Homepage: https://github.com/ikirill/hl-indent
Updated:
Highlight irregular indentation
This modes puts indentation highlights below the starting character
of a line on subsequent lines, like this:
some line
| some other line
| | another line
| | | an indented line
| fourth line
final line
This works in every mode, but is more useful in haskell, which
doesn't require indentation levels to be at multiples of a specific
level.
If the minor mode `hl-indent-mode-blocks' is on, this mode will
instead highlight blocks of indentation like so:
xxxxxxxxxxxxxx
oooooooooooo
oooooooooooo
*******
*******
**********
oooooooooooo
**********
xxxxxxxxxxxxxx
(where different symbols represent different colours).
To use:
Enable `hl-indent-mode'.
There is also `hl-indent-mode-blocks', but it is less useful
because of limited color contrast, depending on face settings.
Screenshot:

Notes:
- You can customize faces `hl-indent-face' (which is `fringe' by
default), and also `hl-indent-block-face-1', from 1 to 6.
- To easily see where `hl-indent-mode' puts its highlights, use the
function `hl-indent--debug-faces' together with either
`hl-indent-mode' or `hl-indent-mode-blocks'.
- FIXME The mode will refuse to turn on in a very very large file,
because right now it examines every single line once, which can
take too long.
- FIXME Indentation highlights override any non-trivial background.
This is a problem for things like comments that might have a
background different from the default background. It also
conflicts with other highlights, like hl-line-mode.