NAT on a router usually requires two interfaces; inside and outside. In a situation where you have only one interface, you can leverage a loopback interface for your 'ip nat outside'. The trick is to force traffic to be routed out the loopback using global policy-based routing. Once the packet is NATed, the router will once again consult the routing table to route packet to destination.
I know it is a strange concept and personally have not found real-life application for it. the last time I encountered this was in a CCIE lab.
Here is the link to Cisco Doc.

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