Explicitly use an empty vector for structural eq

Previous code was technically valid since the moved from vector was
empty but explicitly setting it empty with it's own name will help.

gcc/rust/ChangeLog:

	* expand/rust-derive-eq.cc: Use empty vector explicitly.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>

Author: P-E-P

gcc/rust/expand/rust-derive-eq.cc

@@ -128,9 +128,13 @@ DeriveEq::eq_impls (
   auto eq_impl = builder.trait_impl (eq, std::move (eq_generics.self_type),
 				     std::move (trait_items),
 				     std::move (eq_generics.impl));
+
+  // StructuralEq is a marker trait
+  decltype (trait_items) steq_trait_items = {};
+
   auto steq_impl
     = builder.trait_impl (steq, std::move (steq_generics.self_type),
-			  std::move (trait_items),
+			  std::move (steq_trait_items),
 			  std::move (steq_generics.impl));
 
   return vec (std::move (eq_impl), std::move (steq_impl));