rotating an edge without thining the shape when we rotate edge, they keep the same size so the shape of the mesh change. is it possible to scale the edge to keep the same shape while rotating it ? 
That's the result I want to get with the rotation
You can use "shear" which is part of the 2.8 new gyzmos:



Note that it works for more complex shapes, and you can do it in the direction you want:



For 2.79, you can use CtrlShiftAltS (yes only that), then choose X or Y axis to do nearly the same.