FreeBSD developers are currently weighing the benefits and costs of using the Rust programming language in the FreeBSD base system. The main disadvantage of incorporating Rust into the FreeBSD base system is doubling compilation time. Due to the need to compile the LLVM-based Rustc compiler and all the additional features of Rust, they will find that the compilation time of the base system roughly doubles compared to the status quo.
However, developers have also come up with some solutions to bypass these build time issues.
By allowing Rust code into the base system, they can (re)write various components into this programming language, such as the ZFS daemon (zfsd) instead of using C++, rewriting devd, WiFi userspace code can benefit from being written in Rust, and so on.
Discussions about allowing the use of the Rust programming language in FreeBSD base systems are still ongoing, but for those interested in the prospect of using the Rust programming language in FreeBSD base systems, this mailing list thread can be used to find out.