add task::LocalSet API for running !Send futures

tokio/Cargo.toml

@@ -39,6 +39,7 @@ full = [
   "net",
   "process",
   "rt-core",
+  "rt-util",
   "rt-threaded",
   "signal",
   "stream",
@@ -67,6 +68,7 @@ process = [
 ]
 # Includes basic task execution capabilities
 rt-core = []
+rt-util = []
 rt-threaded = [
   "num_cpus",
   "rt-core",

tokio/src/macros/cfg.rs

@@ -52,6 +52,17 @@ macro_rules! cfg_not_blocking_impl {
     }
 }
 
+/// Enable internal `AtomicWaker` impl
+macro_rules! cfg_atomic_waker_impl {
+    ($($item:item)*) => {
+        $(
+            #[cfg(any(feature = "io-driver", feature = "time",  feature = "rt-util"))]
+            #[cfg(not(loom))]
+            $item
+        )*
+    }
+}
+
 macro_rules! cfg_dns {
     ($($item:item)*) => {
         $(
@@ -220,6 +231,16 @@ macro_rules! cfg_rt_threaded {
     }
 }
 
+macro_rules! cfg_rt_util {
+    ($($item:item)*) => {
+        $(
+            #[cfg(feature = "rt-util")]
+            #[cfg_attr(docsrs, doc(cfg(feature = "rt-util")))]
+            $item
+        )*
+    }
+}
+
 macro_rules! cfg_not_rt_threaded {
     ($($item:item)*) => {
         $( #[cfg(not(feature = "rt-threaded"))] $item )*

tokio/src/runtime/basic_scheduler.rs

@@ -1,5 +1,5 @@
 use crate::park::{Park, Unpark};
-use crate::task::{self, JoinHandle, Schedule, Task};
+use crate::task::{self, JoinHandle, Schedule, ScheduleSendOnly, Task};
 
 use std::cell::UnsafeCell;
 use std::collections::VecDeque;
@@ -304,6 +304,8 @@ impl Schedule for SchedulerPriv {
     }
 }
 
+impl ScheduleSendOnly for SchedulerPriv {}
+
 impl<P> Drop for BasicScheduler<P>
 where
     P: Park,

tokio/src/runtime/blocking/schedule.rs

@@ -1,4 +1,4 @@
-use crate::task::{Schedule, Task};
+use crate::task::{Schedule, ScheduleSendOnly, Task};
 
 /// `task::Schedule` implementation that does nothing. This is unique to the
 /// blocking scheduler as tasks scheduled are not really futures but blocking
@@ -16,3 +16,5 @@ impl Schedule for NoopSchedule {
         unreachable!();
     }
 }
+
+impl ScheduleSendOnly for NoopSchedule {}

tokio/src/runtime/thread_pool/shared.rs

@@ -1,7 +1,7 @@
 use crate::park::Unpark;
-use crate::runtime::Unparker;
 use crate::runtime::thread_pool::slice;
-use crate::task::{self, Schedule, Task};
+use crate::runtime::Unparker;
+use crate::task::{self, Schedule, ScheduleSendOnly, Task};
 
 use std::ptr;
 
@@ -90,3 +90,5 @@ impl Schedule for Shared {
         Self::schedule(self, task);
     }
 }
+
+impl ScheduleSendOnly for Shared {}

tokio/src/sync/mod.rs

@@ -35,7 +35,7 @@ cfg_sync! {
 }
 
 cfg_not_sync! {
-    cfg_resource_drivers! {
+    cfg_atomic_waker_impl! {
         mod task;
         pub(crate) use task::AtomicWaker;
     }