WIP: task::scope

This change adds task::scope as a mechanism for supporting
structured concurrency as described in tokio-rs#1879.

In this version of the change scopes can be created using a freestanding
`task::scope` function, which creates a new detached scope (which is not
coupled to the parent scope), `Scope::detached` and `Scope::with_parent`,
and a `::child()` method on an existing scope handle.

Since a few of of those methods are doing the same thing, some of those
might get dropped before the change is merged. For future extensibility
also a `Scope::with_config` and `ScopeConfigBuilder` are demonstrated in
this change. Those might also be part of the initial change.

tokio/Cargo.toml

@@ -41,10 +41,12 @@ full = [
   "rt-core",
   "rt-util",
   "rt-threaded",
+  "scope",
   "signal",
   "stream",
   "sync",
   "time",
+  "futures"
 ]
 
 blocking = ["rt-core"]
@@ -73,6 +75,7 @@ rt-threaded = [
   "num_cpus",
   "rt-core",
 ]
+scope = []
 signal = [
   "io-driver",
   "lazy_static",
@@ -99,6 +102,7 @@ pin-project-lite = "0.1.1"
 # Everything else is optional...
 fnv = { version = "1.0.6", optional = true }
 futures-core = { version = "0.3.0", optional = true }
+futures = { version = "0.3.0", optional = true }
 lazy_static = { version = "1.0.2", optional = true }
 memchr = { version = "2.2", optional = true }
 mio = { version = "0.6.20", optional = true }
@@ -122,7 +126,7 @@ optional = true
 
 [dev-dependencies]
 tokio-test = { version = "0.2.0", path = "../tokio-test" }
-futures = { version = "0.3.0", features = ["async-await"] }
+futures = { version = "0.3.0", features = ["async-await", "executor"] }
 futures-test = "0.3.0"
 proptest = "0.9.4"
 tempfile = "3.1.0"

tokio/src/macros/cfg.rs

@@ -204,6 +204,16 @@ macro_rules! cfg_process {
     }
 }
 
+macro_rules! cfg_scope {
+    ($($item:item)*) => {
+        $(
+            #[cfg(feature = "scope")]
+            #[cfg_attr(docsrs, doc(cfg(feature = "scope")))]
+            $item
+        )*
+    }
+}
+
 macro_rules! cfg_signal {
     ($($item:item)*) => {
         $(

tokio/src/sync/mod.rs

@@ -460,6 +460,11 @@ cfg_sync! {
     mod task;
     pub(crate) use task::AtomicWaker;
 
+    cfg_unstable! {
+        mod wait_group;
+        pub(crate) use wait_group::{SharedWaitGroup};
+    }
+
     pub mod watch;
 }