Appropriate type for non-Rust userdata

[s5bug]

I'm working with MAME (I have to go through the Lua interface) and I want to make my life a little bit easier when working with MAME values. What's the proper way to represent them?

[MAME]> print(manager)
sol.mame_machine_manager*: 000001f553eba408
[MAME]> print(type(manager))
userdata

Is it with AnyUserData?

#[repr(transparent)]
pub struct MameMachineManager(LuaAnyUserData);

[s5bug]

It's definitely not with LuaAnyUserData: this causes MAME to segfault, despite LuaValue::type_name reporting userdata.

[khvzak]

Do you have a full code?

[s5bug]

@khvzak I can't get a minimal reproduction to you for a while (it involves me setting some time aside to write C), but the question in the title is more important. If LuaAnyUserData is the wrong type for holding non-Rust userdata, then the problem is with me, not mlua. However, I got the impression from jumping around mlua's source code that LuaAnyUserData really means "any userdata from this same dylib" (which means loading up a mlua library in any environment that adds its own userdata & trying to pass that userdata around should crash).