• Make sure your projects mix.exs has the :rustler compiler listed in the project function: compilers: [:rustler] ++ Mix.compilers() If there already is a :compilers list, you should append :rustler to it.
• Add your crate to the rustler_crates attribute in the `project function. See here.
• Your NIF will now build along with your project.

defmodule Mjml do
    use Rustler, otp_app: :mjml, crate: "mjml_nif"

    # When your NIF is loaded, it will override this function.
    def add(_a, _b), do: :erlang.nif_error(:nif_not_loaded)
end

This is a complete example of a NIF written in Rust.