Add pretty printing for exprts in Catch assertions

Without a pretty printer exprt's just show up as ? in
catch assertions, which isn't very helpful.

Author: hannes-steffenhagen-diffblue

unit/testing-utils/catch_pretty_print_expr.h

@@ -0,0 +1,39 @@
+/*******************************************************************\
+
+Module: Pretty print exprts in Catch assertions
+
+Author: Diffblue Ltd.
+
+\*******************************************************************/
+
+#ifndef CPROVER_TESTING_UTILS_CATCH_PRETTY_PRINT_EXPR_H
+#define CPROVER_TESTING_UTILS_CATCH_PRETTY_PRINT_EXPR_H
+
+// this file is expected to be included from `use_catch.hpp`,
+// this include is only here to make editor syntax highlighting
+// work better
+#include <catch/catch.hpp>
+
+#include <ansi-c/expr2c.h>
+#include <util/expr.h>
+#include <util/namespace.h>
+#include <util/symbol_table.h>
+
+namespace Catch // NOLINT
+{
+template <>
+struct StringMaker<exprt> // NOLINT
+{
+  static std::string convert(const exprt &expr)
+  {
+    // expr2c doesn’t capture everything that’s contained
+    // within an expr, but it’s fairly compact.
+    // expr.pretty() could also work and is more precise,
+    // but leads to very large output multiple lines which
+    // makes it hard to see differences.
+    return expr2c(expr, namespacet{symbol_tablet{}});
+  }
+};
+} // namespace Catch
+
+#endif // CPROVER_TESTING_UTILS_CATCH_PRETTY_PRINT_EXPR_H

unit/testing-utils/use_catch.h

@@ -39,4 +39,6 @@ Author: Michael Tautschnig
 /// Add to the end of test tags to mark a test that is expected to fail
 #define XFAIL "[.][!shouldfail]"
 
+#include "catch_pretty_print_expr.h"
+
 #endif // CPROVER_TESTING_UTILS_USE_CATCH_H