Simplify ISUPPORT-derived data wrangling in ERC

* lisp/erc/erc-backend.el (erc--get-isupport-entry): Check server
buffer for `erc-server-parameters' when (re)initializing value.  This
function was previously unreliable from a target buffer on cache
misses.
(erc--with-isupport-data): New macro for accessing and caching data
derived from an ISUPPORT parameter.  Late-arriving params break the
cache.
(erc-server-005): Rewrite pattern as `rx' form, factoring out bol/eol.
* lisp/erc/erc-common.el (erc--isupport-data): New struct to be
subclassed for storing cached ISUPPORT-derived data.
* test/lisp/erc/erc-scenarios-display-message.el: Remove stray
`require'.  (Bug#67220)

lisp/erc/erc-backend.el

@@ -2098,7 +2098,9 @@ primitive value."
                        (erc-with-server-buffer erc--isupport-params)))
             (value (with-memoization (gethash key table)
                      (when-let ((v (assoc (symbol-name key)
-                                          erc-server-parameters)))
+                                          (or erc-server-parameters
+                                              (erc-with-server-buffer
+                                                erc-server-parameters)))))
                        (if (cdr v)
                            (erc--parse-isupport-value (cdr v))
                          '--empty--)))))
@@ -2108,6 +2110,22 @@ primitive value."
     (when table
       (remhash key table))))
 
+;; While it's better to depend on interfaces than specific types,
+;; using `cl-struct-slot-value' or similar to extract a known slot at
+;; runtime would incur a small "ducktyping" tax, which should probably
+;; be avoided when running dozens of times per incoming message.
+(defmacro erc--with-isupport-data (param var &rest body)
+  "Return structured data stored in VAR for \"ISUPPORT\" PARAM.
+Expect VAR's value to be an instance of `erc--isupport-data'.  If
+VAR is uninitialized or stale, evaluate BODY and assign the
+result to VAR."
+  (declare (indent defun))
+  `(erc-with-server-buffer
+     (pcase-let (((,@(list '\` (list param  '\, 'key)))
+                  (erc--get-isupport-entry ',param)))
+       (or (and ,var (eq key (erc--isupport-data-key ,var)) ,var)
+           (setq ,var (progn ,@body))))))
+
 (define-erc-response-handler (005)
   "Set the variable `erc-server-parameters' and display the received message.
 
@@ -2128,8 +2146,11 @@ A server may send more than one 005 message."
             key
             value
             negated)
-        (when (string-match "^\\([A-Z]+\\)=\\(.*\\)$\\|^\\(-\\)?\\([A-Z]+\\)$"
-                            section)
+        (when (string-match
+               (rx bot (| (: (group (+ (any "A-Z"))) "=" (group (* nonl)))
+                          (: (? (group "-")) (group (+ (any "A-Z")))))
+                   eot)
+               section)
           (setq key (or (match-string 1 section) (match-string 4 section))
                 value (match-string 2 section)
                 negated (and (match-string 3 section) '-))

lisp/erc/erc-common.el

@@ -101,6 +101,11 @@
   (contents "" :type string)
   (tags '() :type list))
 
+(cl-defstruct erc--isupport-data
+  "Abstract \"class\" for parsed ISUPPORT data.
+For use with the macro `erc--with-isupport-data'."
+  (key nil :type (or null cons)))
+
 ;; After dropping 28, we can use prefixed "erc-autoload" cookies.
 (defun erc--normalize-module-symbol (symbol)
   "Return preferred SYMBOL for `erc--modules'."

test/lisp/erc/erc-scenarios-display-message.el

@@ -59,6 +59,4 @@
 
     (erc-cmd-QUIT "")))
 
-(eval-when-compile (require 'erc-join))
-
 ;;; erc-scenarios-display-message.el ends here