Symex rename functions: use NODISCARD

These used to have side-effects on their parameters, and will still compile if used that
way, so let's use NODISCARD to enforce that their new return value is consulted when
necessary.

Author: smowton

src/goto-symex/goto_symex_state.h

@@ -18,11 +18,12 @@ Author: Daniel Kroening, [email protected]
 
 #include <analyses/guard.h>
 
+#include <goto-programs/goto_function.h>
 #include <util/invariant.h>
-#include <util/std_expr.h>
-#include <util/ssa_expr.h>
 #include <util/make_unique.h>
-#include <goto-programs/goto_function.h>
+#include <util/nodiscard.h>
+#include <util/ssa_expr.h>
+#include <util/std_expr.h>
 
 #include "call_stack.h"
 #include "field_sensitivity.h"
@@ -97,12 +98,13 @@ class goto_symex_statet final : public goto_statet
   /// A full explanation of SSA (which is why we do this renaming) is in
   /// the SSA section of background-concepts.md.
   template <levelt level = L2>
-  renamedt<exprt, level> rename(exprt expr, const namespacet &ns);
+  NODISCARD renamedt<exprt, level> rename(exprt expr, const namespacet &ns);
 
   /// Version of rename which is specialized for SSA exprt.
   /// Implementation only exists for level L0 and L1.
   template <levelt level>
-  renamedt<ssa_exprt, level> rename_ssa(ssa_exprt ssa, const namespacet &ns);
+  NODISCARD renamedt<ssa_exprt, level>
+  rename_ssa(ssa_exprt ssa, const namespacet &ns);
 
   template <levelt level = L2>
   void rename(typet &type, const irep_idt &l1_identifier, const namespacet &ns);