rt: cleanup and simplify scheduler (scheduler v2.5) (#2273)

A refactor of the scheduler internals focusing on simplifying and
reducing unsafety. There are no fundamental logic changes.

* The state transitions of the core task component are refined and
reduced.
* `basic_scheduler` has most unsafety removed.
* `local_set` has most unsafety removed.
* `threaded_scheduler` limits most unsafety to its queue implementation.

1 files changed, 7 insertions, 6 deletions

diff --git a/tokio/src/util/mod.rs b/tokio/src/util/mod.rs
index 2761f725..c2f572f1 100644
--- a/tokio/src/util/mod.rs
+++ b/tokio/src/util/mod.rs
@@ -3,17 +3,18 @@ cfg_io_driver! {
     pub(crate) mod slab;
 }
 
-cfg_sync! {
-    pub(crate) mod linked_list;
-}
+#[cfg(any(feature = "sync", feature = "rt-core"))]
+pub(crate) mod linked_list;
 
 #[cfg(any(feature = "rt-threaded", feature = "macros", feature = "stream"))]
 mod rand;
 
-cfg_rt_threaded! {
-    mod pad;
-    pub(crate) use pad::CachePadded;
+cfg_rt_core! {
+    mod wake;
+    pub(crate) use wake::{waker_ref, Wake};
+}
 
+cfg_rt_threaded! {
     pub(crate) use rand::FastRand;
 
     mod try_lock;