Redox OS is "*NIX written in Rust." Singularity was a complete reimagining of OS architecture from the ground up. It isn't simply "the same ideas but now in Sing#/CLR!" it was an answer to "what if we designed a clean-sheet OS today?" There's a long paper on it here[0].

But keep in mind Singularity is still a toy OS. It has no use-case, and doesn't aim for real-world goals that would make it commercially successful. I just think that even as toy OS's go, it was one of the more interesting ones of recent times.

[0] https://www.microsoft.com/en-us/research/wp-content/uploads/...