Improve Path error when its extracted multiple times (#1023)

* Improve `Path` error

* Improve docs

* changelog

* Update axum/src/extract/path/mod.rs

Co-authored-by: Jonas Platte <jplatte+git@posteo.de>

* fix test

Co-authored-by: Jonas Platte <jplatte+git@posteo.de>

axum/CHANGELOG.md

@@ -8,8 +8,13 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 # Unreleased
 
 - **added:** Add `WebSocket::protocol` to return the selected WebSocket subprotocol, if there is one. ([#1022])
+- **fixed:** Improve error for `PathRejection::WrongNumberOfParameters` to hint at using
+  `Path<(String, String)>` or `Path<SomeStruct>` ([#1023])
+- **fixed:** `PathRejection::WrongNumberOfParameters` now uses `500 Internal Server Error` since
+  its a programmer error and not a client error ([#1023])
 
 [#1022]: https://github.com/tokio-rs/axum/pull/1022
+[#1023]: https://github.com/tokio-rs/axum/pull/1023
 
 # 0.5.5 (10. May, 2022)
 

axum/src/extract/path/mod.rs

@@ -66,8 +66,7 @@ use std::{
 /// ```
 ///
 /// Path segments also can be deserialized into any type that implements
-/// [`serde::Deserialize`]. Path segment labels will be matched with struct
-/// field names.
+/// [`serde::Deserialize`]. This includes tuples and structs:
 ///
 /// ```rust,no_run
 /// use axum::{
@@ -78,6 +77,7 @@ use std::{
 /// use serde::Deserialize;
 /// use uuid::Uuid;
 ///
+/// // Path segment labels will be matched with struct field names
 /// #[derive(Deserialize)]
 /// struct Params {
 ///     user_id: Uuid,
@@ -90,7 +90,17 @@ use std::{
 ///     // ...
 /// }
 ///
-/// let app = Router::new().route("/users/:user_id/team/:team_id", get(users_teams_show));
+/// // When using tuples the path segments will be matched by their position in the route
+/// async fn users_teams_create(
+///     Path((user_id, team_id)): Path<(String, String)>,
+/// ) {
+///     // ...
+/// }
+///
+/// let app = Router::new().route(
+///     "/users/:user_id/team/:team_id",
+///     get(users_teams_show).post(users_teams_create),
+/// );
 /// # async {
 /// # axum::Server::bind(&"".parse().unwrap()).serve(app.into_make_service()).await.unwrap();
 /// # };
@@ -319,11 +329,19 @@ impl fmt::Display for ErrorKind {
         match self {
             ErrorKind::Message(error) => error.fmt(f),
             ErrorKind::InvalidUtf8InPathParam { key } => write!(f, "Invalid UTF-8 in `{}`", key),
-            ErrorKind::WrongNumberOfParameters { got, expected } => write!(
-                f,
-                "Wrong number of parameters. Expected {} but got {}",
-                expected, got
-            ),
+            ErrorKind::WrongNumberOfParameters { got, expected } => {
+                write!(
+                    f,
+                    "Wrong number of path arguments for `Path`. Expected {} but got {}",
+                    expected, got
+                )?;
+
+                if *expected == 1 {
+                    write!(f, ". Note that multiple parameters must be extracted with a tuple `Path<(_, _)>` or a struct `Path<YourParams>`")?;
+                }
+
+                Ok(())
+            }
             ErrorKind::UnsupportedType { name } => write!(f, "Unsupported type `{}`", name),
             ErrorKind::ParseErrorAtKey {
                 key,
@@ -368,14 +386,13 @@ impl IntoResponse for FailedToDeserializePathParams {
         let (status, body) = match self.0.kind {
             ErrorKind::Message(_)
             | ErrorKind::InvalidUtf8InPathParam { .. }
-            | ErrorKind::WrongNumberOfParameters { .. }
             | ErrorKind::ParseError { .. }
             | ErrorKind::ParseErrorAtIndex { .. }
             | ErrorKind::ParseErrorAtKey { .. } => (
                 StatusCode::BAD_REQUEST,
                 format!("Invalid URL: {}", self.0.kind),
             ),
-            ErrorKind::UnsupportedType { .. } => {
+            ErrorKind::WrongNumberOfParameters { .. } | ErrorKind::UnsupportedType { .. } => {
                 (StatusCode::INTERNAL_SERVER_ERROR, self.0.kind.to_string())
             }
         };
@@ -539,4 +556,19 @@ mod tests {
         let res = client.get("/foo%20bar").send().await;
         assert_eq!(res.text().await, "foo bar");
     }
+
+    #[tokio::test]
+    async fn two_path_extractors() {
+        let app = Router::new().route("/:a/:b", get(|_: Path<String>, _: Path<String>| async {}));
+
+        let client = TestClient::new(app);
+
+        let res = client.get("/a/b").send().await;
+        assert_eq!(res.status(), StatusCode::INTERNAL_SERVER_ERROR);
+        assert_eq!(
+            res.text().await,
+            "Wrong number of path arguments for `Path`. Expected 1 but got 2. \
+            Note that multiple parameters must be extracted with a tuple `Path<(_, _)>` or a struct `Path<YourParams>`",
+        );
+    }
 }