Stream无法取消固定

针对此问题，可以使用box操作符将Stream包装在一个Box指针中，这将使Stream具有固定大小的哈希租借。以下是一个示例：

use std::pin::Pin;
use std::task::{Context, Poll};
use futures::{Stream, StreamExt};
use futures::task::noop_waker;

struct MyStream<T> {
    items: Vec<T>,
}

impl<T> Stream for MyStream<T> {
    type Item = T;

    fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>>> {
        let this = self.get_mut();
        if !this.items.is_empty() {
            Poll::Ready(Some(this.items.remove(0)))
        } else {
            let _ = cx.waker().clone().wake();
            Poll::Pending
        }
    }
}

fn main() {
    let mut my_stream = MyStream { items: vec![1, 2, 3] };
    let mut boxed_stream = Box::pin(my_stream);

    let _ = futures::executor::block_on(async {
        while let Some(item) = boxed_stream.next().await {
            println!("{}", item);
        }
    });
}