Did you consider a port (written in Rust) instead of a NIF?
When you're presenting a GenServer like message passing interface a port is a natural fit, with none of the risks related to linking a NIF into the VM itself.
(admittedly those risks are much lower with Rust than C)
In our case one of these NIF stores is created per user for a specific task; ironically, with the amount of polish that Rustler puts around NIFs I suspect it would have been more work and more risk to go down the port route and manage everything manually.
