I think this EFF document probably provides a more comprehensive answer than what can be provided in a HN comment https://www.eff.org/files/2017/03/10/digital-privacy-border-...

Peter might have good insights on whether the relevant case law has changed since 2017 though.

The only relevant part from that document is this line from page 33: "Foreign visitors have the fewest rights... if a foreign visitor refuses a border agent’s demand to unlock their digital device, provide the device password, or provide social media information, and the agent responds by denying entry, the foreign visitor may have little legal recourse."