Or nothing at all. Rust has an uphill battle since 10 years which shows it's hard to replace C / another language. Might be Rusts borrow checker that prevents adoption, but I doubt it (though it's the reason I use Rust but don't love it).
I think one thing that has helped Rust make the first serious advances towards C that we've seen in a long time is also the development tool chain. Getting started with Rust on any major platform, besides embedded development though that's coming along, is so much easier with Rustup and Cargo than the bizarre (to the uninitiated) work you have to do to setup and actually understand your development tool chain in C as well as C++.
Does rust have any other implementations? It seems like that would help adoption as well.