Add new 'declare' forms for command completion predicates

* doc/lispref/functions.texi (Declare Form): Document the new
`completion' and `modes' declarations.
* lisp/simple.el (completion-with-modes-p): New helper functions.

* lisp/emacs-lisp/byte-run.el (byte-run--set-completion)
(byte-run--set-modes):
(defun-declarations-alist): New declarations for `completion' and
`modes'.

doc/lispref/functions.texi

@@ -2309,6 +2309,16 @@ form @code{(lambda (@var{arg}) @var{body})} in which case that function will
 additionally have access to the macro (or function)'s arguments and it will
 be passed to @code{gv-define-setter}.
 
+@item (completion @var{completion-predicate})
+Declare @var{completion-predicate} as a function to determine whether
+to include the symbol in the list of functions when asking for
+completions in @kbd{M-x}.  @var{completion-predicate} is called with
+two parameters: The first parameter is the symbol, and the second is
+the current buffer.
+
+@item (modes @var{modes})
+Specify that this command is meant to be applicable for @var{modes}
+only.
 @end table
 
 @end defmac

etc/NEWS

@@ -2266,6 +2266,15 @@ back in Emacs 23.1.  The affected functions are: 'make-obsolete',
 
 * Lisp Changes in Emacs 28.1
 
++++
+** New forms to declare how completion should happen has been added.
+'(declare (completion PREDICATE))' can be used as a general predicate
+to say whether the command should be present when completing with
+'M-x TAB'.  '(declare (modes MODE...))' can be used as a short-hand
+way of saying that the command should be present when completing from
+buffers in major modes derived from MODE..., or, if it's a minor mode,
+whether that minor mode is enabled in the current buffer.
+
 +++
 ** The 'interactive' syntax has been extended to allow listing applicable modes.
 Forms like '(interactive "p" dired-mode)' can be used to annotate the

lisp/emacs-lisp/byte-run.el

@@ -143,6 +143,17 @@ The return value of this function is not used."
       (list 'function-put (list 'quote f)
             ''lisp-indent-function (list 'quote val))))
 
+(defalias 'byte-run--set-completion
+  #'(lambda (f _args val)
+      (list 'function-put (list 'quote f)
+            ''completion-predicate val)))
+
+(defalias 'byte-run--set-modes
+  #'(lambda (f _args val)
+      (list 'function-put (list 'quote f)
+            ''completion-predicate `(lambda (_ b)
+                                      (completion-with-modes-p ,val b)))))
+
 ;; Add any new entries to info node `(elisp)Declare Form'.
 (defvar defun-declarations-alist
   (list
@@ -159,7 +170,9 @@ This may shift errors from run-time to compile-time.")
 If `error-free', drop calls even if `byte-compile-delete-errors' is nil.")
    (list 'compiler-macro #'byte-run--set-compiler-macro)
    (list 'doc-string #'byte-run--set-doc-string)
-   (list 'indent #'byte-run--set-indent))
+   (list 'indent #'byte-run--set-indent)
+   (list 'completion #'byte-run--set-completion)
+   (list 'modes #'byte-run--set-modes))
   "List associating function properties to their macro expansion.
 Each element of the list takes the form (PROP FUN) where FUN is
 a function.  For each (PROP . VALUES) in a function's declaration,

lisp/simple.el

@@ -1950,6 +1950,11 @@ to get different commands to edit and resubmit."
          (complete-with-action action obarray string pred)))
      #'commandp t nil 'extended-command-history)))
 
+(defun completion-with-modes-p (modes buffer)
+  (apply #'provided-mode-derived-p
+         (buffer-local-value 'major-mode buffer)
+         modes))
+
 (defun read-extended-command--affixation (command-names)
   (with-selected-window (or (minibuffer-selected-window) (selected-window))
     (mapcar