Primer writer-director and star Shane Carruth is working on director Rian Johnson's next film Looper, which is a time travel thriller starring Bruce Willis, Joseph Gordon-Levitt, Paul Dano, and Emily Blunt. Rian Johnson gave this Tweet today (via Badass News):

Shane Carruth doing some Special Effects work for Looper
"Shane Carruth is working on Looper. If you haven't seen his film Primer, run don't walk...He's doing FX work."
RELATED: Looper Blu-ray and DVD Arrive December 31st

Looper is a time travel movie, set in a near future where time travel doesn't exist but will be invented in a few decades. The story revolves around a group of killers (called Loopers) who work for a crime syndicate in the future. Their bosses send their targets hogtied and blindfolded back in time to the Loopers, and their job is to simply shoot them in the head and dispose of the body. So the target vanishes from the future and the Loopers dispose of a corpse that doesn't technically exist.

Looper is set to begin production in Louisiana early this year.