Use async fn in generated traits!!

The major breaking change is that Channel::execute no longer internally
spawns RPC handlers, because it is no longer possible to place a Send
bound on the return type of Serve::serve. Instead, Channel::execute
returns a stream of RPC handler futures.

Service authors can reproduce the old behavior by spawning each response
handler (the compiler knows whether or not the futures can be spawned;
it's just that the bounds can't be expressed generically):

    channel.execute(server.serve())
           .for_each(|rpc| { tokio::spawn(rpc); })

example-service/src/lib.rs

@@ -4,6 +4,9 @@
 // license that can be found in the LICENSE file or at
 // https://opensource.org/licenses/MIT.
 
+#![allow(incomplete_features)]
+#![feature(async_fn_in_trait)]
+
 use std::env;
 use tracing_subscriber::{fmt::format::FmtSpan, prelude::*};
 

example-service/src/server.rs

@@ -4,6 +4,9 @@
 // license that can be found in the LICENSE file or at
 // https://opensource.org/licenses/MIT.
 
+#![allow(incomplete_features)]
+#![feature(async_fn_in_trait)]
+
 use clap::Parser;
 use futures::{future, prelude::*};
 use rand::{
@@ -34,7 +37,6 @@ struct Flags {
 #[derive(Clone)]
 struct HelloServer(SocketAddr);
 
-#[tarpc::server]
 impl World for HelloServer {
     async fn hello(self, _: context::Context, name: String) -> String {
         let sleep_time =
@@ -44,6 +46,10 @@ impl World for HelloServer {
     }
 }
 
+async fn spawn(fut: impl Future<Output = ()> + Send + 'static) {
+    tokio::spawn(fut);
+}
+
 #[tokio::main]
 async fn main() -> anyhow::Result<()> {
     let flags = Flags::parse();
@@ -66,7 +72,7 @@ async fn main() -> anyhow::Result<()> {
         // the generated World trait.
         .map(|channel| {
             let server = HelloServer(channel.transport().peer_addr().unwrap());
-            channel.execute(server.serve())
+            channel.execute(server.serve()).for_each(spawn)
         })
         // Max 10 channels.
         .buffer_unordered(10)