Function mlua_sys::lua53::lua::lua_upvaluejoin   
source · pub unsafe extern "C-unwind" fn lua_upvaluejoin(
    L: *mut lua_State,
    fidx1: c_int,
    n1: c_int,
    fidx2: c_int,
    n2: c_int
)Available on crate feature 
lua53 only.