Can I restrict viewport render to inside the camera Is it possible to enable viewport rendering, but when in camera mode, restrict it to only inside the camera?

My current workaround is to hit Ctrl+B for render boarders, and select as much of my camera as possible.
Pablo Vazquez has created an addon called the Amaranth Toolset which (among a ton of other useful features) adds a way to do exactly what you are looking for.

Once the addon is installed just go into camera view and hit W > camera as render border.