stack: use LayerFn from upstream (#840)

Tower 0.4 now has a `layer_fn` utility, obviating the need for our _sui
generis_ `layer::mk`.

Author: hawkw

Cargo.lock

@@ -2360,9 +2360,9 @@ dependencies = [
 
 [[package]]
 name = "tower"
-version = "0.4.1"
+version = "0.4.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "9ebe6c299e025c20c08a730be54f816b90089d153a58b8648adea98eee85c82a"
+checksum = "5ed5c471f00236abe8c633860d8234646cb4993eaad7720844f79bb1b94e949b"
 dependencies = [
  "futures-core",
  "futures-util",

linkerd/stack/Cargo.toml

@@ -15,17 +15,9 @@ futures = "0.3.9"
 linkerd-error = { path = "../error" }
 pin-project = "1"
 tokio = { version = "1", features = ["time"] }
+tower = { version = "0.4.2", features = ["util"] }
 tracing = "0.1"
 
-[dependencies.tower]
-version = "0.4"
-# disable tower's tracing `log` integration for performance reasons, since we
-# will consume tower's traces as traces.
-default-features = false
-features = [
-    "util",
-]
-
 [dev-dependencies]
 tower-test = "0.4"
 tokio-test = "0.4"

linkerd/stack/src/layer.rs

@@ -1,21 +1 @@
-pub use tower::layer::Layer;
-
-/// Make a `Layer` from a closure.
-pub fn mk<F>(f: F) -> LayerFn<F> {
-    LayerFn(f)
-}
-
-/// Make a `Layer` from a closure.
-#[derive(Clone, Copy, Debug)]
-pub struct LayerFn<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.0)(inner)
-    }
-}
+pub use tower::layer::{layer_fn as mk, Layer, LayerFn};