As mentioned below, we have two types of refcounted ptrs; one for threads and one for not-threads.

In conjunction with RefCell and Cell and Mutex we get some really useful abstractions that are still safe (runtime safe).

I plan to write another blog post listing what they do exactly (and how they come together to provide neat abstractions), but:

- Box<T> is a boring old owned pointer. It can hand out references that can not live longer than it (checked statically) and it is destroyed when it goes out of scope.

- asterisk-const T and asterisk-mut-T (sorry, I don't know how to use asterisks on HN without it breaking formatting) are your C pointers. Unsafe to dereference, and don't have the lifetime checks associated with them. Good for FFI and building abstractions.

- &T and &mut T are your borrow checked references. These are like regular pointers except they can't statically outlive the thing they borrow from; and follow the rwlock pattern mentioned in the blog post

- Rc is a refcounted shareable pointer. Its main job is ensuring that destructors are run correctly.

- Arc is a version of Rc that uses atomics and is thread safe (but slower)

- RefCell/Cell are ways of (at runtime) ensuring that the rwlock pattern is followed. Cell doesn't use locks or refcounts, but RefCell does. RefCell will keep a track of how many read-references have been taken from it, and make sure that there are none when someone tries to write to it.

- Mutex is kinda-sorta the multithreaded version or RefCell[^]. It's a simple lock, not an RWLock (though there's no reason why you can't use that either), and it gives you thread safe access to mutable data.

- Rc<RefCell<T>> is a common pattern when you want a cycle-free GC-like construct that's going to be bounced around and mutated all over the place. Think Java in a single thread. (Ignoring cycles for now)

- Arc<Mutex<T>> is the thread safe version of the above. Think Java with the appropriate synchronized/volatile keywords everywhere, or Go with mutexes.

All of these use RAII and type safety (especially a thing called "kinds" which makes sure single-threaded abstractions don't get shared across threads, but allows them to be sent across threads depending on the situation) to provide thread safety and memory safety. In many cases the safety requires more and more dynamic checks, for example with Arc<Mutex<T>> it's basically all dynamic checks. Which also means that you're free to throw it around however you want and the compiler won't complain, though of course beware the deadlocks, and if the dynamic checks cause an assertion failure you're going to have to debug it.

Except the first three, these could be written in a separate library. The first one could, too, for the basic features, though there is some additional compiler support that gives it special behavior and makes it easy to work with. We hope to remove that though.

The fact that there's such a variety of things that provide various guarantees can be intimidating at first, but once you get the hang of it it's quite powerful. You can then only pay what you need to pay -- for example if you want refcounting but don't need to worry about threads; you don't pay the cost of the threads. If you want to use shared data with scoped threads, you can even use a plain Mutex with no refcounting. If you want to use the rwlock pattern locally, don't use a thread-safe RWLock, just use RefCell/Cell. Et cetera et cetera.

---------------------

I'm also working on a GC here[1]. Completely untested, the two of us just implemented a design that we hammered out on Saturday and haven't really looked at yet. We're convinced it's broken :P but I'm sure we can fix it. The design is mark-and-sweep, though we're trying to avoid stack scanning and rely on tracing/rooting only (Rust has some neat advanced features that makes this possible to consider). Single threaded only for now.

[1]: http://github.com/manishearth/rust-gc

[^]: Since I'm already calling RefCell a single threaded RWLock, perhaps this cyclic definition isn't perfect. :P Anyway, a Mutex is a mutual exclusion thing which you've probably used that ensures that only one thread can access the inner data at a time.