Infinite loop

[Goncalerta]

Bringing @eddyb's comment on #4 to a new issue so as not to lose track of it.

There is a problem here, AFAICT something like this will repeat ad infinitum without warning (and end up allocating GBs of RAM until stopped by the OOM killer):

let x = quote!(_);
quote(#(#x)*);

If iter::repeat is involved, then its implementation of size_hint suggests this could be detected at runtime by asserting that combined_iterator.size_hint().0 < usize::MAX (but I'm not familiar enough with the implementation to be sure).

[dtolnay]

Take a look at what quote does for this, I think we have a way to catch this at compile time.