Minimal

Cargo.toml

[package]
name = "test_data_fusion"
version = "0.1.0"
edition = "2021"

# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html

[dependencies]
futures = "0.3.24"
futures-util = "0.3.24"

main.rs

use std::pin::Pin;
use std::task::{Context, Poll};
use futures::Stream;
use futures_util::stream;

struct A;
struct B;

impl Stream for A {
    type Item = A;

    fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
        todo!()
    }
}

fn get_stream<'a>() -> impl Stream<Item = A> + Send + Sync + Unpin + 'a {
    stream::iter(vec![A])
}

struct Wrapper<X>(X);

impl<X> Wrapper<X> {
    fn new(x: X) -> Self {
        Wrapper(x)
    }
}

impl<X> Stream for Wrapper<X> {
    type Item = B;

    fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
        todo!()
    }
}

fn execute() -> Pin<Box<dyn Stream<Item = B>>> {
    let stream = get_stream();
    let wrapper = Wrapper(stream);
    Box::pin(wrapper)
}


fn main() {
    println!("Hello, world!");
}