What is the difference between a Rust enum and struct?

While both enum and struct provide ways to encapsulate data, they do so in different ways.

A struct contains fields and every field in the struct is present at all times. This makes struct appropriate when you need to group data together and have access to all components of that data.

An enum contains variants in which a single variant gets represented at a time.

This makes enum appropriate when you have more than one data component, but only want a single component at a time.

A parser is an example where using an enum makes sense because a token may be one of a predefined number of items.