Borrow Errors

As a concrete example of how these borrowing rules prevent memory errors, consider the case of modifying a collection while there are references to its elements:

# // Copyright 2024 Google LLC
# // SPDX-License-Identifier: Apache-2.0
#
fn main() {
    let mut vec = vec![1, 2, 3, 4, 5];
    let elem = &vec[2];
    vec.push(6);
    dbg!(elem);
}

Similarly, consider the case of iterator invalidation:

# // Copyright 2024 Google LLC
# // SPDX-License-Identifier: Apache-2.0
#
fn main() {
    let mut vec = vec![1, 2, 3, 4, 5];
    for elem in &vec {
        vec.push(elem * 2);
    }
}

• In both of these cases, modifying the collection by pushing new elements into it can potentially invalidate existing references to the collection's elements if the collection has to reallocate.