I'm not sure that Rust is really a good example. While promising, the language itself and its standard library are still immature and subject to a huge amount of disruptive change on an ongoing basis.

I've had some experimental code I wrote in Rust become essentially unusable after only a few months due to significant language and standard library changes. I sure hope that somebody wouldn't spend time on a PostgreSQL library for it until the language itself becomes more stable. They'd be wasting a lot of time and effort if they did try to write such a library now.