At least we know it (hopefully) wasn't written.
UPDATE, June 28, 10:06 p.m. ET: JK Y'ALL, IT WAS WRITTEN.
Jay Smooth, creator of the hip-hop radio show "Underground Railroad" on WBAI, tweeted that some of the lines from Shia's so-called freestyle rap were lifted from a track called "Perfectionist" by the Anomolies.
Parts of that Shia Lebeouf rap are plagiarized from this track by the Anomolies https://t.co/D8KxrMvny3 (starting at 0:49)— jay smooth (@jsmooth995) June 29, 2015
Apparently he plagiarized more than one line from the track, including the "gas mask / last laugh" line.
Shia's "gas mask/last laugh" line also bitten from that track, at 1:45. And the Anomolies are awesome and you should find all their stuff.— jay smooth (@jsmooth995) June 29, 2015
It's a dark day for Shia.