That needs a garbage collector to work – the lifetime of String isn't limited, but the lifetime &str's is. That means that the backing storage of String can be

That needs a garbage collector to work – the lifetime of String isn't limited, but the lifetime &str's is. That means that the backing storage of String can be deallocated under it's feet. There has been some discussion about allowing Strings to use COW in case of &'static str which doesn't have this problem, but I'm not sure whether that proposal has any momentum at the moment.