Add AppendHeaders (#927)

* Add `AppendHeaders`

* axum changelog

axum-core/CHANGELOG.md

@@ -7,7 +7,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 # Unreleased
 
-- None.
+- **added:** Add `AppendHeaders` for appending headers to a response rather than overriding them ([#927])
+
+[#927]: https://github.com/tokio-rs/axum/pull/927
 
 # 0.2.1 (03. April, 2022)
 

axum-core/src/response/append_headers.rs

@@ -0,0 +1,65 @@
+use super::{IntoResponse, IntoResponseParts, Response, ResponseParts, TryIntoHeaderError};
+use http::header::{HeaderName, HeaderValue};
+use std::{convert::TryInto, fmt};
+
+/// Append headers to a response.
+///
+/// Returning something like `[("content-type", "foo=bar")]` from a handler will override any
+/// existing `content-type` headers. If instead you want to append headers, use `AppendHeaders`:
+///
+/// ```rust
+/// use axum::{
+///     response::{AppendHeaders, IntoResponse},
+///     http::header::SET_COOKIE,
+/// };
+///
+/// async fn handler() -> impl IntoResponse {
+///     // something that sets the `set-cookie` header
+///     let set_some_cookies = /* ... */
+///     # axum::http::HeaderMap::new();
+///
+///     (
+///         set_some_cookies,
+///         // append two `set-cookie` headers to the response
+///         // without overriding the ones added by `set_some_cookies`
+///         AppendHeaders([
+///             (SET_COOKIE, "foo=bar"),
+///             (SET_COOKIE, "baz=qux"),
+///         ])
+///     )
+/// }
+/// ```
+#[derive(Debug)]
+pub struct AppendHeaders<K, V, const N: usize>(pub [(K, V); N]);
+
+impl<K, V, const N: usize> IntoResponse for AppendHeaders<K, V, N>
+where
+    K: TryInto<HeaderName>,
+    K::Error: fmt::Display,
+    V: TryInto<HeaderValue>,
+    V::Error: fmt::Display,
+{
+    fn into_response(self) -> Response {
+        (self, ()).into_response()
+    }
+}
+
+impl<K, V, const N: usize> IntoResponseParts for AppendHeaders<K, V, N>
+where
+    K: TryInto<HeaderName>,
+    K::Error: fmt::Display,
+    V: TryInto<HeaderValue>,
+    V::Error: fmt::Display,
+{
+    type Error = TryIntoHeaderError<K::Error, V::Error>;
+
+    fn into_response_parts(self, mut res: ResponseParts) -> Result<ResponseParts, Self::Error> {
+        for (key, value) in self.0 {
+            let key = key.try_into().map_err(TryIntoHeaderError::key)?;
+            let value = value.try_into().map_err(TryIntoHeaderError::value)?;
+            res.headers_mut().append(key, value);
+        }
+
+        Ok(res)
+    }
+}

axum-core/src/response/into_response.rs

@@ -392,27 +392,7 @@ where
     V::Error: fmt::Display,
 {
     fn into_response(self) -> Response {
-        let mut res = ().into_response();
-
-        for (key, value) in self {
-            let key = match key.try_into() {
-                Ok(key) => key,
-                Err(err) => {
-                    return (StatusCode::INTERNAL_SERVER_ERROR, err.to_string()).into_response()
-                }
-            };
-
-            let value = match value.try_into() {
-                Ok(value) => value,
-                Err(err) => {
-                    return (StatusCode::INTERNAL_SERVER_ERROR, err.to_string()).into_response()
-                }
-            };
-
-            res.headers_mut().insert(key, value);
-        }
-
-        res
+        (self, ()).into_response()
     }
 }
 

axum-core/src/response/into_response_parts.rs

@@ -161,13 +161,13 @@ pub struct TryIntoHeaderError<K, V> {
 }
 
 impl<K, V> TryIntoHeaderError<K, V> {
-    fn key(err: K) -> Self {
+    pub(super) fn key(err: K) -> Self {
         Self {
             kind: TryIntoHeaderErrorKind::Key(err),
         }
     }
 
-    fn value(err: V) -> Self {
+    pub(super) fn value(err: V) -> Self {
         Self {
             kind: TryIntoHeaderErrorKind::Value(err),
         }

axum-core/src/response/mod.rs

@@ -6,10 +6,12 @@
 
 use crate::body::BoxBody;
 
+mod append_headers;
 mod into_response;
 mod into_response_parts;
 
 pub use self::{
+    append_headers::AppendHeaders,
     into_response::IntoResponse,
     into_response_parts::{IntoResponseParts, ResponseParts, TryIntoHeaderError},
 };

axum/CHANGELOG.md

@@ -7,11 +7,13 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 # Unreleased
 
+- **added:** Add `AppendHeaders` for appending headers to a response rather than overriding them ([#927])
 - **added:** Add `axum::extract::multipart::Field::chunk` method for streaming a single chunk from
   the field ([#901])
 - **fixed:** Fix trailing slash redirection with query parameters ([#936])
 
 [#901]: https://github.com/tokio-rs/axum/pull/901
+[#927]: https://github.com/tokio-rs/axum/pull/927
 [#936]: https://github.com/tokio-rs/axum/pull/936
 
 # 0.5.1 (03. April, 2022)

axum/src/response/mod.rs

@@ -19,7 +19,9 @@ pub use crate::TypedHeader;
 pub use crate::Extension;
 
 #[doc(inline)]
-pub use axum_core::response::{IntoResponse, IntoResponseParts, Response, ResponseParts};
+pub use axum_core::response::{
+    AppendHeaders, IntoResponse, IntoResponseParts, Response, ResponseParts,
+};
 
 #[doc(inline)]
 pub use self::{redirect::Redirect, sse::Sse};