add #[must_use] to more futures and streams (#1309)

tokio/src/io/copy.rs

@@ -6,6 +6,7 @@ use std::task::{Context, Poll};
 use tokio_io::{AsyncRead, AsyncWrite};
 
 #[derive(Debug)]
+#[must_use = "futures do nothing unless you `.await` or poll them"]
 pub struct Copy<'a, R: ?Sized, W: ?Sized> {
     reader: &'a mut R,
     read_done: bool,

tokio/src/io/read.rs

@@ -22,6 +22,7 @@ where
 ///
 /// Created by the [`read`] function.
 #[derive(Debug)]
+#[must_use = "futures do nothing unless you `.await` or poll them"]
 pub struct Read<'a, R: ?Sized> {
     reader: &'a mut R,
     buf: &'a mut [u8],

tokio/src/io/read_exact.rs

@@ -28,6 +28,7 @@ where
 ///
 /// On success the number of bytes is returned
 #[derive(Debug)]
+#[must_use = "futures do nothing unless you `.await` or poll them"]
 pub struct ReadExact<'a, A: ?Sized> {
     reader: &'a mut A,
     buf: &'a mut [u8],

tokio/src/io/write.rs

@@ -7,6 +7,7 @@ use tokio_io::AsyncWrite;
 
 /// A future to write some of the buffer to an `AsyncWrite`.
 #[derive(Debug)]
+#[must_use = "futures do nothing unless you `.await` or poll them"]
 pub struct Write<'a, W: ?Sized> {
     writer: &'a mut W,
     buf: &'a [u8],