struct ComputeHeavy<F: Future, P> {
    source: F,
    prepare: P,
    execute: Option<Box<dyn Future<Output = F::Output>>>
}

impl<F, P> Future for ComputeHeavy<F, P>
where
    F: Future,
    P: Fn(&mut F) -> Poll<()>
{
    // ..

    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        if let Some(exec) = self.execute.as_mut() {
            // poll exec
        }
    
        (self.prepare)(self.source)?;
        self.execute
            .insert(execute(|| self.source.poll(cx))))
            .poll(cx)
    }
}

let mut fut = acceptor.accept(io);
let tls_stream = ComputeHeavy::new(
    // source
    fut,
    // prepare
    |fut| fut.get_mut().unwrap().poll_fill_buf(cx)
).await;