Add AppendHeaders (#927)

* Add `AppendHeaders`

* axum changelog

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)
+    }
+}