It's worth noting that there's several implementations of persistent/immutable/functional data structures for Rust already, and a few basic "building blocks" (namely std::borrow::Cow, noted for its bovine superpowers) are even in the Rust standard library. It's possible that we'll see more of them standardized in the future if a case can be made that they're useful and a consensus is reached wrt. some kind of minimally viable APIs.