bug#46476: Feature request: Right-aligning part of the modeline

[Eli Zaretskii <eliz@gnu.org>]

> From: Paweł Kraśnicki <paul.krasnicki@gmail.com>
> Date: Sat, 13 Feb 2021 18:45:14 +0100
> Cc: 46476@debbugs.gnu.org
> 
> > If people use :align-to, then what would be the advantage of providing
> > %-constructs to produce the same?
> 
> Well, the only advantage is that people wouldn't need to install an external
> package or write a short but advanced piece of Lisp. I think that right
> alignment is such a commonly used feature that it would be good to have it
> accessible very easily. (Center alignment and segments are less popular.)
> I actually got the idea for using %= in particular from Vim, which has right
> alignment under this %-construct.

This currently seems to be somewhat problematic, given the recursive
manner in which the mode line elements are produced by the display
code.

> > That'd require two passes to generate the mode-line display, right?
> 
> I think so. Emacs would have to first expand the modeline constructs in the
> segments separately into some internal data structure, then decide which ones to
> actually display.

Patches welcome.