This might seem to be a pretty straightforward privacy rights case, but of course there really is no right to privacy in the Constitution. It's a question of how far the Fourth Amendment goes in protecting "persons, papers, and effects."
In a unanimous decision, the court decided that your smartphone is the modern equivalent of your papers---even though in important respects your bits and your papers are not equivalent. For example, you may be able to delete your bits, or lock them away, remotely. Nonetheless, the court was unequivocal.
Held: The police generally may not, without a warrant, search digital information on a cell phone seized from an individual who has been arrested.