Laziness in tuple_windows

[aobatact]

I was bit surprised by that Itertools::tuple_windows is not somewhat lazy and has side effect. It is consuming the first tuple when calling Itertools::tuple_windows .

https://doc.rust-lang.org/nightly/core/iter/index.html#laziness says that adapters in Iterator is lazy and the TupleWindows has #[must_use = "iterator adaptors are lazy and do nothing unless consumed] since #227

So the question is

• Is this intended behavior?
• Should (all) adapters in Itertools be lazy?
• Should the laziness be documented?

[aobatact]

I think this is a bug to be fixed...

[Philippe-Cholet]

I looked for (mut text in function definitions and found multiple functions I thought as lazy to actually consume one item before we actually consume them:

• tuple_windows relying on the struct TupleWindows as mentioned before ;
• coalesce, dedup_by, dedup_by_with_count all three relying on the struct "CoalesceBy" ;
• cartesian_product relying on the struct "Product".

CoalesceBy and Product also uses #[must_use = "iterator adaptors are lazy and do nothing unless consumed"].

Now that I think about it, I suppose we could argue that "must_use" implies that the user is supposed to consume the iterator and consume one item at definition is not a big deal since it's "just a moment" before the rest?!