Rust for Solana – EP04: Structs, Enums, Functions, Traits & Modules

Now that you understand the ownership model and data types in Rust, it's time to go deeper into how Rust lets us organize, abstract, and structure logic.

In this episode, you’ll learn:

✅ How to define and use structs

✅ How to model data with enums

✅ How to apply pattern matching

✅ How to write functions and traits

✅ How to organize your code with modules

These tools are the foundation for building scalable Solana smart contracts and apps in Rust.

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🧱 Part 1: Structs in Rust

A struct in Rust is used to group related data.

✨ Defining and Instantiating Structs

struct User {
    username: String,
    age: u8,
}

fn main() {
    let user = User {
        username: String::from("alice"),
        age: 30,
    };

    println!("User: {} is {} years old", user.username, user.age);
}

✨ Struct with Associated Function

impl User {
    fn is_adult(&self) -> bool {
        self.age >= 18
    }
}

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🔀 Part 2: Enums in Rust

Enums allow you to define a type by enumerating possible variants.

✨ Example: Message Enum

enum Message {
    Quit,
    Move { x: i32, y: i32 },
    Write(String),
}

Each variant can carry data or none at all.

✨ Matching Enums

fn process(msg: Message) {
    match msg {
        Message::Quit => println!("Quit"),
        Message::Move { x, y } => println!("Move to ({x}, {y})"),
        Message::Write(text) => println!("Write: {text}"),
    }
}

Enums are powerful for representing state, commands, or network messages — perfect for smart contracts.

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🎭 Part 3: Pattern Matching

match lets you run code based on pattern logic. It works with enums, primitives, tuples, structs, and more.

✨ Match with Option<T>

fn maybe_double(val: Option<i32>) -> i32 {
    match val {
        Some(x) => x * 2,
        None => 0,
    }
}

✨ Match with Structs

let user = User {
    username: "Bob".into(),
    age: 25,
};

match user {
    User { age, .. } if age >= 18 => println!("Adult"),
    _ => println!("Minor"),
}

You can destructure values inside match, if let, and while let for expressive flow control.

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🧮 Part 4: Functions in Rust

Functions are reusable blocks of code defined using the fn keyword.

fn greet(name: &str) -> String {
    format!("Hello, {name}!")
}

✨ Key Points

• All parameters must have types.
• Return type is defined using ->.
• The last expression (without a ;) is returned.

fn add(a: i32, b: i32) -> i32 {
    a + b
}

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🔌 Part 5: Traits in Rust

Traits define shared behavior that types can implement.

✨ Define a Trait

trait Summary {
    fn summarize(&self) -> String;
}

✨ Implement for Struct

impl Summary for User {
    fn summarize(&self) -> String {
        format!("{} ({})", self.username, self.age)
    }
}

✨ Use Traits in Functions

fn notify(item: &impl Summary) {
    println!("Notification: {}", item.summarize());
}

Traits give Rust powerful polymorphism without runtime overhead.

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📦 Part 6: Modules in Rust

Modules let you split your code into organized files and control visibility.

✨ Inline Module

mod greetings {
    pub fn hello() {
        println!("Hello!");
    }
}

fn main() {
    greetings::hello();
}

✨ File-based Modules

src/
├── main.rs
└── utils.rs

main.rs

mod utils;

fn main() {
    utils::welcome();
}

utils.rs

pub fn welcome() {
    println!("Welcome to Rust for Solana!");
}

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📚 Summary Table

Concept	Description
struct	Group related data into one logical unit
enum	Represent multiple related variants
match	Exhaustive pattern matching for control flow
fn	Reusable functions with static typing
trait	Define and share behavior across types
mod	Organize and encapsulate logic across files

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🧪 Practice Exercises

1. Define a Struct

Create a struct Product with name, price, and stock. Implement a method is_in_stock().

2. Enum Challenge

Create an enum OrderStatus with Pending, Shipped, Delivered. Match and print their status.

3. Trait Implementation

Define a Displayable trait with display(&self) -> String. Implement it for Product.

4. Module Exercise

Create a math module with add, subtract, multiply, and divide functions. Use them in main.