Dev Notes

Software Development Resources by David Egan.

Rules of Borrowing in Rust


rust
David Egan

This is a personal note/reminder on borrowing rules in Rust.

Rules for Variables

  • Each value has a variable which is it’s owner.
  • There can only be one owner of a value at a time.
  • When the owner goes out of scope, the value is dropped.

Rules of Borrowing

  • Unlimited borrows for read-only borrows: let a = &x
  • For a read only borrow, the original data is immutable for the duration of the borrow
  • You can only have a single borrow at a time for mutable borrows: let a = &mut x

Example

/// Notes ownership and borrowing.
fn main() -> Result<(), &'static str> {
    let mut a = [1,2,3,4];
    println!("{:?}", a);
    {
        let b = &mut a[0..2];
        // You can't access a at this point because it has been mutably borrowed
        // from. The following line won't compile, with the error message:
        // `cannot borrow `a` as immutable because it is also borrowed as mutable`:
        // println!("a: {:?}", a);
        println!("b: {:?}", b); // b: [1, 2]
        b[0] = 42;
        println!("b: {:?}", b); // b: [42, 2]
    }
    // a is accessible again:
    println!("a: {:?}", a); // a: [42, 2, 3, 4]
    Ok(())
}
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