Add axum_extra::response::ErasedJson (#537)

axum-extra/Cargo.toml

@@ -10,5 +10,10 @@ readme = "README.md"
 repository = "https://github.com/tokio-rs/axum"
 version = "0.1.0"
 
+[features]
+erased-json = ["serde", "serde_json"]
+
 [dependencies]
 axum = { path = "../axum", version = "0.3" }
+serde = { version = "1.0.130", optional = true }
+serde_json = { version = "1.0.71", optional = true }

axum-extra/src/lib.rs

@@ -41,3 +41,5 @@
 #![forbid(unsafe_code)]
 #![cfg_attr(docsrs, feature(doc_cfg))]
 #![cfg_attr(test, allow(clippy::float_cmp))]
+
+pub mod response;

axum-extra/src/response/erased_json.rs

@@ -0,0 +1,67 @@
+use std::convert::Infallible;
+
+use axum::{
+    body::{Bytes, Full},
+    http::{header, HeaderValue, Response, StatusCode},
+    response::IntoResponse,
+};
+use serde::Serialize;
+
+/// A response type that holds a JSON in serialized form.
+///
+/// This allows returning a borrowing type from a handler, or returning different response
+/// types as JSON from different branches inside a handler.
+///
+/// # Example
+///
+/// ```rust
+/// # use axum::{response::IntoResponse};
+/// # use axum_extra::response::ErasedJson;
+/// async fn handler() -> impl IntoResponse {
+///     # let condition = true;
+///     # let foo = ();
+///     # let bar = vec![()];
+///     // ...
+///
+///     if condition {
+///         ErasedJson::new(&foo)
+///     } else {
+///         ErasedJson::new(&bar)
+///     }
+/// }
+/// ```
+#[derive(Debug)]
+pub struct ErasedJson(serde_json::Result<Vec<u8>>);
+
+impl ErasedJson {
+    /// Create an `ErasedJson` by serializing a value.
+    pub fn new<T: Serialize>(val: T) -> Self {
+        Self(serde_json::to_vec(&val))
+    }
+}
+
+impl IntoResponse for ErasedJson {
+    type Body = Full<Bytes>;
+    type BodyError = Infallible;
+
+    fn into_response(self) -> Response<Self::Body> {
+        #[allow(clippy::declare_interior_mutable_const)]
+        const APPLICATION_JSON: HeaderValue = HeaderValue::from_static("application/json");
+
+        let bytes = match self.0 {
+            Ok(res) => res,
+            Err(err) => {
+                return Response::builder()
+                    .status(StatusCode::INTERNAL_SERVER_ERROR)
+                    .header(header::CONTENT_TYPE, "text/plain")
+                    .body(Full::from(err.to_string()))
+                    .unwrap();
+            }
+        };
+
+        let mut res = Response::new(Full::from(bytes));
+        res.headers_mut()
+            .insert(header::CONTENT_TYPE, APPLICATION_JSON);
+        res
+    }
+}

axum-extra/src/response/mod.rs

@@ -0,0 +1,7 @@
+//! Additional types for generating responses.
+
+#[cfg(feature = "erased-json")]
+mod erased_json;
+
+#[cfg(feature = "erased-json")]
+pub use erased_json::ErasedJson;

axum/Cargo.toml

@@ -45,7 +45,7 @@ base64 = { optional = true, version = "0.13" }
 headers = { optional = true, version = "0.3" }
 mime = { optional = true, version = "0.3" }
 multer = { optional = true, version = "2.0.0" }
-serde_json = { version = "1.0", optional = true }
+serde_json = { version = "1.0", optional = true, features = ["raw_value"] }
 sha-1 = { optional = true, version = "0.9.6" }
 tokio-tungstenite = { optional = true, version = "0.16" }