Vendor dependencies

Let's see how I like this workflow.

vendor/tokio/tests/io_write.rs

@@ -0,0 +1,58 @@
+#![warn(rust_2018_idioms)]
+#![cfg(feature = "full")]
+
+use tokio::io::{AsyncWrite, AsyncWriteExt};
+use tokio_test::assert_ok;
+
+use bytes::BytesMut;
+use std::io;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+#[tokio::test]
+async fn write() {
+    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);
+            self.buf.extend(&buf[0..4]);
+            Ok(4).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 n = assert_ok!(wr.write(b"hello world").await);
+    assert_eq!(n, 4);
+    assert_eq!(wr.buf, b"hell"[..]);
+}
+
+#[tokio::test]
+async fn write_cursor() {
+    use std::io::Cursor;
+
+    let mut wr = Cursor::new(Vec::new());
+
+    let n = assert_ok!(wr.write(b"hello world").await);
+    assert_eq!(n, 11);
+    assert_eq!(wr.get_ref().as_slice(), &b"hello world"[..]);
+}