rustdoc-json: Add test for #[automatically_derived] attribute

Author: aDotInTheVoid

tests/rustdoc-json/traits/automatically_derived_attr.rs

@@ -0,0 +1,14 @@
+#[derive(Default)]
+pub struct Derived;
+
+// @is '$.index[*][?(@.inner.for.inner.name == "Derived")].attrs' '["#[automatically_derived]"]'
+
+pub struct ManualImpl;
+
+// @is '$.index[*][?(@.docs == "The manual impl of default")].attrs' []
+/// The manual impl of default
+impl Default for ManualImpl {
+    fn default() -> Self {
+        Self
+    }
+}