In general, rust does not want you to use even a slightly older version.
Everything is pulling you forwards, and even slight inconveniences for the sake of supporting older versions won't be taken.
That's... an extreme culture shock when you're coming from a position of using C++11 because C++17 support is "just not there".
I suppose it helps that getting a new rust isn't the massive pain in the cock that getting a new gcc is.