Introduce tokio_old feature

This feature allows users to rely on the `0.2` version of `tokio` while
the async ecosystem upgrades to the latest version.

futures/Cargo.toml

@@ -19,6 +19,12 @@ log = "0.4"
 [dependencies.futures]
 version = "0.3"
 
+[target.'cfg(not(target_arch = "wasm32"))'.dependencies.tokio_old]
+package = "tokio"
+version = "0.2"
+optional = true
+features = ["rt-core", "rt-threaded", "time", "stream"]
+
 [target.'cfg(not(target_arch = "wasm32"))'.dependencies.tokio]
 version = "0.3"
 optional = true

futures/src/executor.rs

@@ -7,6 +7,9 @@ mod thread_pool;
 #[cfg(all(not(target_arch = "wasm32"), feature = "tokio"))]
 mod tokio;
 
+#[cfg(all(not(target_arch = "wasm32"), feature = "tokio_old"))]
+mod tokio_old;
+
 #[cfg(all(not(target_arch = "wasm32"), feature = "async-std"))]
 mod async_std;
 
@@ -21,6 +24,9 @@ pub use thread_pool::ThreadPool;
 #[cfg(all(not(target_arch = "wasm32"), feature = "tokio"))]
 pub use self::tokio::Tokio;
 
+#[cfg(all(not(target_arch = "wasm32"), feature = "tokio_old"))]
+pub use self::tokio_old::TokioOld;
+
 #[cfg(all(not(target_arch = "wasm32"), feature = "async-std"))]
 pub use self::async_std::AsyncStd;
 

futures/src/executor/tokio_old.rs

@@ -0,0 +1,21 @@
+use crate::Executor;
+
+use futures::Future;
+
+/// An old `tokio` runtime.
+#[cfg_attr(docsrs, doc(cfg(feature = "tokio_old")))]
+pub type TokioOld = tokio_old::runtime::Runtime;
+
+impl Executor for TokioOld {
+    fn new() -> Result<Self, futures::io::Error> {
+        tokio_old::runtime::Runtime::new()
+    }
+
+    fn spawn(&self, future: impl Future<Output = ()> + Send + 'static) {
+        let _ = tokio_old::runtime::Runtime::spawn(self, future);
+    }
+
+    fn enter<R>(&self, f: impl FnOnce() -> R) -> R {
+        tokio_old::runtime::Runtime::enter(self, f)
+    }
+}

futures/src/lib.rs

@@ -15,7 +15,7 @@ pub mod executor;
 pub mod subscription;
 
 #[cfg(all(
-    any(feature = "tokio", feature = "async-std"),
+    any(feature = "tokio", feature = "tokio_old", feature = "async-std"),
     not(target_arch = "wasm32")
 ))]
 #[cfg_attr(docsrs, doc(cfg(any(feature = "tokio", feature = "async-std"))))]

futures/src/time.rs

@@ -41,7 +41,10 @@ where
     }
 }
 
-#[cfg(all(feature = "tokio", not(feature = "async-std")))]
+#[cfg(all(
+    any(feature = "tokio", feature = "tokio_old"),
+    not(feature = "async-std")
+))]
 impl<H, E> subscription::Recipe<H, E> for Every
 where
     H: std::hash::Hasher,
@@ -61,6 +64,9 @@ where
     ) -> futures::stream::BoxStream<'static, Self::Output> {
         use futures::stream::StreamExt;
 
+        #[cfg(feature = "tokio_old")]
+        use tokio_old as tokio;
+
         let start = tokio::time::Instant::now() + self.0;
 
         tokio::time::interval_at(start, self.0)