It is literally being changed constantly. They chose to add it to the Linux kernel but they have to build the features they need into the language as they go. It's not backward compatible like C and C++ either. Sure, there are a couple of versions you could try to pin to in Rust, but then all your libraries need to be pinned to the same version (to be fair, this last part is an assumption).
It's not, especially the part about it being actively changed to adapt to Linux. I've heard several complaints about this already. I may have been a little harsh on the stability concerns but I stand by my assertions in general.
