Earlier quoted context omitted.
I am a huge fan of no required GC, and of precise refcounting. But does Rust have anything on the roadmap to handle truly cyclic structures that don't lend themselves to a weak pointer approach?
One option would be to allocate objects out of an arena[0] and then destroy the entire thing at once when you're finished with the contained objects. [0] http://doc.rust-lang.org/arena/index.html
If anyone is interested, I think it could be interesting to see if the refcounting approach I developed for handling cyclic references in my library "upb" would work as a Rust library:
https://github.com/haberman/upb/blob/master/upb/refcounted.h
The basic idea is that you refcount groups of objects instead of refcounting objects independently. You compute the groups dynamically such that no cycle can span groups. This is sort of like an arena, except that no arena is ever explicitly created, and the collection can be more precise than an arena.
In my library, objects go through a two-phase lifecycle. First they are mutable, then they are frozen, after which no further mutations can be made. When an object is frozen, its outbound pointers are also frozen.
The nice property of this scheme is that you can perfectly compute these "virtual arenas" by computing strongly-connected components at freeze time. This makes the scheme optimal for frozen objects. For mutable objects, the groups are computed more conservatively and objects may not get freed as often as they otherwise would with a perfect scheme (ie. the same downside of an arena).
I would love to know how this scheme might possibly be modeled in Rust. Since Rust also has strong semantics around mutability and immutability, it seems possible that a very nice idiomatic Rust interface could be implemented around this scheme, giving more precise cleanup than an arena while still allowing circular structures.