Thomas Dupond <thomas@dupond.eu> writes:

> I delved a little bit deeper and apparently you should be able to do
> what you want after you have defined the variable
> imenu-generic-expression or the variable imenu-create-index-function.
> Fortran mode uses the former and Org mode the latter (respectively in
> fortran-mode.el and org-compat.el).

Many thanks for your research! It works now:

(add-to-list 'auto-mode-alist '("\\.mini2\\'" . mini2-mode))

(defun mini2-mode-which-func ()
"XXX")

(add-hook 'which-func-functions #'mini2-mode-which-func)

(define-derived-mode mini2-mode
text-mode "Mini2"
"Major mode for editing .mini2 files."
(which-function-mode 1)
(defvar mini2-imenu-generic-expression
(list
(list
nil
"^\*\\ \\(.*\\)$"
1)
(list
nil
"^\*\*\\ \\(.*\\)$"
1)))
(set (make-local-variable 'imenu-generic-expression)
mini2-imenu-generic-expression))

(provide 'mini2)

correctly shows "XXX" in the mode line after

emacs -Q -l mini2-mode.el foo.mini2

Thanks again and best regards

Axel (not Alex)