Borrow Checking

Rust’s borrow checker puts constraints on the ways you can borrow values. 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}");
}

This slide should take about 10 minutes.

• Note that the requirement is that conflicting references not exist at the same point. It does not matter where the reference is dereferenced.
• The above code does not compile because a is borrowed as mutable (through c) and as immutable (through b) at the same time.
• Move the println! statement for b before the scope that introduces c to make the code compile.
• After that change, the compiler realizes that b is only ever used before the new mutable borrow of a through c. This is a feature of the borrow checker called “non-lexical lifetimes”.
• The exclusive reference constraint is quite strong. Rust uses it to ensure that data races do not occur. Rust also relies on this constraint to optimize code. For example, a value behind a shared reference can be safely cached in a register for the lifetime of that reference.
• The borrow checker is designed to accommodate many common patterns, such as taking exclusive references to different fields in a struct at the same time. But, there are some situations where it doesn’t quite “get it” and this often results in “fighting with the borrow checker.”