chore: refine feature flags (#1785)

Removes dependencies between Tokio feature flags. For example, `process`
should not depend on `sync` simply because it uses the `mpsc` channel.
Instead, feature flags represent **public** APIs that become available
with the feature enabled. When the feature is not enabled, the
functionality is removed. If another Tokio component requires the
functionality, it is stays as `pub(crate)`.

The threaded scheduler is now exposed under `rt-threaded`. This feature
flag only enables the threaded scheduler and does not include I/O,
networking, or time. Those features must be explictly enabled.

A `full` feature flag is added that enables all features.

`stdin`, `stdout`, `stderr` are exposed under `io-std`.

Macros are used to scope code by feature flag.

1 files changed, 6 insertions, 5 deletions

diff --git a/tokio/src/sync/mpsc/bounded.rs b/tokio/src/sync/mpsc/bounded.rs
index 523dde75..d635e138 100644
--- a/tokio/src/sync/mpsc/bounded.rs
+++ b/tokio/src/sync/mpsc/bounded.rs
@@ -161,12 +161,13 @@ impl<T> Receiver<T> {
 
 impl<T> Unpin for Receiver<T> {}
 
-#[cfg(feature = "stream")]
-impl<T> futures_core::Stream for Receiver<T> {
-    type Item = T;
+cfg_stream! {
+    impl<T> futures_core::Stream for Receiver<T> {
+        type Item = T;
 
-    fn poll_next(mut self: std::pin::Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<T>> {
-        self.poll_recv(cx)
+        fn poll_next(mut self: std::pin::Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<T>> {
+            self.poll_recv(cx)
+        }
     }
 }