What is a Bill of Attainder?
Basically, it is a type of bill in some part of a legislature that convicts you without trial, and then may proceed to levy punishment.
Our Constitution forbids it in Article 1.
In this case the act itself, and the resultant media storm that ensued, was the punishment itself, intending to muzzle, discredit or otherwise disgrace Rush by using the good offices of the Senate itself.