whitespace-mode, but I can't configure it the way I want. There are others, but some of them are obsolete and I don't want to try them all.
I always indent via tabs and can't stand trailing whitespace. I never combine tabs and spaces (which means that with tabwidth=4 I can indent only multiples of 4, that's fine). I don't want non-indenting spaces to be highlighted as it's highly disturbing. I don't want any non-indenting tabs as there are always an error.
The following pattern express it maybe more clearly
^\t* +- indenting space
[ \t]+$- trailing space or tab
[^\t\n]+\t- non-indenting tab
It should work for all buffers and ideally use some nasty color.
I'm looking for either a package allowing exactly this or a hint what could I modify to get it in the easiest way. I used to hack emacs a lot some 15 years ago, but that's too long ago.