Organizers say it's all about "decency reasons" and keeping crowds moving smoothly.
The rules sparked backlash last year: Halle Berry switched up her look to comply, while Heidi Klum just went her own way.
Cannes has a history of dress code drama too; remember Kristen Stewart going barefoot in protest against the unwritten high-heels mandate?
Now elegant flats are okay, but the festival still keeps things pretty formal.