Added utility class to convert strings into expressions

In turns the error return state into a CATCH exception so the test will
fail without cluttering tests with checks on the flag when it is just setup
code for the actual test.

unit/Makefile

@@ -2,6 +2,7 @@
 
 # Source files for test utilities
 SRC = src/expr/require_expr.cpp \
+      src/ansi-c/c_to_expr.cpp \
       # Empty last line
 
 # Test source files

unit/src/ansi-c/c_to_expr.cpp

@@ -0,0 +1,35 @@
+/*******************************************************************\
+
+ Module: Unit test utilities
+
+ Author: DiffBlue Limited. All rights reserved.
+
+\*******************************************************************/
+
+/// \file
+/// Utility for converting strings in to exprt, throwing a CATCH exception
+/// if this fails in any way.
+///
+#include "c_to_expr.h"
+
+#include <catch.hpp>
+
+c_to_exprt::c_to_exprt():
+  message_handler(
+    std::unique_ptr<message_handlert>(new ui_message_handlert()))
+{
+  language.set_message_handler(*message_handler);
+}
+
+/// Take an input string that should be a valid C rhs expression
+/// \param input_string: The string to convert
+/// \param ns: The global namespace
+/// \return: A constructed expr representing the string
+exprt c_to_exprt::operator()(
+  const std::string &input_string, const namespacet &ns)
+{
+  exprt expr;
+  bool result=language.to_expr(input_string, "",  expr, ns);
+  REQUIRE(!result);
+  return expr;
+}

unit/src/ansi-c/c_to_expr.h

@@ -0,0 +1,35 @@
+/*******************************************************************\
+
+ Module: Unit test utilities
+
+ Author: DiffBlue Limited. All rights reserved.
+
+\*******************************************************************/
+
+/// \file
+/// Utility for converting strings in to exprt, throwing a CATCH exception
+/// if this fails in any way.
+
+#ifndef CPROVER_SRC_ANSI_C_C_TO_EXPR_H
+#define CPROVER_SRC_ANSI_C_C_TO_EXPR_H
+
+#include <memory>
+
+#include <util/expr.h>
+#include <util/message.h>
+#include <util/ui_message.h>
+#include <util/namespace.h>
+#include <ansi-c/ansi_c_language.h>
+
+class c_to_exprt
+{
+public:
+  c_to_exprt();
+  exprt operator()(const std::string &input_string, const namespacet &ns);
+
+private:
+  std::unique_ptr<message_handlert> message_handler;
+  ansi_c_languaget language;
+};
+
+#endif // CPROVER_SRC_ANSI_C_C_TO_EXPR_H