Simultaneous futures compat (#172)

This patch adds opt-in support for futures 0.2.

src/executor/current_thread/mod.rs

@@ -119,6 +119,9 @@ use std::marker::PhantomData;
 use std::rc::Rc;
 use std::time::{Duration, Instant};
 
+#[cfg(feature = "unstable-futures")]
+use futures2;
+
 /// Executes tasks on the current thread
 pub struct CurrentThread<P: Park = ParkThread> {
     /// Execute futures and receive unpark notifications.
@@ -386,6 +389,13 @@ impl tokio_executor::Executor for CurrentThread {
         self.borrow().spawn_local(future);
         Ok(())
     }
+
+    #[cfg(feature = "unstable-futures")]
+    fn spawn2(&mut self, _future: Box<futures2::Future<Item = (), Error = futures2::Never> + Send>)
+        -> Result<(), futures2::executor::SpawnError>
+    {
+        panic!("Futures 0.2 integration is not available for current_thread");
+    }
 }
 
 impl<P: Park> fmt::Debug for CurrentThread<P> {
@@ -591,6 +601,13 @@ impl tokio_executor::Executor for TaskExecutor {
         self.spawn_local(future)
     }
 
+    #[cfg(feature = "unstable-futures")]
+    fn spawn2(&mut self, _future: Box<futures2::Future<Item = (), Error = futures2::Never> + Send>)
+        -> Result<(), futures2::executor::SpawnError>
+    {
+        panic!("Futures 0.2 integration is not available for current_thread");
+    }
+
     fn status(&self) -> Result<(), SpawnError> {
         CURRENT.with(|current| {
             if current.spawn.get().is_some() {