Borrow elements while iterating

An exclusive loop lends each element in turn for mutation:

def main() -> Result[(), AllocError | IoError | OverflowError[i64]]:
    mut numbers = [10, 20]?
    for number in &mut numbers:
        *number = *number + 2
    print(numbers)?
    mut groups = [[1]?, [2, 3]?]?
    for group in &mut groups:
        group.append(4)?
    print(groups)?
    Ok(())

[12, 22]
[[1, 4], [2, 3, 4]]

Use &items when inspection is enough. Copyable elements such as integers and strings are yielded as values; owned elements such as inner lists are shared references. Use &mut items for mutable references, including scalar elements. The outer list cannot grow, shrink, or move during either loop. Afterward, the owner is available again.