If you look under the hood, you'll see that fields still uses the two-argument form of bless. It's just hiding it from you. It happens when you call fields::new().

The examples you see in the docs really just passes the first argument to fields::new, although it gives the first argument different names, and then passes that to fields::new() that uses it as the second argument to bless().

It's always seemed to mind-bending for me, so I don't use it. I can do without the black magic. :)