pub struct Server {
    pub(crate) handle: libaudioverse_sys::LavHandle,
    block_callback: Option<Box<Box<'static + FnMut(f64)>>>,

impl Server {
    /// Set the callback to be called per block of synthesized audio
    pub fn set_block_callback<F>(&mut self, callback: F) -> Result<()>
    where
        F: 'static + FnMut(f64),
    {
        let cb: Box<Box<'static + FnMut(f64)>> = Box::new(Box::new(callback));
        let cb_ptr = Box::into_raw(cb);
        self.block_callback = Some(unsafe { Box::from_raw(cb_ptr) });
        check(unsafe {
            libaudioverse_sys::Lav_serverSetBlockCallback(
                self.handle,
                Some(callback_handler),
                cb_ptr as *mut _,
            )
        })?;
        Ok(())
    }

// actual usage:

let server = ...; // construct a server
...
let environment = ...;
let source = SourceNode::new(&server, &environment);

let cb = |_: f64| {
    let (x, y) = ...; // compute the next position of the source, referencing other variables created after server is constructed
    source.position().set(x, y);
};

// trying to set this callback leads to lifetime errors
server.set_block_callback(cb)