runtime: merge multi & single threaded runtimes (#1716)

Simplify Tokio's runtime construct by combining both Runtime variants
into a single type. The execution style can be controlled by a
configuration setting on `Builder`.

The implication of this change is that there is no longer any way to
spawn `!Send` futures. This, however, is a temporary limitation. A
different strategy will be employed for supporting `!Send` futures.

Included in this patch is a rework of `task::JoinHandle` to support
using this type from both the thread-pool and current-thread executors.

2 files changed, 16 insertions, 5 deletions

diff --git a/tokio/src/signal/registry.rs b/tokio/src/signal/registry.rs
index e70d0495..d608539c 100644
--- a/tokio/src/signal/registry.rs
+++ b/tokio/src/signal/registry.rs
@@ -178,13 +178,13 @@ where
 #[cfg(all(test, not(loom)))]
 mod tests {
     use super::*;
-    use crate::runtime::current_thread::Runtime;
+    use crate::runtime::{self, Runtime};
     use crate::sync::{mpsc, oneshot};
     use futures::{future, StreamExt};
 
     #[test]
     fn smoke() {
-        let mut rt = Runtime::new().unwrap();
+        let mut rt = rt();
         rt.block_on(async move {
             let registry = Registry::new(vec![
                 EventInfo::default(),
@@ -307,4 +307,8 @@ mod tests {
 
         drop(second_rx);
     }
+
+    fn rt() -> Runtime {
+        runtime::Builder::new().current_thread().build().unwrap()
+    }
 }
diff --git a/tokio/src/signal/windows.rs b/tokio/src/signal/windows.rs
index 1e68d628..96e585ba 100644
--- a/tokio/src/signal/windows.rs
+++ b/tokio/src/signal/windows.rs
@@ -174,13 +174,13 @@ impl Stream for CtrlBreak {
 #[cfg(all(test, not(loom)))]
 mod tests {
     use super::*;
-    use crate::runtime::current_thread::Runtime;
+    use crate::runtime::Runtime;
 
     use futures_util::stream::StreamExt;
 
     #[test]
     fn ctrl_c() {
-        let mut rt = Runtime::new().unwrap();
+        let mut rt = rt();
 
         rt.block_on(async {
             let ctrl_c = crate::signal::ctrl_c().expect("failed to create CtrlC");
@@ -198,7 +198,7 @@ mod tests {
 
     #[test]
     fn ctrl_break() {
-        let mut rt = Runtime::new().unwrap();
+        let mut rt = rt();
 
         rt.block_on(async {
             let ctrl_break = super::ctrl_break().expect("failed to create CtrlC");
@@ -213,4 +213,11 @@ mod tests {
             let _ = ctrl_break.into_future().await;
         });
     }
+
+    fn rt() -> Runtime {
+        crate::runtime::Builder::new()
+            .current_thread()
+            .build()
+            .unwrap()
+    }
 }