This is the missing link for people wanting to do FP in Rust and even though this is an issue of standard library and not necessarily one of language, this will be a source of pain for people wanting to do FP in Rust, as FP means working with referential transparency, which begets immutability. This I think is worth mentioning, being the biggest road-block when wanting to adopt an FP style in any language - after all, any language that supports higher-order functions can be used for FP (or OOP) with varying levels of pain.
Again, as an outsider with a superficial understanding of Rust, it seems that Rust will make FP enjoyable in a language that doesn't need a garbage collector. But judging from articles and announcements made, Rust's primary objective to make concurrent programming safer isn't necessarily to make immutability effective, but rather to limit/control both shared reads and writes. And I can think of contexts in which you don't necessarily want that - after all, the shared reads of an immutable value are embarrassingly parallelizable, persistent data-structures working really well in say single-producer multiple-consumers scenarios. Plus immutable values are for having referential transparency and the common concurrency issues that we are commonly seeing are only a symptom. Truthfully, Rust must allow specialized algorithms and low level code, so I expect it to give the possibility of choosing your path instead of forcing a solution down your thrown.
But herein we are ending with a problem also exposed when working with C++. People have been doing FP in C++ with varying levels of success. But the problem with persistent data-structures that are doing structural sharing is one of memory management. It's much harder to implement persistent data-structures without relying on a garbage collector. LISPs that encourage an FP style, Ocaml, Haskell, SML, F#, Scala and all languages advertised as FP I can think about are garbage collected. Rust can use an optional garbage collector for "shared references". But that negates the biggest advantage when using Rust, the community is moving away from shared references and Rc/Arm touched in this article would be awful solutions to this problem.
Therefore I hope seeing progress in seeing a library of immutable data-structures that I could use in Rust or at least advice on how to deal with effectively immutable data-structures or something.
On the bright side - Rust's type system seems to be solid (yay generics, yay type-classes), which means it's just a matter of time before the libraries will get there.