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;
}