Back at it; in the end my problem is that cargo build fails because it can't find openssl (and pkg-config). I've tried to add:
buildInputs = [ pkgs.openssl pkgs.pkg-config ];
I've also tried with nativeBuildInputs, changed nothing.
The doc I've found ( https://nixos.org/manual/nixpkgs/stable/#compiling-rust-applications-with-cargo ) was not very helpful. Where can I find the full description of all the configuration options of buildRustPackage ? Surely there's something I'm doing wrong, but what?
