Vendor dependencies

Let's see how I like this workflow.

vendor/tokio/tests/io_copy.rs

@@ -0,0 +1,87 @@
+#![warn(rust_2018_idioms)]
+#![cfg(feature = "full")]
+
+use bytes::BytesMut;
+use futures::ready;
+use tokio::io::{self, AsyncRead, AsyncReadExt, AsyncWrite, AsyncWriteExt, ReadBuf};
+use tokio_test::assert_ok;
+
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+#[tokio::test]
+async fn copy() {
+    struct Rd(bool);
+
+    impl AsyncRead for Rd {
+        fn poll_read(
+            mut self: Pin<&mut Self>,
+            _cx: &mut Context<'_>,
+            buf: &mut ReadBuf<'_>,
+        ) -> Poll<io::Result<()>> {
+            if self.0 {
+                buf.put_slice(b"hello world");
+                self.0 = false;
+                Poll::Ready(Ok(()))
+            } else {
+                Poll::Ready(Ok(()))
+            }
+        }
+    }
+
+    let mut rd = Rd(true);
+    let mut wr = Vec::new();
+
+    let n = assert_ok!(io::copy(&mut rd, &mut wr).await);
+    assert_eq!(n, 11);
+    assert_eq!(wr, b"hello world");
+}
+
+#[tokio::test]
+async fn proxy() {
+    struct BufferedWd {
+        buf: BytesMut,
+        writer: io::DuplexStream,
+    }
+
+    impl AsyncWrite for BufferedWd {
+        fn poll_write(
+            self: Pin<&mut Self>,
+            _cx: &mut Context<'_>,
+            buf: &[u8],
+        ) -> Poll<io::Result<usize>> {
+            self.get_mut().buf.extend_from_slice(buf);
+            Poll::Ready(Ok(buf.len()))
+        }
+
+        fn poll_flush(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
+            let this = self.get_mut();
+
+            while !this.buf.is_empty() {
+                let n = ready!(Pin::new(&mut this.writer).poll_write(cx, &this.buf))?;
+                let _ = this.buf.split_to(n);
+            }
+
+            Pin::new(&mut this.writer).poll_flush(cx)
+        }
+
+        fn poll_shutdown(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
+            Pin::new(&mut self.writer).poll_shutdown(cx)
+        }
+    }
+
+    let (rd, wd) = io::duplex(1024);
+    let mut rd = rd.take(1024);
+    let mut wd = BufferedWd {
+        buf: BytesMut::new(),
+        writer: wd,
+    };
+
+    // write start bytes
+    assert_ok!(wd.write_all(&[0x42; 512]).await);
+    assert_ok!(wd.flush().await);
+
+    let n = assert_ok!(io::copy(&mut rd, &mut wd).await);
+
+    assert_eq!(n, 1024);
+}