throw error if result_impl.hpp is included

Author: liam-teale

include/common/result.hpp

@@ -1,6 +1,8 @@
 #pragma once
 
+#define I_WANT_RESULT_IMPL
 #include "common/result_impl.hpp"
+#undef I_WANT_RESULT_IMPL
 
 #include <functional>
 

include/common/result_impl.hpp

@@ -6,6 +6,12 @@
 #include <stacktrace>
 #include <variant> // IWYU pragma: keep (std::get is used, not sure why clangd says this header isn't needed)
 
+// No file other than result.hpp should include result_impl.hpp.
+// The include level macro is used to prevent the error being thrown when viewing this file
+#if !defined(I_WANT_RESULT_IMPL) && __INCLUDE_LEVEL__ > 0
+    #error Don't include "result_impl.hpp", include "result.hpp" instead
+#endif
+
 namespace zest {
 
 /**