Rust is exciting, no doubt, and I have high hopes for its adoption, but I've personally not seen/heard of any visible OSS big data style projects using it. I see Frank McSherry's stuff has been mentioned, but I think that's still his pet project (hopefully not putting words in his mouth).

But really I was using C++ as an example of something more fit for these types of projects than Java, it doesn't have to be only C++ of course.

Rust has Frank McSherry (formerly working on Naiad for Microsoft Research) and his work on timely dataflow and differential dataflow: https://github.com/frankmcsherry/blog