4.7 Ownership and borrowing

Explanation

Rust values have one owner. Code may create many immutable borrows of a value, or one mutable borrow, but not both at the same time. This rule makes mutation visible and prevents uncontrolled mutable aliasing.

Function signatures show whether a function reads or changes data. A &[f64] argument reads a slice. A &mut [f64] argument may change a slice.

fn norm2(xs: &[f64]) -> f64 {
    xs.iter().map(|x| *x * *x).sum::<f64>().sqrt()
}

fn normalize(xs: &mut [f64]) {
    let n = norm2(xs);
    if n > 0.0 {
        for x in xs {
            *x /= n;
        }
    }
}

The normalize function first reads the slice to compute the norm, then uses a mutable borrow to change the values.

Things to look up

• Owner
• Immutable borrow
• Mutable borrow
• Aliasing
• Borrow checker

Exercise

Ask an AI agent why Rust rejects two simultaneous mutable borrows of the same vector. Then write a small example that first reads a slice and then mutates it in a separate step.

Notes for the exercise

• Rust does not forbid all aliasing; it controls mutable aliasing.
• Read function signatures before reading the function body.
• Treat borrow-checker messages as information about data flow.