Return result of collect_deref_expr function by value

This allows for `deref_expr_r` to be made constant, which makes it clear
that the set is rebuilt for each assign instruction and is built in one
place only. This also makes it clear that the result of the
`collect_deref_expr` function is its return value.

There is no performance cost to this refactor as Return Value
Optimization (RVO) applies in this case.

Author: thomasspriggs

src/goto-programs/mm_io.cpp

@@ -23,14 +23,14 @@ Date:   April 2017
 
 #include <set>
 
-void collect_deref_expr(
-  const exprt &src,
-  std::set<dereference_exprt> &dest)
+std::set<dereference_exprt> collect_deref_expr(const exprt &src)
 {
-  src.visit_pre([&dest](const exprt &e) {
+  std::set<dereference_exprt> collected;
+  src.visit_pre([&collected](const exprt &e) {
     if(e.id() == ID_dereference)
-      dest.insert(to_dereference_expr(e));
+      collected.insert(to_dereference_expr(e));
   });
+  return collected;
 }
 
 void mm_io(
@@ -47,11 +47,9 @@ void mm_io(
     if(!it->is_assign())
       continue;
 
-    std::set<dereference_exprt> deref_expr_r;
-
     auto &a_lhs = it->assign_lhs();
     auto &a_rhs = it->assign_rhs_nonconst();
-    collect_deref_expr(a_rhs, deref_expr_r);
+    const auto deref_expr_r = collect_deref_expr(a_rhs);
 
     if(mm_io_r.is_not_nil())
     {