Here's why:
* The vowel sound in "rust" is similar to the vowel sound in "cup" or "sun". These are short vowel sounds.
* Long vowel sounds are typically indicated by a single vowel letter followed by a silent "e" at the end of a word. For example, "mate", "pine", and "robe" have long vowel sounds.
Therefore, the vowel sound in "rust" is short, not long.