feat: add response::Error

This type makes for efficient use of the `?` operator when in a function
with multiple return error types that all implement `IntoResponse`.

Signed-off-by: Nathaniel McCallum <nathaniel@profian.com>

axum-core/src/response/error.rs

@@ -0,0 +1,26 @@
+use super::{IntoResponse, Response};
+
+/// An [IntoResponse]-based error type
+///
+/// All types which implement [IntoResponse] can be converted to an [Error].
+/// This makes it useful as a general error type for functions which combine
+/// multiple distinct error types but all of which implement [IntoResponse].
+#[derive(Debug)]
+pub struct Error(Response);
+
+impl<T: IntoResponse> From<T> for Error {
+    #[inline]
+    fn from(value: T) -> Self {
+        Self(value.into_response())
+    }
+}
+
+impl<T: IntoResponse> IntoResponse for Result<T, Error> {
+    #[inline]
+    fn into_response(self) -> Response {
+        match self {
+            Ok(ok) => ok.into_response(),
+            Err(err) => err.0,
+        }
+    }
+}

axum-core/src/response/mod.rs

@@ -9,7 +9,10 @@ use crate::body::BoxBody;
 mod into_response;
 mod into_response_parts;
 
+mod error;
+
 pub use self::{
+    error::Error,
     into_response::IntoResponse,
     into_response_parts::{IntoResponseParts, ResponseParts, TryIntoHeaderError},
 };