Use rename_ssa instead of set_level

The set_level function shouldn't be called directly outside of the
implementation of the rename function. This will make how expressions
are transformed from one level to the other clearer and more abstract.

Author: romainbrenguier

src/goto-symex/symex_start_thread.cpp

@@ -36,7 +36,6 @@ void goto_symext::symex_start_thread(statet &state)
     instruction.get_target();
 
   // put into thread vector
-  std::size_t t=state.threads.size();
   state.threads.push_back(statet::threadt());
   // statet::threadt &cur_thread=state.threads[state.source.thread_nr];
   statet::threadt &new_thread=state.threads.back();
@@ -108,11 +107,9 @@ void goto_symext::symex_start_thread(statet &state)
        (symbol.is_extern && symbol.value.is_nil()))
       continue;
 
-    // get original name
-    ssa_exprt lhs(symbol.symbol_expr());
-
     // get L0 name for current thread
-    lhs.set_level_0(t);
+    const renamedt<ssa_exprt, L0> l0_lhs =
+      state.rename_ssa<L0>(ssa_exprt{symbol.symbol_expr()}, ns);
 
     exprt rhs=symbol.value;
     if(rhs.is_nil())
@@ -125,7 +122,7 @@ void goto_symext::symex_start_thread(statet &state)
     guardt guard{true_exprt{}};
     symex_assign_symbol(
       state,
-      lhs,
+      l0_lhs.value,
       nil_exprt(),
       rhs,
       guard,