Yes - if there are two references as parameters, there is no way for the compiler to decide which the output reference may be borrowed from - and so which lifetime to assign the output, so it must be explicitly indicated by the programmer.
There are a few a few example cases in the doc linked about that walk through the conditions it's necessary to add explicit lifetime annotation.
There are a few a few example cases in the doc linked about that walk through the conditions it's necessary to add explicit lifetime annotation.