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/rust/expand/rust-derive.cc

@@ -160,4 +160,4 @@ DeriveVisitor::setup_impl_generics (
 }
 
 } // namespace AST
-} // namespace Rust
+} // namespace Rust

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

@@ -0,0 +1,12 @@
+// { dg-options "-w" }
+
+#[derive(Clone)] // { dg-error "derive may only be applied to structs, enums and unions" }
+fn foo() {}      // { dg-excess-errors "" }
+
+#[derive(Debug)] // { dg-error "derive may only be applied to structs, enums and unions" }
+trait Bar {}     // { dg-excess-errors "" }
+
+#[derive(Copy, Clone)]
+struct TupleStruct(i32); // This should NOT error
+
+fn main() {}