> Is it basically just a Rust rewrite which also tries to reduce the complexity of their current just-in-time compiler?

My understanding is it's rather more similar to RPython: the developer does not write the JIT, the developer writes the interpreter and the meta-jit generates a JIT from that. The developer can further add various annotations to guide JIT generation for improved performances.

See Laurence Tratt's Fast Enough VMs in Fast Enough Time on rewriting their Converge VM in RPython: http://tratt.net/laurie/blog/entries/fast_enough_vms_in_fast...