It's been moving slowly at first because you need a lot of bindings done before you can do interesting work, and bindings/FFI tend to be fiddly, error prone things that you want to take your time on and get right - that's where you deal with the impedance mismatch between C and Rust and have to get all the implicit rules expressed in the type system (if you can).

It'll go faster once all the bindings are in place and people have more experience with this stuff. I've been greatly looking forward to expanding bcachefs's use of rust, right now it's just in userspace but I've got some initial bindings for bcachefs's core btree API.

Real iterators, closures, better data types, all that stuff is going to be so nice when it can replace pages and pages of macro madness.