Bring a large pot of water to a boil for the haricots verts. Prepare a large bowl of ice water. Salt the boiling water; add the haricots verts. Cook until crisp-tender, about 1 minute. Transfer to the ice water to cool. Drain the beans; cut into thirds. Return the water to a boil for the pasta.
Meanwhile, in a large skillet, heat the oil, two turns of the pan, over medium-high. Add the pancetta, if using, and cook until beginning to brown, 2 to 3 minutes. Add the zucchini and carrots; cook, stirring often, until the vegetables soften, about 5 minutes. Add the leeks and garlic; season with salt and pepper. Cook, stirring often, until the leeks start to soften, about 1 minute. Add the stock; reduce the heat to low. Let the sauce simmer, stirring occasionally, while the pasta cooks.
Add the pasta to the boiling water; cook until almost al dente. Reserve 1/2 cup of the starchy cooking water, then drain the pasta. Return the pasta and reserved cooking water to the pot. Add the cheese and butter; toss to coat. Mix in the sauce, parsley and pine nuts; season. Serve in shallow bowls.
