Function luaL_ref 

pub unsafe extern "C-unwind" fn luaL_ref(
    l: *mut lua_State,
    t: c_int,
) -> c_int

Creates and returns a reference, in the table at index t, for the object at the top of the stack (and pops the object). [-1, +0, m]

A reference is a unique integer key. As long as you do not manually add integer keys into table t, luaL_ref ensures the uniqueness of the key it returns. You can retrieve an object referred by reference r by calling lua_rawgeti(l, t, r). Function luaL_unref frees a reference and its associated object.

If the object at the top of the stack is nil, luaL_ref returns the constant LUA_REFNIL. The constant LUA_NOREF is guaranteed to be different from any reference returned by luaL_ref.