Depends on the kinds of parallelism you want. Rust has built-in support for parallelization, but tasks are mapped 1:1 to threads. Currently M:N threading is also supported, but that will be removed. Consequently, applications requiring lots of tasks (including highly-concurrent websocket servers) won't do well with the built-in mechanism.
Context: https://news.ycombinator.com/item?id=8459888