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, 26 insertions, 29 deletions

diff --git a/tokio/src/sync/mod.rs b/tokio/src/sync/mod.rs
index cf003816..40566c0c 100644
--- a/tokio/src/sync/mod.rs
+++ b/tokio/src/sync/mod.rs
@@ -13,43 +13,40 @@
 //! - [watch](watch/index.html), a single-producer, multi-consumer channel that
 //!   only stores the **most recently** sent value.
 
-macro_rules! debug {
-    ($($t:tt)*) => {
-        if false {
-            println!($($t)*);
-        }
-    }
-}
+cfg_sync! {
+    mod barrier;
+    pub use barrier::{Barrier, BarrierWaitResult};
 
-macro_rules! if_loom {
-    ($($t:tt)*) => {{
-        #[cfg(loom)]
-        const LOOM: bool = true;
-        #[cfg(not(loom))]
-        const LOOM: bool = false;
-
-        if LOOM {
-            $($t)*
-        }
-    }}
-}
+    pub mod mpsc;
 
-mod barrier;
-pub use barrier::{Barrier, BarrierWaitResult};
+    mod mutex;
+    pub use mutex::{Mutex, MutexGuard};
 
-pub mod mpsc;
+    pub mod oneshot;
 
-mod mutex;
-pub use mutex::{Mutex, MutexGuard};
+    pub(crate) mod semaphore;
 
-pub mod oneshot;
+    mod task;
+    pub(crate) use task::AtomicWaker;
 
-pub mod semaphore;
+    pub mod watch;
+}
 
-mod task;
-pub(crate) use task::AtomicWaker;
+cfg_not_sync! {
+    cfg_atomic_waker! {
+        mod task;
+        pub(crate) use task::AtomicWaker;
+    }
 
-pub mod watch;
+    cfg_rt_threaded! {
+        pub(crate) mod oneshot;
+    }
+
+    cfg_signal! {
+        pub(crate) mod mpsc;
+        pub(crate) mod semaphore;
+    }
+}
 
 /// Unit tests
 #[cfg(test)]