expand: Add error check if derive has wrong item

gcc/testsuite/rust/compile/issue-3875.rs

@@ -0,0 +1,26 @@
+#![crate_type = "lib"]
+#![feature(no_core)]
+#![feature(lang_items)]
+#![no_core]
+// { dg-options "-w" }
+
+#[lang = "sized"]
+pub trait Sized {}
+
+#[lang = "clone"]
+pub trait Clone: Sized {
+    fn clone(&self) -> Self;
+}
+
+#[lang = "copy"]
+pub trait Copy: Clone {}
+
+#[derive(Clone)] // { dg-error "derive may only be applied to structs, enums and unions" }
+fn foo() {}
+
+#[derive(Clone)] // { dg-error "derive may only be applied to structs, enums and unions" }
+trait Bar {}
+
+// Valid test: Derive Copy and Clone on a unit struct
+#[derive(Copy, Clone)]
+struct TupleStruct;