In deriving Euler's equations for fluid mechanics, in particular 

$$f=\rho \partial\_t v +\rho v\cdot \nabla v$$

for some body force $f$ (e.g Landau & Lifschitz 2.3) one assumes the continuity equation $\partial\_t\rho=-\nabla\cdot\left(\rho v\right)$ and Newton's second law in the form $F=ma$ so $$f=\rho D\_t v$$ where $D\_t=\partial\_t+v\cdot \nabla$ is the total time derivative (e.g Landau & Lifschitz 2.1).

If one instead uses $F=\dot p$ so that $$f = D\_t\left(\rho v\right)$$ then in order to get to get our initial equation back one must assume $D\_t \rho=0$ but of course $D\_t \rho=\partial\_t\rho+v\cdot \nabla\rho=\nabla\cdot\rho$ and so we have assumed the flow is incompressible. 

Landau implies that he has not assumed incompressibility at this point so how is it that one can choose the first form for Newton's second law without loss of generality?

*(If your answer would appeal to stress-energy tensors or Navier-Stokes etc., if possible please show how the assumption isn't implicitly made.)*

To be more clear, as I have shown above, if you say that $f = D\_t\left(\rho v\right)$ and also that $f=\rho \partial\_t v +\rho v\cdot \nabla v$ then it follows that the flow is incompressible. I don't think that this does imply incompressibility so why is the assumption false?