gv.el: Allow use of ignore as a place (bug#81217)

* lisp/emacs-lisp/gv.el (error): Delete redundant `gv-expander` definition.
(ignore): Add a setter.
* doc/lispref/functions.texi (Calling Functions): Document `ignore`
as a place.
* lisp/xdg.el (xdg-mime-collect-associations): Silence compiler warning.

doc/lispref/functions.texi

@@ -1131,6 +1131,8 @@ This function returns @var{argument} and has no side effects.
 
 @defun ignore &rest arguments
 This function ignores any @var{arguments} and returns @code{nil}.
+When used as a place (@pxref{Generalized Variables}), the assignment has
+simply no effect.
 @end defun
 
 @defun always &rest arguments

etc/NEWS

@@ -171,6 +171,10 @@ To install the grammars, use 'M-x markdown-ts-mode-install-parsers'.
 
 * Lisp Changes in Emacs 32.1
 
++++
+** 'ignore' is now also a place, acting as a "blackhole" like /dev/null.
+E.g. (push (new-elem) (pcase (foo) (0 var1) (1 var2) (_ (ignore))))
+
 +++
 ** 'kill-all-local-variables' can kill locals silently and reset the buffer.
 This function's KILL-PERMANENT argument now accepts the value

lisp/emacs-lisp/gv.el

@@ -576,11 +576,6 @@ See also `incf'."
         (funcall do `(funcall (car ,gv))
                  (lambda (v) `(funcall (cdr ,gv) ,v)))))))
 
-(put 'error 'gv-expander
-     (lambda (do &rest args)
-       (funcall do `(error . ,args)
-                (lambda (v) `(progn ,v (error . ,args))))))
-
 (defun gv-synthetic-place (getter setter)
   "Special place described by its setter and getter.
 GETTER and SETTER (typically obtained via `gv-letplace') get and
@@ -683,11 +678,9 @@ REF must have been previously obtained with `gv-ref'."
 
 ;;; Generalized variables.
 
-;; You'd think no one would write `(setf (error ...) ..)' but it
-;; appears naturally as the result of macroexpansion of things like
-;; (setf (pcase-exhaustive ...)).
-;; We could generalize this to `throw' and `signal', but it seems
-;; preferable to wait until there's a concrete need.
+;; `error' and `ignore' places come in handy in the default branch
+;; of `cond/pcase' places.
+(gv-define-setter ignore (store &rest _args) store) ;; A kind of >/dev/null
 (gv-define-expander error (lambda (_do &rest args) `(error . ,args)))
 
 ;; Some Emacs-related place types.

lisp/xdg.el

@@ -346,10 +346,11 @@ which is expected to be ordered by priority as in
                     (forward-line)
                   (dolist (str (xdg-desktop-strings (match-string 1)))
                     (cl-pushnew str
-                                (cond ((eq sec ?D) defaults)
-                                      ((eq sec ?A) added)
-                                      ((eq sec ?R) removed)
-                                      ((eq sec ?M) cached))
+                                (pcase-exhaustive sec
+                                  (?D defaults)
+                                  (?A added)
+                                  (?R removed)
+                                  (?M cached))
                                 :test #'equal))
                   (while (and (zerop (forward-line))
                               (/= (following-char) ?\[)))))))