Add FromParts impl example

design/src/server/anatomy.md

@@ -840,4 +840,17 @@ pub struct Parts {
 }
 ```
 
-This is commonly used to access types stored within [`Extensions`](https://docs.rs/http/0.2.8/http/struct.Extensions.html) which have been inserted by a middleware.
+This is commonly used to access types stored within [`Extensions`](https://docs.rs/http/0.2.8/http/struct.Extensions.html) which have been inserted by a middleware. Here is the `FromParts` implementation supporting this:
+
+```rust
+impl<Protocol, T> FromParts<Protocol> for Extension<T>
+where
+    T: Clone + Send + Sync + 'static,
+{
+    type Rejection = MissingExtension;
+
+    fn from_parts(parts: &mut http::request::Parts) -> Result<Self, Self::Rejection> {
+        parts.extensions.remove::<T>().map(Extension).ok_or(MissingExtension)
+    }
+}
+```