Add the `always' function

* doc/lispref/functions.texi (Calling Functions): Document it.
* lisp/subr.el (always): New function.

* lisp/emacs-lisp/byte-opt.el (side-effect-free-fns): Mark it as
side effect free.

doc/lispref/functions.texi

@@ -861,6 +861,10 @@ This function returns @var{argument} and has no side effects.
 
 @defun ignore &rest arguments
 This function ignores any @var{arguments} and returns @code{nil}.
+@end defun
+
+@defun always &rest arguments
+This function ignores any @var{arguments} and returns @code{t}.
 @end defun
 
   Some functions are user-visible @dfn{commands}, which can be called

etc/NEWS

@@ -2305,6 +2305,10 @@ back in Emacs 23.1.  The affected functions are: 'make-obsolete',
 
 * Lisp Changes in Emacs 28.1
 
++++
+** New function 'always'.
+This is identical to 'ignore', but returns t instead.
+
 +++
 ** New forms to declare how completion should happen has been added.
 '(declare (completion PREDICATE))' can be used as a general predicate

lisp/emacs-lisp/byte-opt.el

@@ -1348,7 +1348,7 @@ Same format as `byte-optimize--lexvars', with shared structure and contents.")
 	 window-total-height window-total-width window-use-time window-vscroll
 	 window-width zerop))
       (side-effect-and-error-free-fns
-       '(arrayp atom
+       '(always arrayp atom
 	 bignump bobp bolp bool-vector-p
 	 buffer-end buffer-list buffer-size buffer-string bufferp
 	 car-safe case-table-p cdr-safe char-or-string-p characterp

lisp/subr.el

@@ -373,10 +373,17 @@ PREFIX is a string, and defaults to \"g\"."
 
 (defun ignore (&rest _arguments)
   "Do nothing and return nil.
-This function accepts any number of ARGUMENTS, but ignores them."
+This function accepts any number of ARGUMENTS, but ignores them.
+Also see `always'."
   (interactive)
   nil)
 
+(defun always (&rest _arguments)
+  "Do nothing and return t.
+This function accepts any number of ARGUMENTS, but ignores them.
+Also see `ignore'."
+  t)
+
 ;; Signal a compile-error if the first arg is missing.
 (defun error (&rest args)
   "Signal an error, making a message by passing ARGS to `format-message'.