ready_cache: just use pin_project for Pending (#667)

This gets rid of the `Unpin` impl with the weird comment on it.

Alternatively, we could just put a `S: Unpin` bound on `Pending`, but
this changes the public API to require that the service type is `Unpin`.
In practice, it will be, but we could also just avoid the trait bound.

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

tower/Cargo.toml

@@ -56,7 +56,7 @@ limit = ["__common", "tokio/time", "tokio/sync", "tokio-util", "tracing"]
 load = ["__common", "tokio/time", "tracing"]
 load-shed = ["__common"]
 make = ["futures-util", "pin-project-lite", "tokio/io-std"]
-ready-cache = ["futures-core", "futures-util", "indexmap", "tokio/sync", "tracing"]
+ready-cache = ["futures-core", "futures-util", "indexmap", "tokio/sync", "tracing", "pin-project-lite"]
 reconnect = ["make", "tokio/io-std", "tracing"]
 retry = ["__common", "tokio/time"]
 spawn-ready = ["__common", "futures-util", "tokio/sync", "tokio/rt", "util", "tracing"]

tower/src/ready_cache/cache.rs

@@ -85,14 +85,16 @@ enum PendingError<K, E> {
     Inner(K, E),
 }
 
-/// A [`Future`] that becomes satisfied when an `S`-typed service is ready.
-///
-/// May fail due to cancelation, i.e. if the service is evicted from the balancer.
-struct Pending<K, S, Req> {
-    key: Option<K>,
-    cancel: Option<CancelRx>,
-    ready: Option<S>,
-    _pd: std::marker::PhantomData<Req>,
+pin_project_lite::pin_project! {
+    /// A [`Future`] that becomes satisfied when an `S`-typed service is ready.
+    ///
+    /// May fail due to cancelation, i.e. if the service is evicted from the balancer.
+    struct Pending<K, S, Req> {
+        key: Option<K>,
+        cancel: Option<CancelRx>,
+        ready: Option<S>,
+        _pd: std::marker::PhantomData<Req>,
+    }
 }
 
 // === ReadyCache ===
@@ -400,37 +402,35 @@ where
 
 // === Pending ===
 
-// Safety: No use unsafe access therefore this is safe.
-impl<K, S, Req> Unpin for Pending<K, S, Req> {}
-
 impl<K, S, Req> Future for Pending<K, S, Req>
 where
     S: Service<Req>,
 {
     type Output = Result<(K, S, CancelRx), PendingError<K, S::Error>>;
 
-    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
-        let mut fut = self.cancel.as_mut().expect("polled after complete");
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let this = self.project();
+        let mut fut = this.cancel.as_mut().expect("polled after complete");
         if let Poll::Ready(r) = Pin::new(&mut fut).poll(cx) {
             assert!(r.is_ok(), "cancel sender lost");
-            let key = self.key.take().expect("polled after complete");
+            let key = this.key.take().expect("polled after complete");
             return Err(PendingError::Canceled(key)).into();
         }
 
-        match self
+        match this
             .ready
             .as_mut()
             .expect("polled after ready")
             .poll_ready(cx)
         {
             Poll::Pending => Poll::Pending,
             Poll::Ready(Ok(())) => {
-                let key = self.key.take().expect("polled after complete");
-                let cancel = self.cancel.take().expect("polled after complete");
-                Ok((key, self.ready.take().expect("polled after ready"), cancel)).into()
+                let key = this.key.take().expect("polled after complete");
+                let cancel = this.cancel.take().expect("polled after complete");
+                Ok((key, this.ready.take().expect("polled after ready"), cancel)).into()
             }
             Poll::Ready(Err(e)) => {
-                let key = self.key.take().expect("polled after compete");
+                let key = this.key.take().expect("polled after compete");
                 Err(PendingError::Inner(key, e)).into()
             }
         }