bug#62606: [PATCH] function to align mode-line elements to right

[Stefan Monnier via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

> Let's hear from others.  Stefan, Lars, any thoughts?  Does anyone else
> have any comments about this proposed feature?

FWIW, my mode-lines always tend to overflow to the right, so I wouldn't
be able to use that functionality, but I have no doubt that it would be
welcome by several users, based on some of the mode-lines I've
seen elsewhere.

Also, the code is fairly simple and self-contained, so I don't see
a good reason not to include that.

Some comments about the code below:

> +(defcustom mode-line-right-align-edge 'window
> +  "Where function `mode-line-format-right-align' should align to.
> +
> +Must be set to a symbol.  Acceptable values are:
> +- window: align to extreme right of window, regardless of margins
> +  or fringes
> +- right-fringe: align to right-fringe
> +- right-margin: align to right-margin
> +- right: synonym for right-margin (supported because this is how the
> +  display property understands this, see info node `(elisp)Specified
> +  Space'.)"

I think symbols like `right-fringe` should be enclosed in `...'
in docstrings.  Also, I see no reason to encourage the use of an alias,
so I'd drop either `right-margin` or `right` from the doc (especially
since it doesn't really come for free in the code).

As a user I'd wonder if "align to right-fringe" means to align to
the beginning (i.e. left side) or end (i.e. right side) of the
right fringe.

[ And now I wonder: can Emacs have R2L modelines?  If so, what should
  happen in those?  ]

> +(defun mode-line-format-right-align ()
> +  "Right-align all following mode-line constructs.
> +
> +When the symbol `mode-line-format-right-align' appears in
> +`mode-line-format', return a string of one space, with a display
> +property to make it appear long enough to align anything after
> +that symbol to the right of the rendered mode line.  Exactly how
> +far to the right is controlled by `mode-line-right-align-edge'.
> +
> +It is important that the symbol `mode-line-format-right-align' be
> +included in `mode-line-format' (and not another similar construct
> +such as `(:eval (mode-line-format-right-align)').  This is because
> +the symbol `mode-line-format-right-align' is processed by
> +`format-mode-line' as a variable."

AFAICT, this function is internal to the implementation of the
`mode-line-format-right-align` mode-line spec.  So maybe it should use
"--" in its name.

> +  (let* ((rest (cdr (memq 'mode-line-format-right-align
> +			  mode-line-format)))

This is the ugly part of the implementation: the
`mode-line-format-right-align` has to "find itself".
I wonder if/how we could get rid of this wrinkle.

Maybe instead of a `mode-line-format` of the form:

    (..LEFT.. mode-line-format-right-align ..RIGHT..)

an alternative is to use

    (..LEFT.. (:eval (mode-line-format-right-align ..RIGHT..)))

I wonder if it would work as well (I'm worried that it means that
`..RIGHT..` is not processed directly by the C code by goes through the
`format-mode-line` ELisp function to turn it into an ELisp string
first and that could affect the result).

It would have the advantage that it doesn't process `..RIGHT..` twice,
and it should also be usable in `header-line-format` and friends.

It's not great tho :-(


        Stefan