I was also curious. Looking at the code, it seems values are Boxed, but there's a special type called Shared that is an Rc-RefCell (unless Send-enabled.):

                    // Also handle case where target is a `Dynamic` shared value

                    // (returned by a variable resolver, for example)

 Couldn't find any other information about a GC, so guessing this is pure ref-counting. Speaking of potential memory leaks, there's also string interning happening. I agree this seems to be for short-lived contexts right now.