Basically, if you compile your program with a minimally (as minimal as is possible) modified compiler, you get "rumprunnable" output. there's a list of popular packages built to run on rump kernels[1] (Rust is supported[2]). I haven't tried to use the toolchain yet, but it's pretty appealing to me.
The biggest issue is that rumpkernels seem to not be in developed anymore. I can't tell if the entire is just off somewhere being productive with their rumpkernels and are happy or if no one is around at all. The Github organization[3]'s repos all seem to not have been changed in a while either.
Outside of rumpkernels, Nabla containers[4] also look interesting to me. They're built on Solo5[5], but the only microkernels on there are currently MirageOS (for OCaml) and IncludeOS (C++?). Nabla looks amazing (containers look like a great fit and they seemed to have improved ergonomics massively), but the limitations[6] are intense.
[0]: https://github.com/rumpkernel/wiki/wiki/Info:-FAQ
[1]: https://github.com/rumpkernel/rumprun-packages
[2]: https://github.com/rumpkernel/rumprun-packages/tree/master/r...
[3]: https://github.com/rumpkernel
[4]: https://nabla-containers.github.io/