Borrow Checking

Rust's borrow checker puts constraints on the ways you can borrow values. We've already seen that a reference cannot outlive the value it borrows:

fn main() {
    let x_ref = {
        let x = 10;
        &x
    };
    println!("x: {x_ref}");
}

There's also a second main rule that the borrow checker enforces: The aliasing rule. For a given value, at any time:

• You can have one or more shared references to the value, or
• You can have exactly one exclusive reference to the value.

fn main() {
    let mut a: i32 = 10;
    let b: &i32 = &a;

    {
        let c: &mut i32 = &mut a;
        *c = 20;
    }

    println!("a: {a}");
    println!("b: {b}");
}