bug#33435: 27.0.50; A use case for recursive display specifications

[Pip Cet <pipcet@gmail.com>]

This is a feature request: I'd like to be able to display an empty line
as a non-empty line with images in it.

If there's a way to do that with standard display specs, I haven't
found it. It's possible I'm merely missing an obvious way to do so.

I think there are two almost-equivalent approaches to achieving this:

1. interpret a list of display specifications by displaying the first
element of the list, then the second, and so, on, so I could use
(put-text-property beg end 'display `(,image1 ,image2 ,image3 "\n"))

2. allow a single level of recursion in display specifications, so I
could use
(put-text-property beg end 'display (concat (propertize " " 'display
image1) (propertize " " 'display image2) (propertize " " 'display
image3) "\n"))

I've tried simply disabling the checks for recursive display
specifications and (2) appears to work, but I don't know the precise
rationale for their existence, so it's possible that breaks the display
engine somehow.

If the problem is merely the possibility of creating an infinite loop
by setting a string's display property to the string itself (indeed,
in my test build, that crashes emacs), that could be avoided while
still allowing a single level of recursion.

(I'm using this to highlight syntactic indentation with SVG images,
and that's much prettier if empty lines are interpreted as being
indented to the minimum of the preceding and following levels of
indentation, rather than merely being empty.)