bug#36472: 27.0.50; Convey information by showing line numbers using different colors?

[Eli Zaretskii <eliz@gnu.org>]

> Cc: 36472@debbugs.gnu.org
> From: Dmitry Gutov <dgutov@yandex.ru>
> Date: Tue, 2 Jul 2019 18:49:38 +0300
> 
> (defvar display-line-number-renderers-functions nil
>   "The line number (a string) is mapped through all of the functions in 
> this list, in turn. Each receives it as an argument, and then the return 
> value is used. The functions are called in the buffer for which the line 
> numbers are displayed, at the beginning of a line which corresponds to 
> the given number.")
> 
> (add-hook 'display-line-number-renderers 'diff-hl-line-number-renderer)
> 
> (defun diff-hl-line-number-renderer (line-number-string)
>    (cl-case (get-text-property (point) 'diff-hl-indicator-type)
>      ...)

I still have some questions:

  . The argument is a line-number string.  You expect the absolute
    line number there?  When the line-number display style is
    'relative' or 'visual', the absolute line number might not be
    available.
  . What kind of object is the return value, and how should the
    display engine use it?
  . You seem to assume the hook will be called at point, but that is
    not true: it will be called where the display engine is scanning
    the buffer.  So you need that position (the beginning of line or
    something) in the interface, or else you will need to calculate
    the position from the line number, not a nice prospect.
  . The display engine is sometimes called to scan buffer positions
    outside of the window, so you should either detect that or be sure
    to place your properties/overlays not only in the window.  Is that
    a problem?
  . What are the triggers for changing these properties/overlays?  Are
    they determined once and for all, or can they change after the
    buffer has been created and populated with the text?  If some
    changes in the buffer affect visual appearance of screen lines
    that are otherwise unaffected by the changes, it would mean
    disabling redisplay optimizations when this feature is used.  For
    example, with 'relative' style, moving point to another line
    requires to redraw all the lines in the window.

In general, calling Lisp from the display engine means complications
and all kinds of silly precautions, to protect ourselves from crazy
Lisp, so I'm still not very fond if the idea, sorry.