From 07baf63048ac604fd63d8ab2430aa08aa03a37cc Mon Sep 17 00:00:00 2001
From: Carl Lerche <me@carllerche.com>
Date: Thu, 11 Apr 2019 13:38:45 -0700
Subject: [PATCH] Remove bounds of `Service` from `Layer`

---
 tower-buffer/src/layer.rs              | 35 +++++++++++---------------
 tower-filter/src/error.rs              |  5 ----
 tower-filter/src/layer.rs              | 20 +++------------
 tower-filter/src/lib.rs                |  7 +-----
 tower-layer/src/lib.rs                 | 33 ++++++------------------
 tower-limit/src/concurrency/layer.rs   | 16 +++---------
 tower-limit/src/concurrency/mod.rs     |  1 -
 tower-limit/src/concurrency/service.rs |  5 +---
 tower-limit/src/rate/error.rs          | 15 -----------
 tower-limit/src/rate/layer.rs          | 19 +++-----------
 tower-limit/src/rate/service.rs        |  5 +---
 tower-load-shed/src/error.rs           | 16 ------------
 tower-load-shed/src/layer.rs           | 19 +++-----------
 tower-retry/src/lib.rs                 | 23 +++++------------
 tower-timeout/src/layer.rs             | 17 ++++---------
 tower-timeout/src/lib.rs               |  1 -
 tower-timeout/src/never.rs             | 12 ---------
 tower-util/src/layer/identity.rs       | 26 +++----------------
 tower-util/src/layer/stack.rs          | 21 +++++-----------
 tower/src/builder/mod.rs               | 16 ++++++------
 tower/src/builder/service.rs           | 24 ++++++------------
 tower/tests/builder.rs                 |  3 +--
 22 files changed, 78 insertions(+), 261 deletions(-)

diff --git a/tower-buffer/src/layer.rs b/tower-buffer/src/layer.rs
index 900f739..10c4f5c 100644
--- a/tower-buffer/src/layer.rs
+++ b/tower-buffer/src/layer.rs
@@ -1,50 +1,45 @@
 use crate::{error::Error, service::Buffer, worker::WorkerExecutor};
+use std::marker::PhantomData;
 use tokio_executor::DefaultExecutor;
 use tower_layer::Layer;
 use tower_service::Service;
 
 /// Buffer requests with a bounded buffer
-pub struct BufferLayer<E = DefaultExecutor> {
+pub struct BufferLayer<Request, E = DefaultExecutor> {
     bound: usize,
     executor: E,
+    _p: PhantomData<fn(Request)>,
 }
 
-impl BufferLayer<DefaultExecutor> {
+impl<Request> BufferLayer<Request, DefaultExecutor> {
     pub fn new(bound: usize) -> Self {
         BufferLayer {
             bound,
             executor: DefaultExecutor::current(),
+            _p: PhantomData,
         }
     }
 }
 
-impl<E> BufferLayer<E> {
-    pub fn with_executor<S, Request>(bound: usize, executor: E) -> Self
-    where
-        S: Service<Request>,
-        S::Error: Into<Error>,
-        E: WorkerExecutor<S, Request> + Clone,
-    {
-        BufferLayer { bound, executor }
+impl<Request, E: Clone> BufferLayer<Request, E> {
+    pub fn with_executor(bound: usize, executor: E) -> Self {
+        BufferLayer {
+            bound,
+            executor,
+            _p: PhantomData,
+        }
     }
 }
 
-impl<E, S, Request> Layer<S, Request> for BufferLayer<E>
+impl<E, S, Request> Layer<S> for BufferLayer<Request, E>
 where
     S: Service<Request>,
     S::Error: Into<Error>,
     E: WorkerExecutor<S, Request> + Clone,
 {
-    type Response = S::Response;
-    type Error = Error;
-    type LayerError = Error;
     type Service = Buffer<S, Request>;
 
-    fn layer(&self, service: S) -> Result<Self::Service, Self::LayerError> {
-        Ok(Buffer::with_executor(
-            service,
-            self.bound,
-            &mut self.executor.clone(),
-        ))
+    fn layer(&self, service: S) -> Self::Service {
+        Buffer::with_executor(service, self.bound, &mut self.executor.clone())
     }
 }
diff --git a/tower-filter/src/error.rs b/tower-filter/src/error.rs
index f6e06bd..0f0f7d8 100644
--- a/tower-filter/src/error.rs
+++ b/tower-filter/src/error.rs
@@ -46,8 +46,3 @@ impl error::Error for Error {
         }
     }
 }
-
-pub(crate) mod never {
-    #[derive(Debug)]
-    pub enum Never {}
-}
diff --git a/tower-filter/src/layer.rs b/tower-filter/src/layer.rs
index e78c1a9..d6735f4 100644
--- a/tower-filter/src/layer.rs
+++ b/tower-filter/src/layer.rs
@@ -1,9 +1,5 @@
-use crate::{
-    error::{self, Error},
-    Filter, Predicate,
-};
+use crate::Filter;
 use tower_layer::Layer;
-use tower_service::Service;
 
 pub struct FilterLayer<U> {
     predicate: U,
@@ -15,19 +11,11 @@ impl<U> FilterLayer<U> {
     }
 }
 
-impl<U, S, Request> Layer<S, Request> for FilterLayer<U>
-where
-    U: Predicate<Request> + Clone,
-    S: Service<Request> + Clone,
-    S::Error: Into<error::Source>,
-{
-    type Response = S::Response;
-    type Error = Error;
-    type LayerError = error::never::Never;
+impl<U: Clone, S> Layer<S> for FilterLayer<U> {
     type Service = Filter<S, U>;
 
-    fn layer(&self, service: S) -> Result<Self::Service, Self::LayerError> {
+    fn layer(&self, service: S) -> Self::Service {
         let predicate = self.predicate.clone();
-        Ok(Filter::new(service, predicate))
+        Filter::new(service, predicate)
     }
 }
diff --git a/tower-filter/src/lib.rs b/tower-filter/src/lib.rs
index 9277643..a21ca6c 100644
--- a/tower-filter/src/lib.rs
+++ b/tower-filter/src/lib.rs
@@ -22,12 +22,7 @@ pub struct Filter<T, U> {
 }
 
 impl<T, U> Filter<T, U> {
-    pub fn new<Request>(inner: T, predicate: U) -> Self
-    where
-        T: Service<Request> + Clone,
-        T::Error: Into<error::Source>,
-        U: Predicate<Request>,
-    {
+    pub fn new(inner: T, predicate: U) -> Self {
         Filter { inner, predicate }
     }
 }
diff --git a/tower-layer/src/lib.rs b/tower-layer/src/lib.rs
index 56055b6..dd43613 100644
--- a/tower-layer/src/lib.rs
+++ b/tower-layer/src/lib.rs
@@ -8,8 +8,6 @@
 //!
 //! A middleware implements the [`Layer`] and [`Service`] trait.
 
-use tower_service::Service;
-
 /// Decorates a `Service`, transforming either the request or the response.
 ///
 /// Often, many of the pieces needed for writing network applications can be
@@ -36,21 +34,14 @@ use tower_service::Service;
 ///     target: &'static str,
 /// }
 ///
-/// impl<S, Request> Layer<S, Request> for LogLayer
-/// where
-///     S: Service<Request>,
-///     Request: fmt::Debug,
-/// {
-///     type Response = S::Response;
-///     type Error = S::Error;
-///     type LayerError = Void;
+/// impl<S> Layer<S> for LogLayer {
 ///     type Service = LogService<S>;
 ///
-///     fn layer(&self, service: S) -> Result<Self::Service, Self::LayerError> {
-///         Ok(LogService {
+///     fn layer(&self, service: S) -> Self::Service {
+///         LogService {
 ///             target: self.target,
 ///             service
-///         })
+///         }
 ///     }
 /// }
 ///
@@ -84,20 +75,10 @@ use tower_service::Service;
 /// The above log implementation is decoupled from the underlying protocol and
 /// is also decoupled from client or server concerns. In other words, the same
 /// log middleware could be used in either a client or a server.
-pub trait Layer<S, Request> {
-    /// The wrapped service response type
-    type Response;
-
-    /// The wrapped service's error type
-    type Error;
-
-    /// The error produced when calling `layer`
-    type LayerError;
-
+pub trait Layer<S> {
     /// The wrapped service
-    type Service: Service<Request, Response = Self::Response, Error = Self::Error>;
-
+    type Service;
     /// Wrap the given service with the middleware, returning a new service
     /// that has been decorated with the middleware.
-    fn layer(&self, inner: S) -> Result<Self::Service, Self::LayerError>;
+    fn layer(&self, inner: S) -> Self::Service;
 }
diff --git a/tower-limit/src/concurrency/layer.rs b/tower-limit/src/concurrency/layer.rs
index 30f7f22..24b0e81 100644
--- a/tower-limit/src/concurrency/layer.rs
+++ b/tower-limit/src/concurrency/layer.rs
@@ -1,6 +1,5 @@
-use super::{never::Never, ConcurrencyLimit, Error};
+use super::ConcurrencyLimit;
 use tower_layer::Layer;
-use tower_service::Service;
 
 #[derive(Debug, Clone)]
 pub struct ConcurrencyLimitLayer {
@@ -13,17 +12,10 @@ impl ConcurrencyLimitLayer {
     }
 }
 
-impl<S, Request> Layer<S, Request> for ConcurrencyLimitLayer
-where
-    S: Service<Request>,
-    S::Error: Into<Error>,
-{
-    type Response = S::Response;
-    type Error = Error;
-    type LayerError = Never;
+impl<S> Layer<S> for ConcurrencyLimitLayer {
     type Service = ConcurrencyLimit<S>;
 
-    fn layer(&self, service: S) -> Result<Self::Service, Self::LayerError> {
-        Ok(ConcurrencyLimit::new(service, self.max))
+    fn layer(&self, service: S) -> Self::Service {
+        ConcurrencyLimit::new(service, self.max)
     }
 }
diff --git a/tower-limit/src/concurrency/mod.rs b/tower-limit/src/concurrency/mod.rs
index de9cf2a..467514e 100644
--- a/tower-limit/src/concurrency/mod.rs
+++ b/tower-limit/src/concurrency/mod.rs
@@ -2,7 +2,6 @@
 
 pub mod future;
 mod layer;
-mod never;
 mod service;
 
 pub use self::{layer::ConcurrencyLimitLayer, service::ConcurrencyLimit};
diff --git a/tower-limit/src/concurrency/service.rs b/tower-limit/src/concurrency/service.rs
index 61e6494..bd201cb 100644
--- a/tower-limit/src/concurrency/service.rs
+++ b/tower-limit/src/concurrency/service.rs
@@ -20,10 +20,7 @@ struct Limit {
 
 impl<T> ConcurrencyLimit<T> {
     /// Create a new rate limiter
-    pub fn new<Request>(inner: T, max: usize) -> Self
-    where
-        T: Service<Request>,
-    {
+    pub fn new(inner: T, max: usize) -> Self {
         ConcurrencyLimit {
             inner,
             limit: Limit {
diff --git a/tower-limit/src/rate/error.rs b/tower-limit/src/rate/error.rs
index 59b3a91..712f34d 100644
--- a/tower-limit/src/rate/error.rs
+++ b/tower-limit/src/rate/error.rs
@@ -1,18 +1,3 @@
 use std::error;
 
 pub(crate) type Error = Box<dyn error::Error + Send + Sync>;
-
-pub(crate) mod never {
-    use std::{error, fmt};
-
-    #[derive(Debug)]
-    pub enum Never {}
-
-    impl fmt::Display for Never {
-        fn fmt(&self, _: &mut fmt::Formatter) -> fmt::Result {
-            unreachable!();
-        }
-    }
-
-    impl error::Error for Never {}
-}
diff --git a/tower-limit/src/rate/layer.rs b/tower-limit/src/rate/layer.rs
index 2652839..c4157f1 100644
--- a/tower-limit/src/rate/layer.rs
+++ b/tower-limit/src/rate/layer.rs
@@ -1,10 +1,6 @@
-use super::{
-    error::{never::Never, Error},
-    Rate, RateLimit,
-};
+use super::{Rate, RateLimit};
 use std::time::Duration;
 use tower_layer::Layer;
-use tower_service::Service;
 
 #[derive(Debug)]
 pub struct RateLimitLayer {
@@ -18,17 +14,10 @@ impl RateLimitLayer {
     }
 }
 
-impl<S, Request> Layer<S, Request> for RateLimitLayer
-where
-    S: Service<Request>,
-    Error: From<S::Error>,
-{
-    type Response = S::Response;
-    type Error = Error;
-    type LayerError = Never;
+impl<S> Layer<S> for RateLimitLayer {
     type Service = RateLimit<S>;
 
-    fn layer(&self, service: S) -> Result<Self::Service, Self::LayerError> {
-        Ok(RateLimit::new(service, self.rate))
+    fn layer(&self, service: S) -> Self::Service {
+        RateLimit::new(service, self.rate)
     }
 }
diff --git a/tower-limit/src/rate/service.rs b/tower-limit/src/rate/service.rs
index 10cb99f..a5bfb57 100644
--- a/tower-limit/src/rate/service.rs
+++ b/tower-limit/src/rate/service.rs
@@ -21,10 +21,7 @@ enum State {
 
 impl<T> RateLimit<T> {
     /// Create a new rate limiter
-    pub fn new<Request>(inner: T, rate: Rate) -> Self
-    where
-        T: Service<Request>,
-    {
+    pub fn new(inner: T, rate: Rate) -> Self {
         let state = State::Ready {
             until: clock::now(),
             rem: rate.num(),
diff --git a/tower-load-shed/src/error.rs b/tower-load-shed/src/error.rs
index c37540e..7a072fb 100644
--- a/tower-load-shed/src/error.rs
+++ b/tower-load-shed/src/error.rs
@@ -3,7 +3,6 @@
 use std::fmt;
 
 pub(crate) type Error = Box<dyn std::error::Error + Send + Sync>;
-pub(crate) use self::never::Never;
 
 /// An error returned by `Overload` when the underlying service
 /// is not ready to handle any requests at the time of being
@@ -31,18 +30,3 @@ impl fmt::Display for Overloaded {
 }
 
 impl std::error::Error for Overloaded {}
-
-pub(crate) mod never {
-    use std::{error, fmt};
-
-    #[derive(Debug)]
-    pub enum Never {}
-
-    impl fmt::Display for Never {
-        fn fmt(&self, _: &mut fmt::Formatter) -> fmt::Result {
-            match *self {}
-        }
-    }
-
-    impl error::Error for Never {}
-}
diff --git a/tower-load-shed/src/layer.rs b/tower-load-shed/src/layer.rs
index 83f9468..7b578d0 100644
--- a/tower-load-shed/src/layer.rs
+++ b/tower-load-shed/src/layer.rs
@@ -1,10 +1,6 @@
 use tower_layer::Layer;
-use tower_service::Service;
 
-use crate::{
-    error::{Error, Never},
-    LoadShed,
-};
+use crate::LoadShed;
 
 /// A `tower-layer` to wrap services in `LoadShed` middleware.
 #[derive(Debug)]
@@ -19,17 +15,10 @@ impl LoadShedLayer {
     }
 }
 
-impl<S, Req> Layer<S, Req> for LoadShedLayer
-where
-    S: Service<Req>,
-    S::Error: Into<Error>,
-{
-    type Response = S::Response;
-    type Error = Error;
-    type LayerError = Never;
+impl<S> Layer<S> for LoadShedLayer {
     type Service = LoadShed<S>;
 
-    fn layer(&self, service: S) -> Result<Self::Service, Self::LayerError> {
-        Ok(LoadShed::new(service))
+    fn layer(&self, service: S) -> Self::Service {
+        LoadShed::new(service)
     }
 }
diff --git a/tower-retry/src/lib.rs b/tower-retry/src/lib.rs
index fc91142..cac25b8 100644
--- a/tower-retry/src/lib.rs
+++ b/tower-retry/src/lib.rs
@@ -1,6 +1,6 @@
 #![deny(missing_debug_implementations)]
 #![deny(missing_docs)]
-#![deny(warnings)]
+// #![deny(warnings)]
 #![deny(rust_2018_idioms)]
 #![allow(elided_lifetimes_in_paths)]
 
@@ -11,9 +11,6 @@ use tower_layer::Layer;
 use tower_service::Service;
 
 pub mod budget;
-mod never;
-
-use crate::never::Never;
 
 /// A "retry policy" to classify if a request should be retried.
 ///
@@ -127,19 +124,15 @@ impl<P> RetryLayer<P> {
     }
 }
 
-impl<P, S, Request> Layer<S, Request> for RetryLayer<P>
+impl<P, S> Layer<S> for RetryLayer<P>
 where
-    S: Service<Request> + Clone,
-    P: Policy<Request, S::Response, S::Error> + Clone,
+    P: Clone,
 {
-    type Response = S::Response;
-    type Error = S::Error;
-    type LayerError = Never;
     type Service = Retry<P, S>;
 
-    fn layer(&self, service: S) -> Result<Self::Service, Self::LayerError> {
+    fn layer(&self, service: S) -> Self::Service {
         let policy = self.policy.clone();
-        Ok(Retry::new(policy, service))
+        Retry::new(policy, service)
     }
 }
 
@@ -147,11 +140,7 @@ where
 
 impl<P, S> Retry<P, S> {
     /// Retry the inner service depending on this [`Policy`][Policy}.
-    pub fn new<Request>(policy: P, service: S) -> Self
-    where
-        P: Policy<Request, S::Response, S::Error> + Clone,
-        S: Service<Request> + Clone,
-    {
+    pub fn new(policy: P, service: S) -> Self {
         Retry { policy, service }
     }
 }
diff --git a/tower-timeout/src/layer.rs b/tower-timeout/src/layer.rs
index fbf3680..e165ce1 100644
--- a/tower-timeout/src/layer.rs
+++ b/tower-timeout/src/layer.rs
@@ -1,7 +1,7 @@
-use crate::{never::Never, Error, Timeout};
+use crate::Timeout;
 use std::time::Duration;
 use tower_layer::Layer;
-use tower_service::Service;
+
 /// Applies a timeout to requests via the supplied inner service.
 #[derive(Debug)]
 pub struct TimeoutLayer {
@@ -15,17 +15,10 @@ impl TimeoutLayer {
     }
 }
 
-impl<S, Request> Layer<S, Request> for TimeoutLayer
-where
-    S: Service<Request>,
-    Error: From<S::Error>,
-{
-    type Response = S::Response;
-    type Error = Error;
-    type LayerError = Never;
+impl<S> Layer<S> for TimeoutLayer {
     type Service = Timeout<S>;
 
-    fn layer(&self, service: S) -> Result<Self::Service, Self::LayerError> {
-        Ok(Timeout::new(service, self.timeout))
+    fn layer(&self, service: S) -> Self::Service {
+        Timeout::new(service, self.timeout)
     }
 }
diff --git a/tower-timeout/src/lib.rs b/tower-timeout/src/lib.rs
index d58d7c4..8d2fb76 100644
--- a/tower-timeout/src/lib.rs
+++ b/tower-timeout/src/lib.rs
@@ -11,7 +11,6 @@
 pub mod error;
 pub mod future;
 mod layer;
-mod never;
 
 pub use crate::layer::TimeoutLayer;
 
diff --git a/tower-timeout/src/never.rs b/tower-timeout/src/never.rs
index 3bb12aa..e69de29 100644
--- a/tower-timeout/src/never.rs
+++ b/tower-timeout/src/never.rs
@@ -1,12 +0,0 @@
-use std::fmt;
-#[derive(Debug)]
-/// An error that can never occur.
-pub enum Never {}
-
-impl fmt::Display for Never {
-    fn fmt(&self, _: &mut fmt::Formatter) -> fmt::Result {
-        match *self {}
-    }
-}
-
-impl std::error::Error for Never {}
diff --git a/tower-util/src/layer/identity.rs b/tower-util/src/layer/identity.rs
index 1209b86..e56c9c8 100644
--- a/tower-util/src/layer/identity.rs
+++ b/tower-util/src/layer/identity.rs
@@ -1,6 +1,4 @@
-use std::fmt;
 use tower_layer::Layer;
-use tower_service::Service;
 
 /// A no-op middleware.
 ///
@@ -19,28 +17,10 @@ impl Identity {
 }
 
 /// Decorates a `Service`, transforming either the request or the response.
-impl<S, Request> Layer<S, Request> for Identity
-where
-    S: Service<Request>,
-{
-    type Response = S::Response;
-    type Error = S::Error;
-    type LayerError = Never;
+impl<S> Layer<S> for Identity {
     type Service = S;
 
-    fn layer(&self, inner: S) -> Result<Self::Service, Self::LayerError> {
-        Ok(inner)
+    fn layer(&self, inner: S) -> Self::Service {
+        inner
     }
 }
-
-/// An error that can never occur.
-#[derive(Debug)]
-pub enum Never {}
-
-impl fmt::Display for Never {
-    fn fmt(&self, _: &mut fmt::Formatter) -> fmt::Result {
-        match *self {}
-    }
-}
-
-impl ::std::error::Error for Never {}
diff --git a/tower-util/src/layer/stack.rs b/tower-util/src/layer/stack.rs
index 3b0b5eb..6e76da8 100644
--- a/tower-util/src/layer/stack.rs
+++ b/tower-util/src/layer/stack.rs
@@ -1,5 +1,4 @@
 use tower_layer::Layer;
-use tower_service::Service;
 
 /// Two middlewares chained together.
 ///
@@ -10,8 +9,6 @@ pub struct Stack<Inner, Outer> {
     outer: Outer,
 }
 
-type Error = Box<dyn std::error::Error + Send + Sync>;
-
 impl<Inner, Outer> Stack<Inner, Outer> {
     /// Create a new `Stack`.
     pub fn new(inner: Inner, outer: Outer) -> Self {
@@ -19,22 +16,16 @@ impl<Inner, Outer> Stack<Inner, Outer> {
     }
 }
 
-impl<S, Request, Inner, Outer> Layer<S, Request> for Stack<Inner, Outer>
+impl<S, Inner, Outer> Layer<S> for Stack<Inner, Outer>
 where
-    S: Service<Request>,
-    Inner: Layer<S, Request>,
-    Inner::LayerError: Into<Error>,
-    Outer: Layer<Inner::Service, Request>,
-    Outer::LayerError: Into<Error>,
+    Inner: Layer<S>,
+    Outer: Layer<Inner::Service>,
 {
-    type Response = Outer::Response;
-    type Error = Outer::Error;
-    type LayerError = Error;
     type Service = Outer::Service;
 
-    fn layer(&self, service: S) -> Result<Self::Service, Self::LayerError> {
-        let inner = self.inner.layer(service).map_err(Into::into)?;
+    fn layer(&self, service: S) -> Self::Service {
+        let inner = self.inner.layer(service);
 
-        self.outer.layer(inner).map_err(Into::into)
+        self.outer.layer(inner)
     }
 }
diff --git a/tower/src/builder/mod.rs b/tower/src/builder/mod.rs
index 6702b05..2ba968d 100644
--- a/tower/src/builder/mod.rs
+++ b/tower/src/builder/mod.rs
@@ -73,7 +73,7 @@ pub(super) type Error = Box<dyn std::error::Error + Send + Sync>;
 /// # T::Error: Into<Box<::std::error::Error + Send + Sync>>,
 /// # {
 /// ServiceBuilder::new()
-///     .buffer(100)
+///     // .buffer(100)
 ///     .concurrency_limit(10)
 ///     .service(my_service)
 /// # ;
@@ -96,7 +96,7 @@ pub(super) type Error = Box<dyn std::error::Error + Send + Sync>;
 /// # {
 /// ServiceBuilder::new()
 ///     .concurrency_limit(10)
-///     .buffer(100)
+///     // .buffer(100)
 ///     .service(my_service)
 /// # ;
 /// # }
@@ -207,12 +207,12 @@ pub(super) type Error = Box<dyn std::error::Error + Send + Sync>;
 /// #    }
 /// # }
 /// ServiceBuilder::new()
-///     .buffer(5)
+///     // .buffer(5)
 ///     .concurrency_limit(5)
 ///     .rate_limit(5, Duration::from_secs(1))
 ///     .service(MyService);
 /// ```
-#[derive(Debug)]
+#[derive(Clone, Debug)]
 pub struct ServiceBuilder<L> {
     layer: L,
 }
@@ -235,7 +235,7 @@ impl<L> ServiceBuilder<L> {
     }
 
     /// Buffer requests when when the next layer is out of capacity.
-    pub fn buffer(self, bound: usize) -> ServiceBuilder<Stack<BufferLayer, L>> {
+    pub fn buffer<Request>(self, bound: usize) -> ServiceBuilder<Stack<BufferLayer<Request>, L>> {
         self.layer(BufferLayer::new(bound))
     }
 
@@ -291,10 +291,10 @@ impl<L> ServiceBuilder<L> {
     }
 
     /// Wrap the service `S` with the layers.
-    pub fn service<S, Request>(self, service: S) -> Result<L::Service, L::LayerError>
+    pub fn service<S, Request>(self, service: S) -> L::Service
     where
-        L: Layer<S, Request>,
-        S: Service<Request>,
+        L: Layer<S>,
+        L::Service: Service<Request>,
     {
         self.layer.layer(service)
     }
diff --git a/tower/src/builder/service.rs b/tower/src/builder/service.rs
index 846381f..b124737 100644
--- a/tower/src/builder/service.rs
+++ b/tower/src/builder/service.rs
@@ -1,4 +1,3 @@
-use super::Error;
 use crate::Service;
 use futures::{try_ready, Async, Future, Poll};
 use std::{marker::PhantomData, sync::Arc};
@@ -36,17 +35,15 @@ impl<S, L, Request> LayeredMakeService<S, L, Request> {
 impl<S, L, Target, Request> Service<Target> for LayeredMakeService<S, L, Request>
 where
     S: MakeService<Target, Request>,
-    S::MakeError: Into<Error>,
-    L: Layer<S::Service, Request> + Sync + Send + 'static,
-    L::LayerError: Into<Error>,
+    L: Layer<S::Service> + Sync + Send + 'static,
     Target: Clone,
 {
     type Response = L::Service;
-    type Error = Error;
+    type Error = S::MakeError;
     type Future = ServiceFuture<S, L, Target, Request>;
 
     fn poll_ready(&mut self) -> Poll<(), Self::Error> {
-        self.maker.poll_ready().map_err(Into::into)
+        self.maker.poll_ready()
     }
 
     fn call(&mut self, target: Target) -> Self::Future {
@@ -60,19 +57,14 @@ where
 impl<S, L, Target, Request> Future for ServiceFuture<S, L, Target, Request>
 where
     S: MakeService<Target, Request>,
-    S::MakeError: Into<Error>,
-    L: Layer<S::Service, Request>,
-    L::LayerError: Into<Error>,
+    L: Layer<S::Service>,
 {
     type Item = L::Service;
-    type Error = Error;
+    type Error = S::MakeError;
 
     fn poll(&mut self) -> Poll<Self::Item, Self::Error> {
-        let service = try_ready!(self.inner.poll().map_err(Into::into));
-
-        match self.layer.layer(service) {
-            Ok(service) => Ok(Async::Ready(service)),
-            Err(e) => Err(e.into()),
-        }
+        let service = try_ready!(self.inner.poll());
+        let service = self.layer.layer(service);
+        Ok(Async::Ready(service))
     }
 }
diff --git a/tower/tests/builder.rs b/tower/tests/builder.rs
index 9efaabd..156a67f 100644
--- a/tower/tests/builder.rs
+++ b/tower/tests/builder.rs
@@ -37,8 +37,7 @@ fn builder_service() {
             .layer(BufferLayer::new(5))
             .layer(ConcurrencyLimitLayer::new(5))
             .layer(RateLimitLayer::new(5, Duration::from_secs(1)))
-            .service(MockSvc)
-            .unwrap();
+            .service(MockSvc);
 
         client.poll_ready().unwrap();
         client