Why `itertools` is not part of STD?

[Wandalen]

Crate itertools implements a lot of useful and often essential algorithms based on pattern iterator, extending STD.
Why is it not part of STD?
Is there reason why it should be not a part of STD?
Is that harmful to have in STD something what is not totally necessary?

[hellow554]

One reason, quoting the docs:

However, if your feature involves heap allocation, such as storing elements in a Vec<T>, then it can't be accepted into libcore, and you should propose it for itertools directly instead.

[Wandalen]

I see. I believe most things from the crate should be in std.

[scottmcm]

I suggest closing this issue, since it's not really specific enough to ever close, and the OP also asked it in a URLO thread: https://users.rust-lang.org/t/why-itertools-is-not-part-of-std/76016/27?u=scottmcm

To copy my reply to here:

The core answer is that itertools can make breaking changes, but std cannot.

That means that anything we add to Iterator we're stuck with in exactly that form forever. And since most of the provided things have no need to be overridden, there's no need to have them in core, so they might as well be in another crate if there's any question about it.

One common situation where that happens is things that might want lending iterators (formerly streaming iterators) in the future. For example, group_by says

This type implements IntoIterator (it is not an iterator itself), because the group iterators need to borrow from this value. It should be stored in a local variable or temporary and iterated.

So it's plausible that std doesn't want that right now, because it wants to leave the API space available for something that can be a (lending) iterator, once such a thing is possible.

Also, a bunch of the itertools things need extra types, not just the method on iterator. Things like MinMaxResult and EitherOrBoth are important for making some of them work, but it's not clear that core wants to have those.

[jswrenn]

That's an excellent answer! I also want to add: Things in itertools sometimes do make their way over to libcore! If I had my druthers, it'd happen more often. Unfortunately, because itertools is so widely used, adding methods with the same name to Iterator can cause widespread breakage.

[Wandalen]

Sure. Thanks for answer.