I wrote this crate last night while waiting for some long-running ops task to finish 
There is a practical side to it (method delegation is somewhat common), but it also happens to be a pretty good example of (ab)using the macro_rules system to implement pretty much anything you can imagine.
The style I use to structure the macros (essentially a pushdown automaton) is based on the helix macros, which I spoke about last year at Ruby Kaigi.
See the reddit post for links to real world code example, link to slides and videos etc. (I am only allowed to include two links in my post here.)