Fix .source() of composite rejections (#2030)

Co-authored-by: David Pedersen <david.pdrsn@gmail.com>

axum-core/src/macros.rs

@@ -203,7 +203,7 @@ macro_rules! __composite_rejection {
             fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
                 match self {
                     $(
-                        Self::$variant(inner) => Some(inner),
+                        Self::$variant(inner) => inner.source(),
                     )+
                 }
             }
@@ -211,6 +211,46 @@ macro_rules! __composite_rejection {
     };
 }
 
+#[cfg(test)]
+mod composite_rejection_tests {
+    use self::defs::*;
+    use crate::Error;
+    use std::error::Error as _;
+
+    #[allow(dead_code, unreachable_pub)]
+    mod defs {
+        use crate::{__composite_rejection, __define_rejection};
+
+        __define_rejection! {
+            #[status = BAD_REQUEST]
+            #[body = "error message 1"]
+            pub struct Inner1;
+        }
+        __define_rejection! {
+            #[status = BAD_REQUEST]
+            #[body = "error message 2"]
+            pub struct Inner2(Error);
+        }
+        __composite_rejection! {
+            pub enum Outer { Inner1, Inner2 }
+        }
+    }
+
+    /// The implementation of `.source()` on `Outer` should defer straight to the implementation
+    /// on its inner type instead of returning the inner type itself, because the `Display`
+    /// implementation on `Outer` already forwards to the inner type and so it would result in two
+    /// errors in the chain `Display`ing the same thing.
+    #[test]
+    fn source_gives_inner_source() {
+        let rejection = Outer::Inner1(Inner1);
+        assert!(rejection.source().is_none());
+
+        let msg = "hello world";
+        let rejection = Outer::Inner2(Inner2(Error::new(msg)));
+        assert_eq!(rejection.source().unwrap().to_string(), msg);
+    }
+}
+
 #[rustfmt::skip]
 macro_rules! all_the_tuples {
     ($name:ident) => {