How can compile-time errors be used to limit the possible permutations of a struct in Rust?

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In Rust, compile-time errors can be used to limit the possible permutations of a struct by using the concept of enums with associated values. An enum is a way to define a type by enumerating its possible variants, and associated values allow you to attach data of different types to these variants.

For example, suppose you have a struct called Person that can have a different set of attributes depending on whether that person is a student, employee or retiree. Instead of creating separate structs for each of these types, you can define an enum called PersonType that represents the different types of people:

enum PersonType {
    Student(String, u32),
    Employee(String, u32),
    Retiree(String),
}

In this enum, each variant contains different associated values depending on the type of person. Student has a string for their name and an unsigned integer for their age, Employee has a string for their name and an unsigned integer for their employee ID, and Retiree only has a string for their name.

Now, when you create a Person, you can specify their PersonType and provide the associated values accordingly:

struct Person {
    ptype: PersonType,
}

let student = Person {
    ptype: PersonType::Student(String::from("John Doe"), 20),
};

let retiree = Person {
    ptype: PersonType::Retiree(String::from("Jane Smith")),
};

If you try to create a Person struct without providing the necessary associated values for each variant of PersonType, Rust's type checker will give you a compile-time error. This limits the possible permutations of the Person struct and ensures that only valid combinations of attributes can be created at compile-time.