Add layer_fn

tower-layer/src/layer_fn.rs

@@ -0,0 +1,23 @@
+use super::Layer;
+
+/// Returns a new `LayerFn` with the given closure.
+pub fn layer_fn<T>(f: T) -> LayerFn<T> {
+    LayerFn { f }
+}
+
+/// A `Layer` implemented by a closure.
+#[derive(Clone, Copy, Debug)]
+pub struct LayerFn<F> {
+    f: F,
+}
+
+impl<F, S, Out> Layer<S> for LayerFn<F>
+where
+    F: Fn(S) -> Out,
+{
+    type Service = Out;
+
+    fn layer(&self, inner: S) -> Self::Service {
+        (self.f)(inner)
+    }
+}

tower-layer/src/lib.rs

@@ -14,9 +14,14 @@
 //! A middleware implements the [`Layer`] and [`Service`] trait.
 
 mod identity;
+mod layer_fn;
 mod stack;
 
-pub use self::{identity::Identity, stack::Stack};
+pub use self::{
+    identity::Identity,
+    layer_fn::{layer_fn, LayerFn},
+    stack::Stack,
+};
 
 /// Decorates a `Service`, transforming either the request or the response.
 ///