How to Rig a Stiff Leg?

I’m about to build an IK/FK rig for a character with a stiff leg. In other words, the knee doesn’t bend. Ever.

In the interests of not reinventing the wheel, I thought I’d ask if anyone else has ever built a rig like this or if any pre-existing rigging tool (Rigify, Auto-rig) has an option for stiff legs.

Is it not as simple as using one bone instead of two ?