impl blocks: attaching methods and associated functions to types

Developers from OOP backgrounds are unsure how to define constructors and methods on Rust structs without classes.

Use impl blocks to define methods (&self, &mut self) and associated functions (no self) like constructors:

#[derive(Debug)]
struct Rectangle {
    width: f64,
    height: f64,
}

impl Rectangle {
    // Associated function — called as Rectangle::new()
    pub fn new(width: f64, height: f64) -> Self {
        Rectangle { width, height }
    }

    // Immutable method — borrows self
    pub fn area(&self) -> f64 {
        self.width * self.height
    }

    // Mutable method — requires mut binding
    pub fn scale(&mut self, factor: f64) {
        self.width *= factor;
        self.height *= factor;
    }

    // Consuming method — takes ownership
    pub fn into_square(self) -> Rectangle {
        let side = self.width.max(self.height);
        Rectangle { width: side, height: side }
    }
}

let mut r = Rectangle::new(3.0, 4.0);
println!("Area: {}", r.area());
r.scale(2.0);

Rust separates data (struct fields) from behavior (impl blocks). Multiple impl blocks for the same type are allowed and often used to separate trait implementations from inherent methods.