stream: update features and doc for broadcast/watch stream (#3504)

tokio-stream/Cargo.toml

@@ -25,12 +25,13 @@ time = ["tokio/time"]
 net = ["tokio/net"]
 io-util = ["tokio/io-util"]
 fs = ["tokio/fs"]
+sync = ["tokio/sync", "tokio-util"]
 
 [dependencies]
 futures-core = { version = "0.3.0" }
 pin-project-lite = "0.2.0"
 tokio = { version = "1.0", features = ["sync"] }
-tokio-util = { version = "0.6.3" }
+tokio-util = { version = "0.6.3", optional = true }
 
 [dev-dependencies]
 tokio = { version = "1.0", features = ["full", "test-util"] }

tokio-stream/src/macros.rs

@@ -38,6 +38,16 @@ macro_rules! cfg_time {
     }
 }
 
+macro_rules! cfg_sync {
+    ($($item:item)*) => {
+        $(
+            #[cfg(feature = "sync")]
+            #[cfg_attr(docsrs, doc(cfg(feature = "sync")))]
+            $item
+        )*
+    }
+}
+
 macro_rules! ready {
     ($e:expr $(,)?) => {
         match $e {

tokio-stream/src/wrappers.rs

@@ -1,17 +1,25 @@
 //! Wrappers for Tokio types that implement `Stream`.
 
+/// Error types for the wrappers.
+pub mod errors {
+    cfg_sync! {
+        pub use crate::wrappers::broadcast::BroadcastStreamRecvError;
+    }
+}
+
 mod mpsc_bounded;
 pub use mpsc_bounded::ReceiverStream;
 
 mod mpsc_unbounded;
 pub use mpsc_unbounded::UnboundedReceiverStream;
 
-mod broadcast;
-pub use broadcast::BroadcastStream;
-pub use broadcast::BroadcastStreamRecvError;
+cfg_sync! {
+    mod broadcast;
+    pub use broadcast::BroadcastStream;
 
-mod watch;
-pub use watch::WatchStream;
+    mod watch;
+    pub use watch::WatchStream;
+}
 
 cfg_time! {
     mod interval;

tokio-stream/src/wrappers/broadcast.rs

@@ -12,6 +12,7 @@ use std::task::{Context, Poll};
 ///
 /// [`tokio::sync::broadcast::Receiver`]: struct@tokio::sync::broadcast::Receiver
 /// [`Stream`]: trait@crate::Stream
+#[cfg_attr(docsrs, doc(cfg(feature = "sync")))]
 pub struct BroadcastStream<T> {
     inner: ReusableBoxFuture<(Result<T, RecvError>, Receiver<T>)>,
 }

tokio-stream/src/wrappers/watch.rs

@@ -12,6 +12,7 @@ use tokio::sync::watch::error::RecvError;
 ///
 /// [`tokio::sync::watch::Receiver`]: struct@tokio::sync::watch::Receiver
 /// [`Stream`]: trait@crate::Stream
+#[cfg_attr(docsrs, doc(cfg(feature = "sync")))]
 pub struct WatchStream<T> {
     inner: ReusableBoxFuture<(Result<(), RecvError>, Receiver<T>)>,
 }

tokio/src/sync/broadcast.rs

@@ -163,6 +163,11 @@ pub struct Sender<T> {
 /// Must not be used concurrently. Messages may be retrieved using
 /// [`recv`][Receiver::recv].
 ///
+/// To turn this receiver into a `Stream`, you can use the [`BroadcastStream`]
+/// wrapper.
+///
+/// [`BroadcastStream`]: https://docs.rs/tokio-stream/0.1/tokio_stream/wrappers/struct.BroadcastStream.html
+///
 /// # Examples
 ///
 /// ```

tokio/src/sync/watch.rs

@@ -61,6 +61,11 @@ use std::ops;
 /// Receives values from the associated [`Sender`](struct@Sender).
 ///
 /// Instances are created by the [`channel`](fn@channel) function.
+///
+/// To turn this receiver into a `Stream`, you can use the [`WatchStream`]
+/// wrapper.
+///
+/// [`WatchStream`]: https://docs.rs/tokio-stream/0.1/tokio_stream/wrappers/struct.WatchStream.html
 #[derive(Debug)]
 pub struct Receiver<T> {
     /// Pointer to the shared state