Vendor dependencies

Let's see how I like this workflow.

vendor/tokio/tests/io_write_buf.rs

@@ -0,0 +1,56 @@
+#![warn(rust_2018_idioms)]
+#![cfg(feature = "full")]
+
+use tokio::io::{AsyncWrite, AsyncWriteExt};
+use tokio_test::assert_ok;
+
+use bytes::BytesMut;
+use std::cmp;
+use std::io::{self, Cursor};
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+#[tokio::test]
+async fn write_all() {
+    struct Wr {
+        buf: BytesMut,
+        cnt: usize,
+    }
+
+    impl AsyncWrite for Wr {
+        fn poll_write(
+            mut self: Pin<&mut Self>,
+            _cx: &mut Context<'_>,
+            buf: &[u8],
+        ) -> Poll<io::Result<usize>> {
+            assert_eq!(self.cnt, 0);
+
+            let n = cmp::min(4, buf.len());
+            let buf = &buf[0..n];
+
+            self.cnt += 1;
+            self.buf.extend(buf);
+            Ok(buf.len()).into()
+        }
+
+        fn poll_flush(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<io::Result<()>> {
+            Ok(()).into()
+        }
+
+        fn poll_shutdown(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<io::Result<()>> {
+            Ok(()).into()
+        }
+    }
+
+    let mut wr = Wr {
+        buf: BytesMut::with_capacity(64),
+        cnt: 0,
+    };
+
+    let mut buf = Cursor::new(&b"hello world"[..]);
+
+    assert_ok!(wr.write_buf(&mut buf).await);
+    assert_eq!(wr.buf, b"hell"[..]);
+    assert_eq!(wr.cnt, 1);
+    assert_eq!(buf.position(), 4);
+}