Rust doesn't require reference counting to make sharing immutable datastructures safe. The interface would require lifetimes for safety, though, and might be generally unpleasant to use.
Oh? Lifetimes are that flexible? It's been a long time since I looked into Rust, so I had no idea. I would thought it was difficult to decide when to free an object, if it is linked to by two separate data structures and one of them goes out of scope.
Rust solves this dilemma with lifetimes. As long as a reference exists to data, Rust statically ensures that you cannot perform any operation that frees or moves it in memory. In the case of multiple threads, one can also assign each thread a lifetime, and perform the same analysis to maintain existence guarantees. Things get a bit trickier if you want to actually mutate the shared reference, but in the case of truly immutable data that's really all you need.
They are not. Sending something to another thread requires the thing to send to implement some kinds (I think Send + Sync).
To have those guarantees the data would need to stored in an atomically reference counted structure (or something else with the same guarantees), which is just like you said.
And you would need to watch out for reference cycles. I guess they are quite easy to create with Clojure.
> To have those guarantees the data would need to stored in an atomically reference counted structure (or something else with the same guarantees), which is just like you said.
That is not true.
`Send` is having its `'static` bound removed, so sharing data will no longer require reference counting for threads with bounded lifetime: https://github.com/rust-lang/rfcs/pull/458 (the RFC hasn't been officially accepted but it's been referenced as a foregone conclusion several times by Rust core team members).
There is already an experimental library exploiting this for `Sync` data, which includes immutable substructures: https://github.com/nikomatsakis/rayon
The only reason people aren't more aware of this is because the functionality isn't exposed in the standard library yet. The language is perfectly capable.
