peekable 0.5.0

Buffered peek for `Read` and async readers (tokio, futures): inspect incoming bytes without consuming them. Useful for protocol detection, length-prefixed framing, and header inspection.
Documentation 
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use super::{AsyncPeekable, Buffer, DefaultBuffer};
use ::tokio::io::{AsyncRead, AsyncReadExt};

use pin_project_lite::pin_project;
use std::{
  future::Future,
  io,
  marker::PhantomPinned,
  pin::Pin,
  task::{Context, Poll},
};

pin_project! {
  /// Future for the [`peek_to_string`](super::AsyncPeekExt::peek_to_string) method.
  #[derive(Debug)]
  #[must_use = "futures do nothing unless you `.await` or poll them"]
  pub struct PeekToString<'a, R, B = DefaultBuffer> {
    peekable: &'a mut AsyncPeekable<R, B>,
    // This is the buffer we were provided. It will be replaced with an empty string
    // while reading to postpone utf-8 handling until after reading.
    output: &'a mut String,
    // Make this future `!Unpin` for compatibility with async trait methods.
    #[pin]
    _pin: PhantomPinned,
  }
}

pub(crate) fn peek_to_string<'a, R, B>(
  peekable: &'a mut AsyncPeekable<R, B>,
  string: &'a mut String,
) -> PeekToString<'a, R, B>
where
  R: AsyncRead + Unpin,
{
  PeekToString {
    peekable,
    output: string,
    _pin: PhantomPinned,
  }
}

impl<A, B> Future for PeekToString<'_, A, B>
where
  A: AsyncRead + Unpin,
  B: Buffer,
{
  type Output = io::Result<usize>;

  fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
    let me = self.project();

    let s = match core::str::from_utf8(me.peekable.buffer.as_slice()) {
      Ok(s) => s,
      Err(e) => return Poll::Ready(Err(io::Error::new(io::ErrorKind::InvalidData, e))),
    };

    let original_buf_len = me.output.len();
    let peek_buf_len = me.peekable.buffer.len();
    me.output.push_str(s);
    let fut = me.peekable.reader.read_to_string(me.output);
    ::tokio::pin!(fut);
    match fut.poll(cx) {
      Poll::Ready(Ok(read)) => {
        me.peekable
          .buffer
          .extend_from_slice(&me.output.as_bytes()[original_buf_len + peek_buf_len..])?;
        Poll::Ready(Ok(peek_buf_len + read))
      }
      Poll::Ready(Err(e)) => {
        me.peekable
          .buffer
          .extend_from_slice(&me.output.as_bytes()[original_buf_len + peek_buf_len..])?;
        Poll::Ready(Err(e))
      }
      Poll::Pending => Poll::Pending,
    }
  }
}