Very interesting that they opted for a rewrite in Rust instead of adjusting the existing codebase.

I wonder how long it'll take them writing a compositor from scratch.

Not the whole codebase, only the window manager (compositor is the Wayland equivalent). Other components are written in C and will remain so for the foreseeable future. Those components gained Wayland support in the last couple of years, you can try Xfce in a labwc session, there are of course several things to improve, but the compositor is the last big piece missing.