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.

4 files changed, 8 insertions, 18 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)
+        }
     }
 }
 
diff --git a/tokio/src/sync/mpsc/chan.rs b/tokio/src/sync/mpsc/chan.rs
index 03f35339..4030e380 100644
--- a/tokio/src/sync/mpsc/chan.rs
+++ b/tokio/src/sync/mpsc/chan.rs
@@ -299,12 +299,6 @@ where
             // second time here.
             try_recv!();
 
-            debug!(
-                "recv; rx_closed = {:?}; is_idle = {:?}",
-                rx_fields.rx_closed,
-                self.inner.semaphore.is_idle()
-            );
-
             if rx_fields.rx_closed && self.inner.semaphore.is_idle() {
                 Ready(None)
             } else {
diff --git a/tokio/src/sync/mpsc/list.rs b/tokio/src/sync/mpsc/list.rs
index eecc4da3..dc956403 100644
--- a/tokio/src/sync/mpsc/list.rs
+++ b/tokio/src/sync/mpsc/list.rs
@@ -169,7 +169,6 @@ impl<T> Tx<T> {
     }
 
     pub(crate) unsafe fn reclaim_block(&self, mut block: NonNull<Block<T>>) {
-        debug!("+ reclaim_block({:p})", block);
         // The block has been removed from the linked list and ownership
         // is reclaimed.
         //
@@ -206,7 +205,6 @@ impl<T> Tx<T> {
         }
 
         if !reused {
-            debug!(" + block freed {:p}", block);
             let _ = Box::from_raw(block.as_ptr());
         }
     }
@@ -226,7 +224,6 @@ impl<T> Rx<T> {
     pub(crate) fn pop(&mut self, tx: &Tx<T>) -> Option<block::Read<T>> {
         // Advance `head`, if needed
         if !self.try_advancing_head() {
-            debug!("+ !self.try_advancing_head() -> false");
             return None;
         }
 
@@ -276,8 +273,6 @@ impl<T> Rx<T> {
     }
 
     fn reclaim_blocks(&mut self, tx: &Tx<T>) {
-        debug!("+ reclaim_blocks()");
-
         while self.free_head != self.head {
             unsafe {
                 // Get a handle to the block that will be freed and update
@@ -316,7 +311,6 @@ impl<T> Rx<T> {
     /// Effectively `Drop` all the blocks. Should only be called once, when
     /// the list is dropping.
     pub(super) unsafe fn free_blocks(&mut self) {
-        debug!("+ free_blocks()");
         debug_assert_ne!(self.free_head, NonNull::dangling());
 
         let mut cur = Some(self.free_head);
@@ -331,7 +325,6 @@ impl<T> Rx<T> {
 
         while let Some(block) = cur {
             cur = block.as_ref().load_next(Relaxed);
-            debug!(" + free: block = {:p}", block);
             drop(Box::from_raw(block.as_ptr()));
         }
     }
diff --git a/tokio/src/sync/mpsc/mod.rs b/tokio/src/sync/mpsc/mod.rs
index 7927dde6..60ae60cd 100644
--- a/tokio/src/sync/mpsc/mod.rs
+++ b/tokio/src/sync/mpsc/mod.rs
@@ -1,3 +1,5 @@
+#![cfg_attr(not(feature = "sync"), allow(dead_code, unreachable_pub))]
+
 //! A multi-producer, single-consumer queue for sending values across
 //! asynchronous tasks.
 //!