Create BufStream from a BufReader + BufWriter (#1609)

This is handy if developers want to construct the inner buffers with a
particular capacity, and still end up with a `BufStream` at the end.

tokio-io/src/io/buf_reader.rs

@@ -26,10 +26,10 @@ use std::{cmp, fmt};
 #[pin_project]
 pub struct BufReader<R> {
     #[pin]
-    inner: R,
-    buf: Box<[u8]>,
-    pos: usize,
-    cap: usize,
+    pub(super) inner: R,
+    pub(super) buf: Box<[u8]>,
+    pub(super) pos: usize,
+    pub(super) cap: usize,
 }
 
 impl<R: AsyncRead> BufReader<R> {

tokio-io/src/io/buf_stream.rs

@@ -55,6 +55,40 @@ impl<RW: AsyncRead + AsyncWrite> BufStream<RW> {
     }
 }
 
+impl<RW> From<BufReader<BufWriter<RW>>> for BufStream<RW> {
+    fn from(b: BufReader<BufWriter<RW>>) -> Self {
+        BufStream(b)
+    }
+}
+
+impl<RW> From<BufWriter<BufReader<RW>>> for BufStream<RW> {
+    fn from(b: BufWriter<BufReader<RW>>) -> Self {
+        // we need to "invert" the reader and writer
+        let BufWriter {
+            inner:
+                BufReader {
+                    inner,
+                    buf: rbuf,
+                    pos,
+                    cap,
+                },
+            buf: wbuf,
+            written,
+        } = b;
+
+        BufStream(BufReader {
+            inner: BufWriter {
+                inner,
+                buf: wbuf,
+                written,
+            },
+            buf: rbuf,
+            pos,
+            cap,
+        })
+    }
+}
+
 impl<RW: AsyncRead + AsyncWrite> AsyncWrite for BufStream<RW> {
     fn poll_write(
         self: Pin<&mut Self>,

tokio-io/src/io/buf_writer.rs

@@ -31,9 +31,9 @@ use std::task::{Context, Poll};
 #[pin_project]
 pub struct BufWriter<W> {
     #[pin]
-    inner: W,
-    buf: Vec<u8>,
-    written: usize,
+    pub(super) inner: W,
+    pub(super) buf: Vec<u8>,
+    pub(super) written: usize,
 }
 
 impl<W: AsyncWrite> BufWriter<W> {