Make C++ code call Rust's instance of object

help
#1

I have a way to create, from Rust, an instance of an OpenVpn object in C++. It is represented by the opaque struct OpenVpnInstance. I want to make the C++ OpenVpn object call open_vpn_receive when it has data. However, obviously, it should call into my OpenVpn struct instance, so it can insert into the OpenVpn::data Vec of buffers.

#[repr(C)]
pub struct OpenVpnInstance {
    _private: [u8; 0],
}

type VpnReceive = extern "C" fn(*mut OpenVpnInstance, *mut c_uchar, *mut size_t) -> u8;

extern "C" {
    pub fn openvpn_new(uri: *const c_char) -> *mut OpenVpnInstance;
    pub fn openvpn_set_on_receive(instance: *mut OpenVpnInstance, on_receive: VpnReceive);

struct OpenVpn{
    data: Vec<Vec<u8>>,
    instance: *mut interface::OpenVpnInstance
}

extern "C" fn open_vpn_receive(instance: *mut interface::OpenVpnInstance, data: *mut c_uchar, size: *mut size_t) -> u8 {
     //This function should be able to call `openvpn.data.insert` for the `openvpn: OpenVpn` that has this `instance`
}

How I can achieve that?