Cook pasta salted boiling water, according to package instructions.
Meanwhile heat a frying pan on medium high heat and melt butter in it. Once the butter has melted add in the garlic, and rosemary. Cook for 1-2 minutes until garlic starts to get a little color on it.
Stir in cream and season with salt. Bring the mixture to a simmer then turn heat down to medium-low to simmer for 2-3 minutes. Once slightly thickened take off the heat.
Combine the cooked pasta and the creamy garlic sauce together in frying pan, if large enough, or in a large bowl.
Notes
The amount of salt you will need will also be dependent on how much salt you add to the water used to cook the pasta