode): Add helper function for `erc--update-modules'.
(erc--update-modules): Always resolve module names and only
conditionally attempt to require corresponding features.
* test/lisp/erc/erc-tests.el (erc-tests--modules): Add manifest for
asserting built-in modules and features.  This is easier to verify
visually than looking at the custom-type set for `erc-modules'.
(erc-modules--initialize): New test.
(erc-modules--internal-property): Add test.
(erc--normalize-module-symbol): New test.
(erc--find-mode): New test.
(erc--update-modules) Adapt to new paradigm and make more
comprehensive.  (Bug#60954.)

lisp/erc/erc-common.el

@@ -85,40 +85,13 @@
   (contents "" :type string)
   (tags '() :type list))
 
-;; TODO move goodies modules here after 29 is released.
-(defconst erc--features-to-modules
-  '((erc-pcomplete completion pcomplete)
-    (erc-capab capab-identify)
-    (erc-join autojoin)
-    (erc-page page ctcp-page)
-    (erc-sound sound ctcp-sound)
-    (erc-stamp stamp timestamp)
-    (erc-services services nickserv))
-  "Migration alist mapping a library feature to module names.
-Keys need not be unique: a library may define more than one
-module.  Sometimes a module's downcased alias will be its
-canonical name.")
-
-(defconst erc--modules-to-features
-  (let (pairs)
-    (pcase-dolist (`(,feature . ,names) erc--features-to-modules)
-      (dolist (name names)
-        (push (cons name feature) pairs)))
-    (nreverse pairs))
-  "Migration alist mapping a module's name to its home library feature.")
-
-(defconst erc--module-name-migrations
-  (let (pairs)
-    (pcase-dolist (`(,_ ,canonical . ,rest) erc--features-to-modules)
-      (dolist (obsolete rest)
-        (push (cons obsolete canonical) pairs)))
-    pairs)
-  "Association list of obsolete module names to canonical names.")
-
+;; After dropping 28, we can use prefixed "erc-autoload" cookies.
 (defun erc--normalize-module-symbol (symbol)
-  "Return preferred SYMBOL for `erc-modules'."
-  (setq symbol (intern (downcase (symbol-name symbol))))
-  (or (cdr (assq symbol erc--module-name-migrations)) symbol))
+  "Return preferred SYMBOL for `erc--modules'."
+  (while-let ((canonical (get symbol 'erc--module))
+              ((not (eq canonical symbol))))
+    (setq symbol canonical))
+  symbol)
 
 (defun erc--assemble-toggle (localp name ablsym mode val body)
   (let ((arg (make-symbol "arg")))

lisp/erc/erc-page.el

@@ -34,6 +34,7 @@
   "React to CTCP PAGE messages."
   :group 'erc)
 
+;;;###autoload(put 'ctcp-page 'erc--module 'page)
 ;;;###autoload(autoload 'erc-page-mode "erc-page")
 (define-erc-module page ctcp-page
   "Process CTCP PAGE requests from IRC."

lisp/erc/erc-pcomplete.el

@@ -56,6 +56,8 @@ add this string to nicks completed."
   "If t, order nickname completions with the most recent speakers first."
   :type 'boolean)
 
+;;;###autoload(put 'Completion 'erc--module 'completion)
+;;;###autoload(put 'pcomplete 'erc--module 'completion)
 ;;;###autoload(autoload 'erc-completion-mode "erc-pcomplete" nil t)
 (define-erc-module pcomplete Completion
   "In ERC Completion mode, the TAB key does completion whenever possible."

lisp/erc/erc-services.el

@@ -102,6 +102,7 @@ You can also use \\[erc-nickserv-identify-mode] to change modes."
 	 (when (featurep 'erc-services)
 	   (erc-nickserv-identify-mode val))))
 
+;;;###autoload(put 'nickserv 'erc--module 'services)
 ;;;###autoload(autoload 'erc-services-mode "erc-services" nil t)
 (define-erc-module services nickserv
   "This mode automates communication with services."

lisp/erc/erc-sound.el

@@ -47,6 +47,7 @@
 
 (require 'erc)
 
+;;;###autoload(put 'ctcp-sound 'erc--module 'sound)
 ;;;###autoload(autoload 'erc-sound-mode "erc-sound")
 (define-erc-module sound ctcp-sound
   "In ERC sound mode, the client will respond to CTCP SOUND requests

lisp/erc/erc-stamp.el

@@ -147,6 +147,10 @@ from entering them and instead jump over them."
   "ERC timestamp face."
   :group 'erc-faces)
 
+;; New libraries should only autoload the minor mode for a module's
+;; preferred name (rather than its alias).
+
+;;;###autoload(put 'timestamp 'erc--module 'stamp)
 ;;;###autoload(autoload 'erc-timestamp-mode "erc-stamp" nil t)
 (define-erc-module stamp timestamp
   "This mode timestamps messages in the channel buffers."

lisp/erc/erc.el

@@ -1839,9 +1839,9 @@ buffer rather than a server buffer.")
   ;; each item is in the format '(old . new)
   (delete-dups (mapcar #'erc--normalize-module-symbol mods)))
 
-(defcustom erc-modules '(netsplit fill button match track completion readonly
-                                  networks ring autojoin noncommands irccontrols
-                                  move-to-prompt stamp menu list)
+(defcustom erc-modules '( autojoin button completion fill irccontrols
+                          list match menu move-to-prompt netsplit
+                          networks noncommands readonly ring stamp track)
   "A list of modules which ERC should enable.
 If you set the value of this without using `customize' remember to call
 \(erc-update-modules) after you change it.  When using `customize', modules
@@ -1849,12 +1849,20 @@ removed from the list will be disabled."
   :get (lambda (sym)
          ;; replace outdated names with their newer equivalents
          (erc-migrate-modules (symbol-value sym)))
-  :initialize #'custom-initialize-default
+  ;; Expect every built-in module to have the symbol property
+  ;; `erc--module' set to its canonical symbol (often itself).
+  :initialize (lambda (symbol exp)
+                ;; Use `cdddr' because (set :greedy t . ,entries)
+                (dolist (entry (cdddr (get 'erc-modules 'custom-type)))
+                  (when-let* (((eq (car entry) 'const))
+                              (s (cadddr entry))) ; (const :tag "..." ,s)
+                    (put s 'erc--module s)))
+                (custom-initialize-reset symbol exp))
   :set (lambda (sym val)
          ;; disable modules which have just been removed
          (when (and (boundp 'erc-modules) erc-modules val)
            (dolist (module erc-modules)
-             (unless (member module val)
+             (unless (memq module val)
                (let ((f (intern-soft (format "erc-%s-mode" module))))
                  (when (and (fboundp f) (boundp f))
                    (when (symbol-value f)
@@ -1866,7 +1874,15 @@ removed from the list will be disabled."
                                           (when (symbol-value f)
                                             (funcall f 0))
                                           (kill-local-variable f)))))))))
-         (set sym val)
+         (let (built-in third-party)
+           (dolist (v val)
+             (setq v (erc--normalize-module-symbol v))
+             (if (get v 'erc--module)
+                 (push v built-in)
+               (push v third-party)))
+           ;; Calling `set-default-toplevel-value' complicates testing
+           (set sym (append (sort built-in #'string-lessp)
+                            (nreverse third-party))))
          ;; this test is for the case where erc hasn't been loaded yet
          (when (fboundp 'erc-update-modules)
            (erc-update-modules)))
@@ -1880,7 +1896,6 @@ removed from the list will be disabled."
            capab-identify)
     (const :tag "completion: Complete nicknames and commands (programmable)"
            completion)
-    (const :tag "hecomplete: Complete nicknames and commands (obsolete, use \"completion\")" hecomplete)
     (const :tag "dcc: Provide Direct Client-to-Client support" dcc)
     (const :tag "fill: Wrap long lines" fill)
     (const :tag "identd: Launch an identd server on port 8113" identd)
@@ -1897,11 +1912,11 @@ removed from the list will be disabled."
     (const :tag "networks: Provide data about IRC networks" networks)
     (const :tag "noncommands: Don't display non-IRC commands after evaluation"
            noncommands)
+    (const :tag "notifications: Desktop alerts on PRIVMSG or mentions"
+           notifications)
     (const :tag
            "notify: Notify when the online status of certain users changes"
            notify)
-    (const :tag "notifications: Send notifications on PRIVMSG or nickname mentions"
-           notifications)
     (const :tag "page: Process CTCP PAGE requests from IRC" page)
     (const :tag "readonly: Make displayed lines read-only" readonly)
     (const :tag "replace: Replace text in messages" replace)
@@ -1914,8 +1929,8 @@ removed from the list will be disabled."
     (const :tag "smiley: Convert smileys to pretty icons" smiley)
     (const :tag "sound: Play sounds when you receive CTCP SOUND requests"
            sound)
-    (const :tag "stamp: Add timestamps to messages" stamp)
     (const :tag "spelling: Check spelling" spelling)
+    (const :tag "stamp: Add timestamps to messages" stamp)
     (const :tag "track: Track channel activity in the mode-line" track)
     (const :tag "truncate: Truncate buffers to a certain size" truncate)
     (const :tag "unmorse: Translate morse code in messages" unmorse)
@@ -1929,18 +1944,28 @@ Except ignore all local modules, which were introduced in ERC 5.5."
   (erc--update-modules)
   nil)
 
+(defun erc--find-mode (sym)
+  (setq sym (erc--normalize-module-symbol sym))
+  (if-let* ((mode (intern-soft (concat "erc-" (symbol-name sym) "-mode")))
+            ((or (boundp mode)
+                 (and (fboundp mode)
+                      (autoload-do-load (symbol-function mode) mode)))))
+      mode
+    (and (require (or (get sym 'erc--feature)
+                      (intern (concat "erc-" (symbol-name sym))))
+                  nil 'noerror)
+         (setq mode (intern-soft (concat "erc-" (symbol-name sym) "-mode")))
+         (fboundp mode)
+         mode)))
+
 (defun erc--update-modules ()
   (let (local-modes)
     (dolist (module erc-modules local-modes)
-      (require (or (alist-get module erc--modules-to-features)
-                   (intern (concat "erc-" (symbol-name module))))
-               nil 'noerror) ; some modules don't have a corresponding feature
-      (let ((mode (intern-soft (concat "erc-" (symbol-name module) "-mode"))))
-        (unless (and mode (fboundp mode))
-          (error "`%s' is not a known ERC module" module))
-        (if (custom-variable-p mode)
-            (funcall mode 1)
-          (push mode local-modes))))))
+      (if-let ((mode (erc--find-mode module)))
+          (if (custom-variable-p mode)
+              (funcall mode 1)
+            (push mode local-modes))
+        (error "`%s' is not a known ERC module" module)))))
 
 (defun erc-setup-buffer (buffer)
   "Consults `erc-join-buffer' to find out how to display `BUFFER'."

test/lisp/erc/erc-tests.el

@@ -1224,6 +1224,85 @@
     (kill-buffer "baznet")
     (kill-buffer "#chan")))
 
+(defconst erc-tests--modules
+  '( autoaway autojoin button capab-identify completion dcc fill identd
+     irccontrols keep-place list log match menu move-to-prompt netsplit
+     networks noncommands notifications notify page readonly
+     replace ring sasl scrolltobottom services smiley sound
+     spelling stamp track truncate unmorse xdcc))
+
+;; Ensure that `:initialize' doesn't change the ordering of the
+;; members because otherwise the widget's state is "edited".
+
+(ert-deftest erc-modules--initialize ()
+  ;; This is `custom--standard-value' from Emacs 28.
+  (should (equal (eval (car (get 'erc-modules 'standard-value)) t)
+                 erc-modules)))
+
+;; Ensure the `:initialize' function for `erc-modules' successfully
+;; tags all built-in modules with the internal property `erc--module'.
+
+(ert-deftest erc-modules--internal-property ()
+  (let (ours)
+    (mapatoms (lambda (s)
+                (when-let ((v (get s 'erc--module))
+                           ((eq v s)))
+                  (push s ours))))
+    (should (equal (sort ours #'string-lessp) erc-tests--modules))))
+
+(ert-deftest erc--normalize-module-symbol ()
+  (dolist (mod erc-tests--modules)
+    (should (eq (erc--normalize-module-symbol mod) mod)))
+  (should (eq (erc--normalize-module-symbol 'pcomplete) 'completion))
+  (should (eq (erc--normalize-module-symbol 'Completion) 'completion))
+  (should (eq (erc--normalize-module-symbol 'ctcp-page) 'page))
+  (should (eq (erc--normalize-module-symbol 'ctcp-sound) 'sound))
+  (should (eq (erc--normalize-module-symbol 'timestamp) 'stamp))
+  (should (eq (erc--normalize-module-symbol 'nickserv) 'services)))
+
+;; Worrying about which library a module comes from is mostly not
+;; worth the hassle so long as ERC can find its minor mode.  However,
+;; bugs involving multiple modules living in the same library may slip
+;; by because a module's loading problems may remain hidden on account
+;; of its place in the default ordering.
+
+(ert-deftest erc--find-mode ()
+  (let* ((package (if-let* ((found (getenv "ERC_PACKAGE_NAME"))
+                            ((string-prefix-p "erc-" found)))
+                      (intern found)
+                    'erc))
+         (prog
+          `(,@(and (featurep 'compat)
+                   `((progn
+                       (require 'package)
+                       (let ((package-load-list '((compat t) (,package t))))
+                         (package-initialize)))))
+            (require 'erc)
+            (let ((mods (mapcar #'cadddr
+                                (cdddr (get 'erc-modules 'custom-type))))
+                  moded)
+              (setq mods
+                    (sort mods (lambda (a b) (if (zerop (random 2)) a b))))
+              (dolist (mod mods)
+                (unless (keywordp mod)
+                  (push (if-let ((mode (erc--find-mode mod)))
+                            mod
+                          (list :missing mod))
+                        moded)))
+              (message "%S"
+                       (sort moded
+                             (lambda (a b)
+                               (string< (symbol-name a) (symbol-name b))))))))
+         (proc (start-process "erc--module-mode-autoloads"
+                              (current-buffer)
+                              (concat invocation-directory invocation-name)
+                              "-batch" "-Q"
+                              "-eval" (format "%S" (cons 'progn prog)))))
+    (set-process-query-on-exit-flag proc t)
+    (while (accept-process-output proc 10))
+    (goto-char (point-min))
+    (should (equal (read (current-buffer)) erc-tests--modules))))
+
 (ert-deftest erc-migrate-modules ()
   (should (equal (erc-migrate-modules '(autojoin timestamp button))
                  '(autojoin stamp button)))
@@ -1234,17 +1313,28 @@
   (let (calls
         erc-modules
         erc-kill-channel-hook erc-kill-server-hook erc-kill-buffer-hook)
+
+    ;; This `lbaz' module is unknown, so ERC looks for it via the
+    ;; symbol proerty `erc--feature' and, failing that, by
+    ;; `require'ing its "erc-" prefixed symbol.
+    (should-not (intern-soft "erc-lbaz-mode"))
+
     (cl-letf (((symbol-function 'require)
-               (lambda (s &rest _) (push s calls)))
+               (lambda (s &rest _)
+                 (when (eq s 'erc--lbaz-feature)
+                   (fset (intern "erc-lbaz-mode") ; local module
+                         (lambda (n) (push (cons 'lbaz n) calls))))
+                 (push s calls)))
 
               ;; Local modules
-              ((symbol-function 'erc-fake-bar-mode)
-               (lambda (n) (push (cons 'fake-bar n) calls)))
+              ((symbol-function 'erc-lbar-mode)
+               (lambda (n) (push (cons 'lbar n) calls)))
+              ((get 'lbaz 'erc--feature) 'erc--lbaz-feature)
 
               ;; Global modules
-              ((symbol-function 'erc-fake-foo-mode)
-               (lambda (n) (push (cons 'fake-foo n) calls)))
-              ((get 'erc-fake-foo-mode 'standard-value) 'ignore)
+              ((symbol-function 'erc-gfoo-mode)
+               (lambda (n) (push (cons 'gfoo n) calls)))
+              ((get 'erc-gfoo-mode 'standard-value) 'ignore)
               ((symbol-function 'erc-autojoin-mode)
                (lambda (n) (push (cons 'autojoin n) calls)))
               ((get 'erc-autojoin-mode 'standard-value) 'ignore)
@@ -1255,20 +1345,28 @@
                (lambda (n) (push (cons 'completion n) calls)))
               ((get 'erc-completion-mode 'standard-value) 'ignore))
 
+      (ert-info ("Unknown module")
+        (setq erc-modules '(lfoo))
+        (should-error (erc--update-modules))
+        (should (equal (pop calls) 'erc-lfoo))
+        (should-not calls))
+
       (ert-info ("Local modules")
-        (setq erc-modules '(fake-foo fake-bar))
-        (should (equal (erc--update-modules) '(erc-fake-bar-mode)))
-        ;; Bar the feature is still required but the mode is not activated
-        (should (equal (nreverse calls)
-                       '(erc-fake-foo (fake-foo . 1) erc-fake-bar)))
+        (setq erc-modules '(gfoo lbar lbaz))
+        ;; Don't expose the mode here
+        (should (equal (mapcar #'symbol-name (erc--update-modules))
+                       '("erc-lbaz-mode" "erc-lbar-mode")))
+        ;; Lbaz required because unknown.
+        (should (equal (nreverse calls) '((gfoo . 1) erc--lbaz-feature)))
+        (fmakunbound (intern "erc-lbaz-mode"))
+        (unintern (intern "erc-lbaz-mode") obarray)
         (setq calls nil))
 
-      (ert-info ("Module name overrides")
-        (setq erc-modules '(completion autojoin networks))
+      (ert-info ("Global modules") ; `pcomplete' resolved to `completion'
+        (setq erc-modules '(pcomplete autojoin networks))
         (should-not (erc--update-modules)) ; no locals
-        (should (equal (nreverse calls) '( erc-pcomplete (completion . 1)
-                                           erc-join (autojoin . 1)
-                                           erc-networks (networks . 1))))
+        (should (equal (nreverse calls)
+                       '((completion . 1) (autojoin . 1) (networks . 1))))
         (setq calls nil)))))
 
 (ert-deftest erc--merge-local-modes ()