Add NestedUri (#161)

Fixes https://github.com/tokio-rs/axum/issues/159

src/extract/mod.rs

@@ -278,6 +278,7 @@ pub use self::{
     path::Path,
     query::Query,
     raw_query::RawQuery,
+    request_parts::NestedUri,
     request_parts::{Body, BodyStream},
 };
 #[doc(no_inline)]

src/extract/rejection.rs

@@ -108,6 +108,16 @@ define_rejection! {
     pub struct InvalidFormContentType;
 }
 
+define_rejection! {
+    #[status = INTERNAL_SERVER_ERROR]
+    #[body = "`NestedUri` extractor used for route that isn't nested"]
+    /// Rejection type used if you try and extract [`NestedUri`] from a route that
+    /// isn't nested.
+    ///
+    /// [`NestedUri`]: crate::extract::NestedUri
+    pub struct NotNested;
+}
+
 /// Rejection type for [`Path`](super::Path) if the capture route
 /// param didn't have the expected type.
 #[derive(Debug)]

src/extract/request_parts.rs

@@ -1,4 +1,4 @@
-use super::{rejection::*, take_body, FromRequest, RequestParts};
+use super::{rejection::*, take_body, Extension, FromRequest, RequestParts};
 use async_trait::async_trait;
 use bytes::Bytes;
 use futures_util::stream::Stream;
@@ -74,6 +74,48 @@ where
     }
 }
 
+/// Extractor that gets the request URI for a nested service.
+///
+/// This is necessary since [`Uri`](http::Uri), when used as an extractor, will
+/// always be the full URI.
+///
+/// # Example
+///
+/// ```
+/// use axum::{prelude::*, extract::NestedUri, http::Uri};
+///
+/// let api_routes = route(
+///     "/users",
+///     get(|uri: Uri, NestedUri(nested_uri): NestedUri| async {
+///         // `uri` is `/api/users`
+///         // `nested_uri` is `/users`
+///     }),
+/// );
+///
+/// let app = nest("/api", api_routes);
+/// # async {
+/// # axum::Server::bind(&"".parse().unwrap()).serve(app.into_make_service()).await.unwrap();
+/// # };
+/// ```
+#[derive(Debug, Clone)]
+pub struct NestedUri(pub Uri);
+
+#[async_trait]
+impl<B> FromRequest<B> for NestedUri
+where
+    B: Send,
+{
+    type Rejection = NotNested;
+
+    async fn from_request(req: &mut RequestParts<B>) -> Result<Self, Self::Rejection> {
+        let uri = Extension::<Self>::from_request(req)
+            .await
+            .map_err(|_| NotNested)?
+            .0;
+        Ok(uri)
+    }
+}
+
 #[async_trait]
 impl<B> FromRequest<B> for Version
 where