feat(server): add const_service and service_fn helpers

- `const_service` creates a `NewService` that clones references to the
  wrapped service.
- `service_fn` creates a `Service` from a function. Useful with closures.

src/server/mod.rs

@@ -7,6 +7,7 @@
 mod compat_impl;
 #[cfg(feature = "compat")]
 pub mod compat;
+mod service;
 
 use std::cell::RefCell;
 use std::fmt;
@@ -46,6 +47,8 @@ feat_server_proto! {
     };
 }
 
+pub use self::service::{const_service, service_fn};
+
 /// An instance of the HTTP protocol, and implementation of tokio-proto's
 /// `ServerProto` trait.
 ///

src/server/service.rs

@@ -0,0 +1,64 @@
+use std::marker::PhantomData;
+use std::sync::Arc;
+
+use futures::IntoFuture;
+use tokio_service::{NewService, Service};
+
+/// Create a `Service` from a function.
+pub fn service_fn<F, R, S>(f: F) -> ServiceFn<F, R>
+where
+    F: Fn(R) -> S,
+    S: IntoFuture,
+{
+    ServiceFn {
+        f: f,
+        _req: PhantomData,
+    }
+}
+
+/// Create a `NewService` by sharing references of `service.
+pub fn const_service<S>(service: S) -> ConstService<S> {
+    ConstService {
+        svc: Arc::new(service),
+    }
+}
+
+#[derive(Debug)]
+pub struct ServiceFn<F, R> {
+    f: F,
+    _req: PhantomData<fn() -> R>,
+}
+
+impl<F, R, S> Service for ServiceFn<F, R>
+where
+    F: Fn(R) -> S,
+    S: IntoFuture,
+{
+    type Request = R;
+    type Response = S::Item;
+    type Error = S::Error;
+    type Future = S::Future;
+
+    fn call(&self, req: Self::Request) -> Self::Future {
+        (self.f)(req).into_future()
+    }
+}
+
+#[derive(Debug)]
+pub struct ConstService<S> {
+    svc: Arc<S>,
+}
+
+impl<S> NewService for ConstService<S>
+where
+    S: Service,
+{
+    type Request = S::Request;
+    type Response = S::Response;
+    type Error = S::Error;
+    type Instance = Arc<S>;
+
+    fn new_service(&self) -> ::std::io::Result<Self::Instance> {
+        Ok(self.svc.clone())
+    }
+}