Well it's not just about Rust. What we have here is pluggable schedulers leveraging the BPF functionality within the linux kernel. They can be written in watever language you like and they are running in the user space.
eBPF doesn't run in user space in the context of eBPF in the linux kernel. It's verified so that the kernel can be sure it won't loop forever and then gets JIT'ed and run in kernel space.
There are some user space BPF vms like https://github.com/iovisor/ubpf and Solana.