Update toolchain to nightly-2019-08-21 (existential_type deprecated)

`existential_type` deprecated by rust-lang/rust#63180
in favour of `#![feature(type_alias_impl_trait)]`.

This change updates the code to use the new syntax and the circle ci configuration.

Author: sdbondi

.circleci/config.yml

@@ -1,7 +1,7 @@
 version: 2
 
 defaults:
-  rust_image: &rust_image quay.io/tarilabs/rust_tari-build-with-deps:nightly-2019-07-31
+  rust_image: &rust_image quay.io/tarilabs/rust_tari-build-with-deps:nightly-2019-08-21
 
 jobs:
   test-docs:
@@ -70,7 +70,7 @@ jobs:
       - run:
           name: Tari source code
           command: |
-            TOOLCHAIN_VERSION=nightly-2019-07-31
+            TOOLCHAIN_VERSION=nightly-2019-08-21
             rustup component add --toolchain $TOOLCHAIN_VERSION rustfmt
             cargo fmt --all -- --check
             cargo test --all

base_layer/p2p/src/executor/builder.rs

@@ -81,10 +81,9 @@ where
 
 impl<N> IntoFuture for StackBuilder<N> {
     type Error = ();
+    type Future = impl Future<Item = (), Error = ()> + Send;
     type Item = ();
 
-    existential type Future: Future<Item = (), Error = ()> + Send;
-
     fn into_future(self) -> Self::Future {
         future::join_all(self.futures).map(|_| ())
     }

base_layer/p2p/src/executor/transport.rs

@@ -356,10 +356,9 @@ mod test {
             struct EchoService(Option<oneshot::Receiver<()>>);
             impl Service<String> for EchoService {
                 type Error = ();
+                type Future = impl Future<Item = String, Error = ()>;
                 type Response = String;
 
-                existential type Future: Future<Item = String, Error = ()>;
-
                 fn poll_ready(&mut self) -> Poll<(), Self::Error> {
                     Ok(().into())
                 }

base_layer/p2p/src/lib.rs

@@ -21,8 +21,8 @@
 //  USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 // Used to eliminate the need for boxing futures in many cases.
-// Detais here: https://github.com/rust-lang/rfcs/pull/2071
-#![feature(existential_type)]
+// Tracking issue: https://github.com/rust-lang/rust/issues/63063
+#![feature(type_alias_impl_trait)]
 
 #[macro_use]
 extern crate futures;

base_layer/p2p/src/services/comms_outbound/service.rs

@@ -78,10 +78,9 @@ impl CommsOutboundService {
 
 impl Service<CommsOutboundRequest> for CommsOutboundService {
     type Error = CommsOutboundServiceError;
+    type Future = impl Future<Item = Self::Response, Error = Self::Error>;
     type Response = Result<CommsOutboundResponse, CommsOutboundServiceError>;
 
-    existential type Future: Future<Item = Self::Response, Error = Self::Error>;
-
     fn poll_ready(&mut self) -> Poll<(), Self::Error> {
         Ok(().into())
     }

base_layer/p2p/src/services/liveness/service.rs

@@ -125,10 +125,9 @@ impl LivenessService {
 
 impl Service<LivenessRequest> for LivenessService {
     type Error = ();
+    type Future = impl Future<Item = Self::Response, Error = Self::Error>;
     type Response = Result<LivenessResponse, LivenessError>;
 
-    existential type Future: Future<Item = Self::Response, Error = Self::Error>;
-
     fn poll_ready(&mut self) -> Poll<(), Self::Error> {
         Ok(().into())
     }

rust-toolchain

@@ -1 +1 @@
-nightly-2019-07-31
+nightly-2019-08-21