It did not move from Rust to C++. Simias worked together with us to port it back to a C++-based emulator, and we also managed to improve several things in the process.

We are still keeping the two codebases moderately the same so that it's easy to backport stuff back and forth inbetween the Rust/C++ versions.

Mednafen/Beetle PSX right now is a more fleshed out PlayStation emulator that has seen more years in development and therefore is more mature, while Rustation is still a relatively new emulator with lots of parts (CDROM/SPU/etc) still not implemented. Backporting the renderer as a pseudo plugin to a more mature emulator has the benefit that the GPU part can be nearly finalized so that most of the hard work should be done by the time Rustation gets to the stage where the other elements of the emulation are mostly complete.

In short, it helps out both projects and we work closely together with the author. The port to C++ was mostly a team effort between him, us and several other guys.