layer: Add an identity layer (#195)

Tied to #175

tower-layer/src/util/identity.rs

@@ -0,0 +1,47 @@
+use std::error::Error;
+use std::fmt;
+use tower_service::Service;
+use Layer;
+
+/// A no-op middleware.
+///
+/// When wrapping a `Service`, the `Identity` layer returns the provided
+/// service without modifying it.
+#[derive(Debug, Default, Clone)]
+pub struct Identity {
+    _p: (),
+}
+
+impl Identity {
+    /// Create a new `Identity` value
+    pub fn new() -> Identity {
+        Identity { _p: () }
+    }
+}
+
+/// 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;
+    type Service = S;
+
+    fn layer(&self, inner: S) -> Result<Self::Service, Self::LayerError> {
+        Ok(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 {}

tower-layer/src/util/mod.rs

@@ -3,8 +3,10 @@
 use Layer;
 
 mod chain;
+mod identity;
 
 pub use self::chain::{Chain, ChainError};
+pub use self::identity::Identity;
 
 /// An extension trait for `Layer`'s that provides a variety of convenient
 /// adapters.