The experimental Singularity OS [0] from Microsoft implemented processes in the same address space using so-called "Software Isolated Processes" or SIPs [1]. Singularity was a microkernel design with the filesystem, networking, drivers implemented outside the kernel in a variant of C# called Sing#. It seems Sing# had ownership semantics similar to Rust which allowed SIPs to share memory by transferring ownership through lightweight message passing.

[0] https://en.wikipedia.org/wiki/Singularity_%28operating_syste...

[1] https://courses.cs.washington.edu/courses/cse551/15sp/papers...