Right. IIRC, now your choices are, roughly:

    |args| expr // upvars captured by reference, can't be called after function has gone out of scope

    move |args| expr // upvars moved from function to the closure context (or copied if trivially copyable)

This is simple and good enough for most use cases. If you want more complex schemes, you have to implement them manually, e.g. to reference count the upvars, like Apple blocks do by default, wrap them in Rc and capture that.

Accepting closures is a bit more complicated though.

That's not quite right. In the non-move case, the capture is inferred per upvar. See my other comment for details (https://news.ycombinator.com/item?id=9047766)