I've read about Pyret on hackernews: http://www.pyret.org/
and found the 'where' statement very compeling. Functions can end with a where that contains small unit tests.
From the documentation example:
fun sum(l): cases(List) l: | empty => 0 | link(first, rest) => first + sum(rest) end where: sum() is 0 sum([1, 2, 3]) is 6 end
It's quite similar to the doctests ideas I guess - but not intended to be documentation like them.
I ended up disliking docttests because of this doc+test duality by the way: it often ends up as a not so good documentation and not so good tests.
Anyways, having a dedicated keyword to append after a function some tests as part of the language has benefits imho: