add task-local reference counted smart pointers

[thestinger]

To provide a reference counted pointer type with deterministic
destruction once managed boxes are switched over to a garbage
collector. Unlike managed boxes, these can be moved instead of just
copied/cloned which is helpful for avoiding reference counts.

Needs #5601 to be fixed in order for safety to be provided without the current ugly workaround of making the pointers contain Option<@()> and Option<@mut ()> (which are just set to None).

@brson: r?

[brson]

Awesome. This is an important topic and I want to think about it a bit before r+ing, maybe get the ok from @pcwalton who has opinions on this subject.

We're going to need something like this in the runtime so once it does not depend on the local heap we may end up moving it to core::unstable and reexporting it, or maybe just making it part of core.

I've been creating a lot of these container types with borrow_* methods lately and feel like this needs to be extracted out into a set of traits and types for containers that obey borrow semantics. Not that we need that implemented for this pull request.

🌞

[thestinger]

@nikomatsakis and I came to the conclusion that the restriction of only containing Owned types is stricter than it has to be to prevent cycles.

An Rc can only be cyclic if it contains a non-Const type like @mut since the self-reference has to be added after construction.

An RcMut is neither Owned or Const, so only one of those bounds is needed to prevent cycles.

Therefore we can have two constructors (one for T: Owned, one for T: Const) and keep the guarantee of never having cycles. This will make Rc usable for a recursive persistent data structure.

[thestinger]

Once #6251 lands I can remove the use of @ from these.

[brson]

This looks fine to me, but I'd like someone else to sign off on it and #[non_owned].

FWIW this code could avoid using the init intrinsics by allocating with ~ instead of malloc. When you allocate the pointer you just move T into the ~ then cast the ~ to *c_void. When destroying just cast the *c_void back to ~ and Rust takes care of the rest.

[thestinger]

@brson: I think using ~ is required anyway if I'm going to allow Const objects that aren't Owned. It needs to use the local heap if you put @ inside one of these. It's a shame to end up with two sets of metadata but at least it will be gone for non-managed values eventually.