This example wasn't entirely clear to me what was inserted where. If it could be reformatted to be:

```ruby
  corrector.insert_before(node.loc.operator, 'and nil ')
end

corrector = Corrector.new(processed_source.buffer, [correction])
expect(corrector.rewrite).to eq 'true and nil and false'
```

I think it would be easier to understand for someone new to your code.