What does this golang code do? [duplicate]

Question

I was reading DigitalOcean's golang client. I noticed that they create an instance of their *Op struct in a _ variable. Example: https://github.com/digitalocean/godo/blob/master/droplets.go#L32

var _ DropletsService = &DropletsServiceOp{}

Why is this line needed?

Answer 1

This line is a compile time check that *DropletsServiceOp satisfies the DropletsService interface.

The line has no effect on the execution of the program.