Add a new buffer-local variable `minor-modes'

* lisp/emacs-lisp/easy-mmode.el (define-minor-mode): Keep
`minor-modes' updated.
* src/buffer.c (bset_minor_modes, Fmake_indirect_buffer)
(reset_buffer, init_buffer_once): Initialise `minor-modes'.
(syms_of_buffer): Add `minor-modes' as a new permanently-local
variable.

* src/buffer.h (struct buffer): Add minor_modes_.

doc/lispref/modes.texi

@@ -1454,6 +1454,11 @@ used only with Diff mode.
 other minor modes in effect.  It should be possible to activate and
 deactivate minor modes in any order.
 
+@defvar minor-modes
+This buffer-local variable lists the currently enabled minor modes in
+the current buffer, and is a list if symbols.
+@end defvar
+
 @defvar minor-mode-list
 The value of this variable is a list of all minor mode commands.
 @end defvar

etc/NEWS

@@ -2266,6 +2266,11 @@ back in Emacs 23.1.  The affected functions are: 'make-obsolete',
 
 * Lisp Changes in Emacs 28.1
 
++++
+** New buffer-local variable 'minor-modes'.
+This permanently buffer-local variable holds a list of currently
+enabled minor modes in the current buffer (as a list of symbols).
+
 ** The 'values' variable is now obsolete.
 
 ---

lisp/emacs-lisp/easy-mmode.el

@@ -317,6 +317,10 @@ or call the function `%s'."))))
                    nil)
                   (t
                    t)))
+           ;; Keep `minor-modes' up to date.
+           (setq minor-modes (delq ',modefun minor-modes))
+           (when ,getter
+             (push ',modefun minor-modes))
            ,@body
            ;; The on/off hooks are here for backward compatibility only.
            (run-hooks ',hook (if ,getter ',hook-on ',hook-off))

src/buffer.c

@@ -292,6 +292,11 @@ bset_major_mode (struct buffer *b, Lisp_Object val)
   b->major_mode_ = val;
 }
 static void
+bset_minor_modes (struct buffer *b, Lisp_Object val)
+{
+  b->minor_modes_ = val;
+}
+static void
 bset_mark (struct buffer *b, Lisp_Object val)
 {
   b->mark_ = val;
@@ -893,6 +898,7 @@ CLONE nil means the indirect buffer's state is reset to default values.  */)
       bset_file_truename (b, Qnil);
       bset_display_count (b, make_fixnum (0));
       bset_backed_up (b, Qnil);
+      bset_minor_modes (b, Qnil);
       bset_auto_save_file_name (b, Qnil);
       set_buffer_internal_1 (b);
       Fset (intern ("buffer-save-without-query"), Qnil);
@@ -967,6 +973,7 @@ reset_buffer (register struct buffer *b)
   b->clip_changed = 0;
   b->prevent_redisplay_optimizations_p = 1;
   bset_backed_up (b, Qnil);
+  bset_minor_modes (b, Qnil);
   BUF_AUTOSAVE_MODIFF (b) = 0;
   b->auto_save_failure_time = 0;
   bset_auto_save_file_name (b, Qnil);
@@ -5151,6 +5158,7 @@ init_buffer_once (void)
   bset_auto_save_file_name (&buffer_local_flags, make_fixnum (-1));
   bset_read_only (&buffer_local_flags, make_fixnum (-1));
   bset_major_mode (&buffer_local_flags, make_fixnum (-1));
+  bset_minor_modes (&buffer_local_flags, make_fixnum (-1));
   bset_mode_name (&buffer_local_flags, make_fixnum (-1));
   bset_undo_list (&buffer_local_flags, make_fixnum (-1));
   bset_mark_active (&buffer_local_flags, make_fixnum (-1));
@@ -5617,6 +5625,11 @@ The default value (normally `fundamental-mode') affects new buffers.
 A value of nil means to use the current buffer's major mode, provided
 it is not marked as "special".  */);
 
+  DEFVAR_PER_BUFFER ("minor-modes", &BVAR (current_buffer, minor_modes),
+		     Qnil,
+		     doc: /* Minor modes currently active in the current buffer.
+This is a list of symbols, or nil if there are no minor modes active.  */);
+
   DEFVAR_PER_BUFFER ("mode-name", &BVAR (current_buffer, mode_name),
                      Qnil,
 		     doc: /* Pretty name of current buffer's major mode.

src/buffer.h

@@ -338,6 +338,9 @@ struct buffer
   /* Symbol naming major mode (e.g., lisp-mode).  */
   Lisp_Object major_mode_;
 
+  /* Symbol listing all currently enabled minor modes.  */
+  Lisp_Object minor_modes_;
+
   /* Pretty name of major mode (e.g., "Lisp"). */
   Lisp_Object mode_name_;