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use std::pin::Pin;
use cfg_if::cfg_if;
use crate::future::Future;
use crate::io::{self, SeekFrom};
use crate::task::{Context, Poll};
cfg_if! {
if #[cfg(feature = "docs")] {
use std::ops::{Deref, DerefMut};
#[doc(hidden)]
pub struct ImplFuture<T>(std::marker::PhantomData<T>);
/// Allows seeking through a byte stream.
///
/// This trait is a re-export of [`futures::io::AsyncSeek`] and is an async version of
/// [`std::io::Seek`].
///
/// The [provided methods] do not really exist in the trait itself, but they become
/// available when the prelude is imported:
///
/// ```
/// # #[allow(unused_imports)]
/// use async_std::prelude::*;
/// ```
///
/// [`std::io::Seek`]: https://doc.rust-lang.org/std/io/trait.Seek.html
/// [`futures::io::AsyncSeek`]:
/// https://docs.rs/futures-preview/0.3.0-alpha.17/futures/io/trait.AsyncSeek.html
/// [provided methods]: #provided-methods
pub trait Seek {
/// Attempt to seek to an offset, in bytes, in a stream.
fn poll_seek(
self: Pin<&mut Self>,
cx: &mut Context<'_>,
pos: SeekFrom,
) -> Poll<io::Result<u64>>;
/// Seeks to a new position in a byte stream.
///
/// Returns the new position in the byte stream.
///
/// A seek beyond the end of stream is allowed, but behavior is defined by the
/// implementation.
///
/// # Examples
///
/// ```no_run
/// # fn main() -> std::io::Result<()> { async_std::task::block_on(async {
/// #
/// use async_std::fs::File;
/// use async_std::io::SeekFrom;
/// use async_std::prelude::*;
///
/// let mut file = File::open("a.txt").await?;
///
/// let file_len = file.seek(SeekFrom::End(0)).await?;
/// #
/// # Ok(()) }) }
/// ```
fn seek(&mut self, pos: SeekFrom) -> ImplFuture<io::Result<u64>>
where
Self: Unpin
{
unreachable!()
}
}
impl<T: Seek + Unpin + ?Sized> Seek for Box<T> {
fn poll_seek(
self: Pin<&mut Self>,
cx: &mut Context<'_>,
pos: SeekFrom,
) -> Poll<io::Result<u64>> {
unreachable!()
}
}
impl<T: Seek + Unpin + ?Sized> Seek for &mut T {
fn poll_seek(
self: Pin<&mut Self>,
cx: &mut Context<'_>,
pos: SeekFrom,
) -> Poll<io::Result<u64>> {
unreachable!()
}
}
impl<P> Seek for Pin<P>
where
P: DerefMut + Unpin,
<P as Deref>::Target: Seek,
{
fn poll_seek(
self: Pin<&mut Self>,
cx: &mut Context<'_>,
pos: SeekFrom,
) -> Poll<io::Result<u64>> {
unreachable!()
}
}
} else {
pub use futures_io::AsyncSeek as Seek;
}
}
#[doc(hidden)]
pub trait SeekExt: futures_io::AsyncSeek {
fn seek(&mut self, pos: SeekFrom) -> SeekFuture<'_, Self>
where
Self: Unpin,
{
SeekFuture { seeker: self, pos }
}
}
impl<T: futures_io::AsyncSeek + ?Sized> SeekExt for T {}
#[doc(hidden)]
#[allow(missing_debug_implementations)]
pub struct SeekFuture<'a, T: Unpin + ?Sized> {
seeker: &'a mut T,
pos: SeekFrom,
}
impl<T: SeekExt + Unpin + ?Sized> Future for SeekFuture<'_, T> {
type Output = io::Result<u64>;
fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
let pos = self.pos;
Pin::new(&mut *self.seeker).poll_seek(cx, pos)
}
}