use std::future::Future;
use std::task::{Context, Poll};
use std::pin::Pin;

enum State {
    A(i32),
}

struct MyFuture {
    rx: Pin<Box<dyn Future<Output=i32> + Send>>,
    state: State,
}

impl Future for MyFuture {
    type Output = ();

    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        loop {
            match &mut self.state {
                State::A(n) => {
                    match Pin::new(&mut self.rx).poll(cx) {
                        Poll::Ready(_) => {
                            *n = 10; // 只要有这句就报错
                        }
                        _ => unimplemented!(),
                    }
//                    *n = 10; // 放这里一样报错
                },
            }
        }
    }
}

fn main() {}