Diagnose when test features are missing
diff --git a/tests/features/mod.rs b/tests/features/mod.rs
new file mode 100644
index 0000000..5df4889
--- /dev/null
+++ b/tests/features/mod.rs
@@ -0,0 +1,13 @@
+#[cfg(not(all(
+    feature = "derive",
+    feature = "full",
+    feature = "parsing",
+    feature = "printing",
+    feature = "visit",
+    feature = "visit-mut",
+    feature = "fold",
+    feature = "clone-impls",
+    feature = "extra-traits",
+    feature = "proc-macro",
+)))]
+mod error;