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use std::pin::Pin;
use std::future::Future;

use crate::task::{Context, Poll};

/// Creates a new future wrapping around a function returning [`Poll`].

///

/// Polling the returned future delegates to the wrapped function.

///

/// # Examples

///

/// ```

/// # async_std::task::block_on(async {

/// #

/// use async_std::future;

/// use async_std::task::{Context, Poll};

///

/// fn poll_greeting(_: &mut Context<'_>) -> Poll<String> {

///     Poll::Ready("hello world".to_string())

/// }

///

/// assert_eq!(future::poll_fn(poll_greeting).await, "hello world");

/// #

/// # })

/// ```

pub async fn poll_fn<F, T>(f: F) -> T
where
    F: FnMut(&mut Context<'_>) -> Poll<T>,
{
    let fut = PollFn { f };
    fut.await
}

struct PollFn<F> {
    f: F,
}

impl<F> Unpin for PollFn<F> {}

impl<T, F> Future for PollFn<F>
where
    F: FnMut(&mut Context<'_>) -> Poll<T>,
{
    type Output = T;

    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<T> {
        (&mut self.f)(cx)
    }
}