MacRuby does support AOT compilation, although I don't think it's a very production-ready feature yet and would likely be incompatible with a fair percent of existing ruby code
This isn't in MacRuby — it's in Objective-C. And it's explicitly distinct from garbage collection. This is an alternative to garbage collection that has generally been considered kind of impractical.
(And MacRuby's AOT compilation works pretty well from what I've seen.)
