Add struct NoContent as a self-described shortcut

We cannot change the behavior of `<() as IntoResponse>::into_response`,
so add a newtype struct to explicitly use 204 No Content status.

Refs: #2363, #2393

axum/CHANGELOG.md

@@ -7,6 +7,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 # Unreleased
 
+- **added:** Add `NoContent` as a self-described shortcut for `StatusCode::NO_CONTENT`.
 - **added:** Add support for WebSockets over HTTP/2 ([#2894]).
   They can be enabled by changing `get(ws_endpoint)` handlers to `any(ws_endpoint)`
 - **added:** Add `MethodFilter::CONNECT`, `routing::connect[_service]`

axum/src/response/mod.rs

@@ -1,6 +1,6 @@
 #![doc = include_str!("../docs/response.md")]
 
-use http::{header, HeaderValue};
+use http::{header, HeaderValue, StatusCode};
 
 mod redirect;
 
@@ -59,6 +59,31 @@ impl<T> From<T> for Html<T> {
     }
 }
 
+/// An empty response with 204 No Content status.
+///
+/// Due to historical and implementation reasons, the `IntoResponse` implementation of `()`
+/// (unit type) returns an empty response with 200 [`StatusCode::OK`] status.
+/// If you specifically want a 204 [`StatusCode::NO_CONTENT`] status, you can use either `StatusCode` type
+/// directly, or this shortcut struct for self-documentation.
+///
+/// ```
+/// use axum::{extract::Path, response::NoContent};
+///
+/// async fn delete_user(Path(user): Path<String>) -> Result<NoContent, String> {
+///     // ...access database...
+/// # drop(user);
+///     Ok(NoContent)
+/// }
+/// ```
+#[derive(Debug, Clone, Copy)]
+pub struct NoContent;
+
+impl IntoResponse for NoContent {
+    fn into_response(self) -> Response {
+        StatusCode::NO_CONTENT.into_response()
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use crate::extract::Extension;
@@ -223,4 +248,12 @@ mod tests {
             .route("/", get(header_array_extension_body))
             .route("/", get(header_array_extension_mixed_body));
     }
+
+    #[test]
+    fn no_content() {
+        assert_eq!(
+            super::NoContent.into_response().status(),
+            StatusCode::NO_CONTENT,
+        )
+    }
 }