It's about safely exposing innards. Are you are familiar with the substring problem in Java? Before 1.7.0_06, substrings where created using an offset into the

It's about safely exposing innards. Are you are familiar with the substring problem in Java? Before 1.7.0_06, substrings where created using an offset into the buffer of the original string. Nice O(1) substring creation, the textbook example for benefits of immutability. Unfortunately, if you have long-lived, but short substrings taken out of short-lived, but long parent strings, the garbage collector won't find much to collect, since the substrings keep references to their parent buffers. This was no showstopper, but without immutability or in an environment with unmanaged memory it might very well be. This Rust snippet is about similar cases of subcomponent sharing, just on a shorter timescale: the subcomponent is exposed in a way that allows the compiler to enforce that nobody keeps a reference for longer than it is guaranteed to be valid. You may look, you may even touch, but if you want to look again you have to ask again. Other languages get by without this by garbage collection, by excessive precautionary copying or by simply hoping that the caller won't do anything stupid. Rust is unique in achieving the same runtime behavior as the latter but without the uncertainty. As a bonus, those un-keepable references cannot escape across concurrency borders so that in-place mutability becomes a lot more manageable.