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

/// 创建一个 future，它包装返回 [`Poll`] 的函数。
///
/// 将 future 委托轮询到包装的函数。
/// 如果返回的 future 是固定的，那么被包装函数的捕获环境也被固定在原地，因此只要闭包不移出它的捕获，它就可以正确地为它们创建固定引用。
///
///
/// # Examples
///
/// ```
/// # async fn run() {
/// use core::future::poll_fn;
/// use std::task::{Context, Poll};
///
/// fn read_line(_cx: &mut Context<'_>) -> Poll<String> {
///     Poll::Ready("Hello, World!".into())
/// }
///
/// let read_future = poll_fn(read_line);
/// assert_eq!(read_future.await, "Hello, World!".to_owned());
/// # }
/// ```
#[stable(feature = "future_poll_fn", since = "1.64.0")]
pub fn poll_fn<T, F>(f: F) -> PollFn<F>
where
    F: FnMut(&mut Context<'_>) -> Poll<T>,
{
    PollFn { f }
}

/// Future，它包装返回 [`Poll`] 的函数。
///
/// 该 `struct` 由 [`poll_fn()`] 创建。
/// 有关更多信息，请参见其文档。
#[must_use = "futures do nothing unless you `.await` or poll them"]
#[stable(feature = "future_poll_fn", since = "1.64.0")]
pub struct PollFn<F> {
    f: F,
}

#[stable(feature = "future_poll_fn", since = "1.64.0")]
impl<F: Unpin> Unpin for PollFn<F> {}

#[stable(feature = "future_poll_fn", since = "1.64.0")]
impl<F> fmt::Debug for PollFn<F> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("PollFn").finish()
    }
}

#[stable(feature = "future_poll_fn", since = "1.64.0")]
impl<T, F> Future for PollFn<F>
where
    F: FnMut(&mut Context<'_>) -> Poll<T>,
{
    type Output = T;

    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<T> {
        // SAFETY: 我们不会离开固定领域。
        (unsafe { &mut self.get_unchecked_mut().f })(cx)
    }
}