expand: Added a more complete test .

This patch adds a regression test ensuring that deriving on invalid items
(such as functions or traits) triggers a diagnostic error instead of an
Internal Compiler Error (ICE). It also verifies that Tuple Structs are
accepted as valid targets for derivation.

Fixes #3875

gcc/testsuite/ChangeLog:

	* rust/compile/issue-3875.rs: New test.

Signed-off-by: Jayant Chauhan <[email protected]>

Author: Pasta-coder

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

@@ -0,0 +1,25 @@
+#![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;