Structs

struct User {
    active: bool,
    username: String,
    email: String,
    sign_in_count: u64,
}

fn main() {
    // create an instance
    let _user1 = User {
        active: true,
        username: String::from("someusername123"),
        email: String::from("someone@example.com"),
        sign_in_count: 1,
    };
}

Struct fields follow the general rule of everything being private by default unless annotated with pub⮳.

struct User {
    active: bool,
    username: String,
    email: String,
}

fn build_user(email: String, username: String) -> User {
    User {
        active: true,
        username, // instead of username: username - field init shorthand
        email,    // same
    }
}

fn main() {
    let user1 = build_user("<user@example.com>".into(), "user".to_string());

    // struct update
    let _user2 = User {
        email: String::from("another@example.com"),
        ..user1 /* the remaining fields not explicitly set should have the
                 * same value as the fields in the given instance. */
    };
}

// Tuple struct
struct Color(i32, i32, i32);

// Unit-like struct
struct AlwaysEqual; // <-- no fields

fn main() {
    let _black = Color(0, 0, 0);
    let _s = AlwaysEqual;
}

struct Rectangle {
    width: u32,
    height: u32,
}

impl Rectangle {
    // implementation block (multiple allowed for a given struct)
    // Method
    fn area(&self) -> u32 {
        // short for self: &Self, an alias for the type that the impl block is
        // for
        self.width * self.height
    }

    // Associated Functions - NO self, &self, or &mut self
    // often use for constructors: SomeType::new(...)
    fn square(size: u32) -> Self {
        Self {
            width: size,
            height: size,
        }
    }
}

fn main() {
    let sq = Rectangle::square(5);
    println!("area: {}", sq.area());
}