According to the Office of Dietary Supplements at the National Institutes of Health, rust, or iron(III) oxide, is not harmful to healthy individuals with normal intestinal function. As Wikipedia explains, the ingested iron uptake is regulated through several mechanisms that maintain proper circulatory iron.
In general, it is not likely for a healthy person to consume enough iron to cause any health issues. However, according to the NIH ODS, consuming over 20 milligrams per kilogram of body weight can lead to acute iron poisoning. MedlinePlus advises medical help in case of greyish skin color, weight loss, anorexia, fatigue, dizziness, nausea, headache, vomiting or shortness of breath.