stack: Add NewService::into_make_service (#618)

This change adds a utility for casting a NewService into a MakeService.

Author: olix0r

linkerd/stack/src/new_service.rs

@@ -1,11 +1,21 @@
 use crate::FutureService;
+use futures::future;
+use linkerd2_error::Never;
+use std::task::{Context, Poll};
 use tower::util::{Oneshot, ServiceExt};
 
 /// Immediately and infalliby creates (usually) a Service.
 pub trait NewService<T> {
     type Service;
 
     fn new_service(&self, target: T) -> Self::Service;
+
+    fn into_make_service(self) -> IntoMakeService<Self>
+    where
+        Self: Sized,
+    {
+        IntoMakeService { new_service: self }
+    }
 }
 
 /// A Layer that modifies inner `MakeService`s to be exposd as a `NewService`.
@@ -18,6 +28,12 @@ pub struct FromMakeService<S> {
     make_service: S,
 }
 
+/// Modifies inner `MakeService`s to be exposd as a `NewService`.
+#[derive(Clone, Copy, Debug)]
+pub struct IntoMakeService<N> {
+    new_service: N,
+}
+
 // === impl NewService ===
 
 impl<F, T, S> NewService<T> for F
@@ -53,3 +69,17 @@ where
         FutureService::new(self.make_service.clone().oneshot(target))
     }
 }
+
+impl<T, N: NewService<T>> tower::Service<T> for IntoMakeService<N> {
+    type Response = N::Service;
+    type Error = Never;
+    type Future = future::Ready<Result<N::Service, Never>>;
+
+    fn poll_ready(&mut self, _: &mut Context<'_>) -> Poll<Result<(), Never>> {
+        Poll::Ready(Ok(()))
+    }
+
+    fn call(&mut self, target: T) -> Self::Future {
+        future::ok(self.new_service.new_service(target))
+    }
+}