Add layer_fn

tower-layer/src/layer_fn.rs

@@ -0,0 +1,25 @@
+use super::Layer;
+
+/// Returns a new `impl Layer` with the given closure.
+pub fn layer_fn<F, S, T>(f: F) -> impl Layer<S, Service = T> + Clone
+where
+    F: Fn(S) -> T + Clone,
+{
+    LayerFn { f }
+}
+
+#[derive(Clone, Copy, Debug)]
+struct LayerFn<F> {
+    f: F,
+}
+
+impl<F, S, T> Layer<S> for LayerFn<F>
+where
+    F: Fn(S) -> T,
+{
+    type Service = T;
+
+    fn layer(&self, inner: S) -> Self::Service {
+        (self.f)(inner)
+    }
+}

tower-layer/src/lib.rs

@@ -14,9 +14,10 @@
 //! 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, stack::Stack};
 
 /// Decorates a `Service`, transforming either the request or the response.
 ///