I found this line:
return (<-chan Delivery)(deliveries), nil
https://github.com/streadway/amqp/blob/master/channel.go#L1089
What does it do? Why the double parentheses?
It's a type conversion. In your case, it converts chan Delivery (two-way channel of Delivery values) to <-chan Delivery (receive-only channel of Delivery values).
It is a type conversion. Returns deliveries as a read-only channel.
Its a type conversion from two-way channel to send-only channel and in case you provide it can be omitted. Conversion would be done implicitly.