Improve error message of MissingExtension rejections (#72)

Now includes the name of missing type.

CHANGELOG.md

@@ -12,6 +12,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Implement `Deref` most extractors ([#56](https://github.com/tokio-rs/axum/pull/56))
 - Return `405 Method Not Allowed` for unsupported method for route ([#63](https://github.com/tokio-rs/axum/pull/63))
 - Add extractor for remote connection info ([#55](https://github.com/tokio-rs/axum/pull/55))
+- Improve error message of `MissingExtension` rejections ([#72](https://github.com/tokio-rs/axum/pull/72))
 - Improve documentation for routing ([#71](https://github.com/tokio-rs/axum/pull/71))
 - Clarify required response body type when routing to `tower::Service`s ([#69](https://github.com/tokio-rs/axum/pull/69))
 - Add `axum::body::box_body` to converting an `http_body::Body` to `axum::body::BoxBody` ([#69](https://github.com/tokio-rs/axum/pull/69))

src/extract/mod.rs

@@ -831,7 +831,12 @@ where
             .extensions()
             .ok_or(ExtensionsAlreadyExtracted)?
             .get::<T>()
-            .ok_or(MissingExtension)
+            .ok_or_else(|| {
+                MissingExtension::from_err(format!(
+                    "Extension of type `{}` was not found. Perhaps you forgot to add it?",
+                    std::any::type_name::<T>()
+                ))
+            })
             .map(|x| x.clone())?;
 
         Ok(Extension(value))

src/extract/rejection.rs

@@ -66,7 +66,7 @@ define_rejection! {
     #[body = "Missing request extension"]
     /// Rejection type for [`Extension`](super::Extension) if an expected
     /// request extension was not found.
-    pub struct MissingExtension;
+    pub struct MissingExtension(BoxError);
 }
 
 define_rejection! {