Looks like the pact was a smart move after the West’s failure to support the initiative to create an anti-fascist coalition (in the hope that Hitler would attack the Soviet Union); its announcement even caused a government crisis in Japan.
That would mean the West hoped that Hitler would attack Poland as that was the only possible way that Hitler could even get to the Soviet Union at the time the pact was negotiated.
The Soviet Union wanted to modernize and expand. Hitler gave them that opportunity. If anything, it was the Soviet Union that hoped that Hitler and Western Europe would duke it out thereby weakening all their enemies.
I would go so far as to say that it is completely rational for Poland to doubt the intentions of Germany, Austria, Hungary, and Russia to this day.