Function Lnewmetatable 

pub unsafe extern "C" fn Lnewmetatable(
    state: lua_State,
    type_name: *const u8,
) -> bool

If the registry already has the key type_name, returns false. Otherwise, creates a new table to be used as a metatable for userdata, adds it to the registry with key type_name, and returns true.

In both cases pushes onto the stack the final value associated with type_name in the registry.