I use Lua via OpenResty / lua-nginx-module and in personal C projects for plugins and scripting. Some of these seem useful, like utf8 support and const variables.
For the multiple user data values feature, I've always found lightuserdata more useful than userdata, because it's not often that you need just one bunch of simple memory that can be freed without any other work. Rather, I almost always have more complex data, such as things that need manual cleanup like sockets, handles, etc. Or, the structure has pointers to other things that must also be cleaned up. What I'd like instead is the ability to pass a destructor callback to `lua_pushlightuserdata` so that it get's called when the pointer falls off the stack.
I don't use Lua threads or coroutines currently, so I don't have much to add there. What I do wish is that there were a way to clone or pass objects from one lua state to another to support parallelism. Basically, I'm thinking of it like the work Eric Snow is doing on subinterpreters in Python. Passing values (by copying, no shared memory) between lua instances.
One use case I had is loading a plugin that you then want to run multiple instances of in parallel with different arguments. Ideally, you wouldn't have to load the plugin multiple times (therefore calling the module level code multiple times). So I'd like to copy the entire lua state and then run each one with different arguments. There's no userdata, coroutines, or lua threads, so I don't have to worry about things that aren't possible to copy.
I got as far as looking into how to copy functions and then got busy with other things and stopped. Is there anyone else out there trying to introspect Lua function structures and copy all the opcodes, upvalues, etc?