Borrow Rules

Rules Summary

1. Multiple immutable borrows (ref) are allowed per statement
2. Mutable borrow (mut ref) must be exclusive
3. Mutable and immutable borrows cannot overlap
4. Cannot move a value while it is borrowed
5. Temporary borrows release at end of statement

Multiple Immutable Borrows (OK)

fn add_refs(a: ref i32, b: ref i32): i32
{
    return @a + @b
}

fn main(): i32
{
    let x = 10
    let y = 20
    print(add_refs(ref x, ref y))  // borrow x and y simultaneously
    return 0
}

Duplicate Mutable Borrow (Error)

fn main(): i32
{
    let mut value = 10
    let r1 = mut ref value
    let r2 = mut ref value  // ERROR: cannot mutably borrow more than once
    return 0
}

Borrow + Move (Error)

fn main(): i32
{
    let xs = [1, 2, 3]
    let r = ref xs
    let ys = xs            // ERROR: cannot move borrowed value
    return 0
}

Assign While Borrowed (Error)

fn main(): i32
{
    let mut value = 10
    let r = ref value
    value = 20             // ERROR: cannot assign to borrowed value
    return 0
}