Add option_layer (#1696)

axum-extra/src/middleware.rs

@@ -0,0 +1,44 @@
+//! Additional middleware utilities.
+
+use crate::either::Either;
+use tower_layer::Identity;
+
+/// Convert an `Option<Layer>` into a [`Layer`].
+///
+/// If the layer is a `Some` it'll be applied, otherwise not.
+///
+/// # Example
+///
+/// ```
+/// use axum_extra::middleware::option_layer;
+/// use axum::{Router, routing::get};
+/// use std::time::Duration;
+/// use tower_http::timeout::TimeoutLayer;
+///
+/// # let option_timeout = Some(Duration::new(10, 0));
+/// let timeout_layer = option_timeout.map(TimeoutLayer::new);
+///
+/// let app = Router::new()
+///     .route("/", get(|| async {}))
+///     .layer(option_layer(timeout_layer));
+/// # let _: Router = app;
+/// ```
+///
+/// # Difference between this and [`tower::util::option_layer`]
+///
+/// [`tower::util::option_layer`] always changes the error type to [`BoxError`] which requires
+/// using [`HandleErrorLayer`] when used with axum, even if the layer you're applying uses
+/// [`Infallible`].
+///
+/// `axum_extra::middleware::option_layer` on the other hand doesn't change the error type so can
+/// be applied directly.
+///
+/// [`Layer`]: tower_layer::Layer
+/// [`BoxError`]: tower::BoxError
+/// [`HandleErrorLayer`]: axum::error_handling::HandleErrorLayer
+/// [`Infallible`]: std::convert::Infallible
+pub fn option_layer<L>(layer: Option<L>) -> Either<L, Identity> {
+    layer
+        .map(Either::E1)
+        .unwrap_or_else(|| Either::E2(Identity::new()))
+}