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, 2 insertions, 11 deletions

diff --git a/tokio/src/time/clock.rs b/tokio/src/time/clock.rs
index 7727f404..e44f4925 100644
--- a/tokio/src/time/clock.rs
+++ b/tokio/src/time/clock.rs
@@ -4,15 +4,7 @@
 //! `test-util` feature flag is enabled, the values returned for `now()` are
 //! configurable.
 
-#[cfg(feature = "test-util")]
-pub(crate) use self::variant::now;
-pub(crate) use self::variant::Clock;
-#[cfg(feature = "test-util")]
-#[allow(unreachable_pub)] // https://github.com/rust-lang/rust/issues/57411
-pub use self::variant::{advance, pause, resume};
-
-#[cfg(not(feature = "test-util"))]
-mod variant {
+cfg_not_test_util! {
     use crate::time::Instant;
 
     #[derive(Debug, Clone)]
@@ -40,8 +32,7 @@ mod variant {
     }
 }
 
-#[cfg(feature = "test-util")]
-mod variant {
+cfg_test_util! {
     use crate::time::{Duration, Instant};
 
     use std::cell::Cell;