Gluster did this about a decade ago, though not in Rust. Actually twice - home-grown NFSv3 and Ganesha-based NFSv4. At Facebook we used the NFSv3 version to run dozens of clusters serving tens or perhaps hundreds of thousands of machines. My own involvement with that piece was minimal (I was a maintainer of Gluster overall for a while) but it worked pretty well for those users who needed a real mountable filesystem with real files instead of vaguely file-like objects that let you down every time you try to step beyond whole-file read and write.