When data is immutably borrowed, it also freezes. Frozen data can't be modified via the original object until all references to it go out of scope:

fn main() {
    let mut _mutable_integer = 7i32;

    {
        // Borrow `_mutable_integer`
        let _large_integer = &_mutable_integer;

        // Error! `_mutable_integer` is frozen in this scope
        _mutable_integer = 50;
        // FIXME ^ Comment out this line

        // `_large_integer` goes out of scope
    }

    // Ok! `_mutable_integer` is not frozen in this scope
    _mutable_integer = 3;
}