use std::os::raw::c_void;
use std::ptr::NonNull;
type virErrorPtr = *const ();

type virErrorFunc = Option<unsafe extern "C" fn(userData: *mut c_void, error: virErrorPtr)>;
extern "C" {
    fn virSetErrorFunc(userData: *mut c_void, handler: virErrorFunc);
}

struct Error(virErrorPtr);

static mut HANDLER: Option<NonNull<Fn(Error)>> = None;

fn set_error_func<F: Fn(Error) + 'static>(f: F) {
    unsafe extern "C" fn callback<F: Fn(Error)>(userData: *mut c_void, error: virErrorPtr) {
        let f = userData as *const F;
        let wrapped_error = Error(error);
        (*f)(wrapped_error)
    }
    let data = Box::into_raw(Box::new(f));
    unsafe {
        HANDLER.take().map(|p| Box::from_raw(p.as_ptr())); //drop the old one
        HANDLER = Some(NonNull::new_unchecked(data));
        virSetErrorFunc(data as _, Some(callback::<F>))
    }
}

fn main() {}