EllisLab/CodeIgniter — pull request #2292 by bliker (2013-02-28 11:32:30+00:00)
Title: Validation rules can also be set by numeric array 

My experience is that passing associative array to form validation is messy and unreadable.
so instead

array(
'field' => 'password',
'label' => 'Password',
'rules' => 'required'
)

you just do

array('username', 'Username', 'required')

You can still set it the first way. So reverse compatibility is preserved.