Document multipart error mapping

src/convert/multipart.rs

@@ -1,11 +1,51 @@
+//! Maps [`MultipartError`] into [`AppError`] with
+//! [`AppErrorKind::BadRequest`], preserving the original message.
+//!
+//! Intended for Axum multipart form parsing so that client mistakes are
+//! surfaced as bad requests.
+
 #![cfg(all(feature = "axum", feature = "multipart"))]
 
 use axum::extract::multipart::MultipartError;
 
-use crate::AppError;
+use crate::{AppError, AppErrorKind};
 
 impl From<MultipartError> for AppError {
     fn from(err: MultipartError) -> Self {
-        AppError::bad_request(format!("Multipart error: {err}"))
+        AppError::with(AppErrorKind::BadRequest, format!("Multipart error: {err}"))
+    }
+}
+
+#[cfg(all(test, feature = "axum", feature = "multipart"))]
+mod tests {
+    use axum::{
+        body::Body,
+        extract::{FromRequest, multipart::Multipart},
+        http::Request
+    };
+
+    use crate::{AppError, AppErrorKind};
+
+    #[tokio::test]
+    async fn multipart_error_maps_to_bad_request() {
+        let boundary = "XBOUNDARY";
+        let request = Request::builder()
+            .header(
+                "content-type",
+                format!("multipart/form-data; boundary={boundary}")
+            )
+            .body(Body::from("not-a-multipart-body"))
+            .expect("request");
+
+        let mut multipart = Multipart::from_request(request, &())
+            .await
+            .expect("extractor");
+
+        let err = multipart.next_field().await.expect_err("error");
+        let expected = format!("Multipart error: {err}");
+        let app_err: AppError = err.into();
+
+        assert_eq!(app_err.kind, AppErrorKind::BadRequest);
+        assert_eq!(app_err.message.as_deref(), Some(expected.as_str()));
     }
 }