gccrs: add error check if derive has wrong item

Derive may only be applied to structs, enums and unions.

gcc/rust/ChangeLog:

	* expand/rust-expand-visitor.cc(ExpandVisitor::expand_inner_items):
	Add check and error.

gcc/testsuite/ChangeLog:

	* rust/compile/issue-3971.rs: New test.

Signed-off-by: Lucas Ly Ba <lucas.ly-ba@outlook.com>

Author: lucasly-ba

gcc/rust/expand/rust-expand-visitor.cc

@@ -18,6 +18,7 @@
 
 #include "rust-expand-visitor.h"
 #include "rust-ast-fragment.h"
+#include "rust-diagnostics.h"
 #include "rust-item.h"
 #include "rust-proc-macro.h"
 #include "rust-attributes.h"
@@ -180,6 +181,15 @@ ExpandVisitor::expand_inner_items (
 
 	      if (current.is_derive ())
 		{
+		  if (item.get_item_kind () != AST::Item::Kind::Enum
+		      && item.get_item_kind () != AST::Item::Kind::Struct
+		      && item.get_item_kind () != AST::Item::Kind::Union)
+		    {
+		      rust_error_at (current.get_locus (),
+				     "derive may only be applied to structs, "
+				     "enums and unions");
+		      return;
+		    }
 		  current.parse_attr_to_meta_item ();
 		  attr_it = attrs.erase (attr_it);
 		  // Get traits to derive in the current attribute

gcc/testsuite/rust/compile/issue-3971.rs

@@ -0,0 +1,11 @@
+#[derive(PartialOrd, PartialEq)]
+// { dg-error "derive may only be applied to structs, enums and unions" "" { target *-*-* } .-1 }
+// { dg-error "could not resolve trait .PartialOrd." "" { target *-*-* } .-2 }
+// { dg-error "could not resolve trait .PartialEq." "" { target *-*-* } .-3 }
+
+pub fn check_ge(a: Foo, b: Foo) -> bool {
+// { dg-error "could not resolve type path .Foo." "" { target *-*-* } .-1 }
+// { dg-error "could not resolve type path .Foo." "" { target *-*-* } .-2 }
+    a >= b
+}
+