Fix discarding of field_sensitivity::apply result

Not using the result is certainly a bug, so we mark it as NODISCARD.

Author: romainbrenguier

src/goto-symex/field_sensitivity.h

@@ -9,6 +9,8 @@ Author: Michael Tautschnig
 #ifndef CPROVER_GOTO_SYMEX_FIELD_SENSITIVITY_H
 #define CPROVER_GOTO_SYMEX_FIELD_SENSITIVITY_H
 
+#include <util/nodiscard.h>
+
 class exprt;
 class ssa_exprt;
 class namespacet;
@@ -82,6 +84,7 @@ class field_sensitivityt
   /// \param expr: an expression to be (recursively) transformed.
   /// \param write: set to true if the expression is to be used as an lvalue.
   /// \return the transformed expression
+  NODISCARD
   exprt
   apply(const namespacet &ns, goto_symex_statet &state, exprt expr, bool write)
     const;

src/goto-symex/symex_assign.cpp

@@ -259,7 +259,8 @@ static assignmentt rewrite_with_to_field_symbols(
           update.new_value().type());
       }
 
-      state.field_sensitivity.apply(ns, state, field_sensitive_lhs, true);
+      field_sensitive_lhs = state.field_sensitivity.apply(
+        ns, state, std::move(field_sensitive_lhs), true);
 
       if(field_sensitive_lhs.id() != ID_symbol)
         break;