#![feature(async_await, futures_api)]

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

struct CustomFuture<F> {
    inner: F,
}

impl<F: Unpin> CustomFuture<F> {
    fn inner_pinned<'a>(self: Pin<&'a mut Self>) -> Pin<&'a mut F> {
        unsafe { self.map_unchecked_mut(|custom_fut| &mut custom_fut.inner) }
    }
}

impl<F, T, E> Future for CustomFuture<F>
where
    F: Future<Output = Result<T, E>> + Unpin,
{
    type Output = F::Output;

    fn poll(self: Pin<&mut Self>, waker: &mut Context<'_>) -> Poll<Self::Output> {
        match self.inner_pinned().poll(waker) {
            v @ Poll::Ready(Ok(_)) => v,
            v @ Poll::Pending => v,
            e @ Poll::Ready(Err(_)) => e,
        }
    }
}

fn main() {}