I am using _t suffix for enums and primitive types to distinguish them from variables. I put them into namespaces, so I don't care about _t reservations.
To justify it. Very often variable name is an allusion to typedefed type. Like std::size_t size;, array_t array etc. I have found that it's easier to pick up decent name for a variable, when type contains _t suffix. It also reminds me that it's a typedefed primitive and not some other beast like class for example.
