Tin doesn't rust. Rust is another name for a compound known as iron oxide. Iron, water and oxygen need to react together for rust to be produced. Because tin doesn't contain iron, it cannot rust.
Tin is a metal most often found in and mined from a mineral called cassiterite. On its own, tin is too soft for most metal-working purposes, so it is often mixed with stronger metals. Tin only breaks down when it is exposed to strong acids. Since it doesn't rust, a thin layer of tin is sometimes used to coat other metals, such as steel, to prevent or slow the presence of rust. It is most commonly used to coat steel cans.