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, 5 insertions, 3 deletions

diff --git a/tokio/src/prelude.rs b/tokio/src/prelude.rs
index 8675c577..6e62c519 100644
--- a/tokio/src/prelude.rs
+++ b/tokio/src/prelude.rs
@@ -12,6 +12,8 @@
 //! The prelude may grow over time as additional items see ubiquitous use.
 
 pub use crate::io::{AsyncBufRead, AsyncRead, AsyncWrite};
-#[cfg(feature = "io-util")]
-#[doc(no_inline)]
-pub use crate::io::{AsyncBufReadExt as _, AsyncReadExt as _, AsyncWriteExt as _};
+
+cfg_io_util! {
+    #[doc(no_inline)]
+    pub use crate::io::{AsyncBufReadExt as _, AsyncReadExt as _, AsyncWriteExt as _};
+}