introduce typet::add_subtype()

This introduces a new method typet::add_subtype(), identical in behavior to
typet::subtype() nonconst, to allow current users of this method to add a
subtype to a given type.  The name of the method reflects that the subtype
is added.

Author: kroening

src/util/type.h

@@ -54,8 +54,8 @@ class typet:public irept
     return static_cast<const typet &>(get_sub().front());
   }
 
-public:
   // This method will be protected eventually.
+public:
   typet &subtype()
   {
     subt &sub=get_sub();
@@ -64,6 +64,20 @@ class typet:public irept
     return static_cast<typet &>(sub.front());
   }
 
+public:
+  // This method allows the construction of a type with a subtype by
+  // starting from a type without subtype.  It avoids copying the contents
+  // of the type.  The primary use-case are parsers, where a copy could be
+  // too expensive.  Consider type_with_subtypet(id, subtype) for other
+  // use-cases.
+  typet &add_subtype()
+  {
+    subt &sub = get_sub();
+    if(sub.empty())
+      sub.resize(1);
+    return static_cast<typet &>(sub.front());
+  }
+
   bool has_subtypes() const
   { return !get_sub().empty(); }