Diagnose when test features are missing
diff --git a/tests/features/error.rs b/tests/features/error.rs
new file mode 100644
index 0000000..10ac889
--- /dev/null
+++ b/tests/features/error.rs
@@ -0,0 +1 @@
+"Hello! You want: cargo test --release --all-features"
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;