Remove explicit generics from tasks layers

This defines an IdentityLayer<R> which is used to bind the request type
on the service, which helps with type inference.

crates/tower/src/utils.rs

@@ -13,6 +13,7 @@
 // limitations under the License.
 
 use opentelemetry::{KeyValue, Value};
+use tower::{Layer, Service};
 
 /// A simple static key-value pair.
 #[derive(Clone, Debug)]
@@ -31,3 +32,67 @@ where
 /// make or enrich spans.
 #[derive(Clone, Debug)]
 pub struct FnWrapper<F>(pub F);
+
+/// A no-op layer that has the request type bound.
+#[derive(Clone, Copy, Debug)]
+pub struct IdentityLayer<R> {
+    _request: std::marker::PhantomData<R>,
+}
+
+impl<R> Default for IdentityLayer<R> {
+    fn default() -> Self {
+        Self {
+            _request: std::marker::PhantomData,
+        }
+    }
+}
+
+/// A no-op service that has the request type bound.
+#[derive(Clone, Copy, Debug)]
+pub struct IdentityService<R, S> {
+    _request: std::marker::PhantomData<R>,
+    inner: S,
+}
+
+impl<R, S> Default for IdentityService<R, S>
+where
+    S: Default,
+{
+    fn default() -> Self {
+        Self {
+            _request: std::marker::PhantomData,
+            inner: S::default(),
+        }
+    }
+}
+
+impl<R, S> Layer<S> for IdentityLayer<R> {
+    type Service = IdentityService<R, S>;
+
+    fn layer(&self, inner: S) -> Self::Service {
+        IdentityService {
+            _request: std::marker::PhantomData,
+            inner,
+        }
+    }
+}
+
+impl<S, R> Service<R> for IdentityService<R, S>
+where
+    S: Service<R>,
+{
+    type Response = S::Response;
+    type Error = S::Error;
+    type Future = S::Future;
+
+    fn poll_ready(
+        &mut self,
+        cx: &mut std::task::Context<'_>,
+    ) -> std::task::Poll<Result<(), Self::Error>> {
+        self.inner.poll_ready(cx)
+    }
+
+    fn call(&mut self, req: R) -> Self::Future {
+        self.inner.call(req)
+    }
+}