> We already mark functions as being side-effect-free, so it seems like
> code like
>
> (if (cl-oddp (% (random) 2))
>     (setq ...))
>
> would be "safe" together with the safep markup for assignments we
> already have.  We could make a safe restricted language subset for use
> both here and in similar circumstances.

That is a good point, but I think more tagging should be done.  Ideally
this would read as regular elisp (which is kind of ironic considering
that we are discussing an .eld file), so it would be nice if
mode-specific modifications could be done by just writing

--8<---------------cut here---------------start------------->8---
(when (derived-mode-p 'c-mode)
  (setq tab-width 8))
--8<---------------cut here---------------end--------------->8---

or something like that.

>> No, what I had in mind was not to trigger warnings but either to
>> highlight unused variables or provide a command that would check it for
>> you.
>
> Oh, right.  That's another good idea.  😀

One idea would be to use Flymake.