Make tokio unstable opt in (#73)

* Make tokio unstable opt in

We use `unhandled_panic` to forward host software panics as test
failures.

Forcing this is a bit invasive for consumers of the crate, so
instead we make it enabled conditionally.

* conditional compilation

README.md

@@ -58,11 +58,17 @@ sim.client("client", async move {
 sim.run();
 ```
 
-`RUSTFLAG="--cfg tokio_unstable"` is necessary to compile the project.
-
 See `ping_pong` in [udp.rs](tests/udp.rs) for a networking example. For more
 examples, check out the [tests](tests) directory.
 
+### tokio_unstable
+
+Turmoil uses [unhandled_panic] to forward host panics as test failures. See
+[unstable features] to opt in.
+
+[unhandled_panic]: https://docs.rs/tokio/latest/tokio/runtime/struct.Builder.html#method.unhandled_panic
+[unstable features]: https://docs.rs/tokio/latest/tokio/#unstable-features
+
 ## License
 
 This project is licensed under the [MIT license](LICENSE).

src/rt.rs

@@ -83,12 +83,12 @@ impl Rt {
 }
 
 fn init() -> (Runtime, LocalSet) {
-    let tokio = tokio::runtime::Builder::new_current_thread()
-        .enable_time()
-        .start_paused(true)
-        .unhandled_panic(tokio::runtime::UnhandledPanic::ShutdownRuntime)
-        .build()
-        .unwrap();
+    let mut builder = tokio::runtime::Builder::new_current_thread();
+
+    #[cfg(tokio_unstable)]
+    builder.unhandled_panic(tokio::runtime::UnhandledPanic::ShutdownRuntime);
+
+    let tokio = builder.enable_time().start_paused(true).build().unwrap();
 
     tokio.block_on(async {
         // Sleep to "round" `Instant::now()` to the closest `ms`
@@ -100,6 +100,9 @@ fn init() -> (Runtime, LocalSet) {
 
 fn new_local() -> LocalSet {
     let mut local = LocalSet::new();
+
+    #[cfg(tokio_unstable)]
     local.unhandled_panic(tokio::runtime::UnhandledPanic::ShutdownRuntime);
+
     local
 }