bug#66317: Project mode-line

[Dmitry Gutov]

On 03/10/2023 11:07, Eshel Yaron via Bug reports for GNU Emacs, the 
Swiss army knife of text editors wrote:
> Hi,
>
> Juri Linkov<juri@linkov.net>  writes:
>
> > Here is the implementation of the mode-line project indicator prepended
> > before the vc indicator with the project menu discussed in bug#63469:
> >
> > diff --git a/lisp/progmodes/project.el b/lisp/progmodes/project.el
> > index 2e6ae89a443..8798feb2c56 100644
> > --- a/lisp/progmodes/project.el
> > +++ b/lisp/progmodes/project.el
> > @@ -2010,5 +2102,20 @@ project-uniquify-dirname-transform
> >             (file-relative-name dirname root))))
> >       dirname))
> >
> > +;;; Project mode-line
> > +
> > +(let ((form '(:eval (when-let (project (project-current))
> > +                      (concat " "
> > +                              (propertize
> > +                               (project-name project)
> > +                               'mouse-face 'mode-line-highlight
> > +                               'local-map
> > +                               (make-mode-line-mouse-map
> > +                                'down-mouse-1
> > +                             `(menu-item "Project"
> > +                                            ,menu-bar-project-menu))))))))
> > +  (when-let (pos (seq-position mode-line-format '(vc-mode vc-mode)))
> > +    (cl-pushnew form (nthcdr pos mode-line-format))))
> > +
> >   (provide 'project)
> >   ;;; project.el ends here
> This looks nice (I actually have something similar in my
> `mode-line-format` already), but I think that modifying
> `mode-line-format` when project.el is loaded is a bit problematic.
> Perhaps a better alternative would be to define a variable that holds
> this mode-line construct, along with an autoloaded function that people
> can call from their init files to actually add this construct to
> `mode-line-format`.  WDYT?

I also think this would be a more conservative choice (keeping the 
runtime cost of using project.el to zero by default).

- A public defvar with the mode-line spec for the project element (which 
could be used by alternative mode-lines as well).
- Either a public function, or a defcustom (nil by default) where the 
setter does the same thing. The latter seems a bit user-friendlier.