VecDeque::insert requires feature collections, even though VecDeque as a whole and most of its methods are marked as stable. Is there a reason for this, or is it just an oversight? The same is true of shrink_to_fit as well.
Well, it's definitely a bug that they require the "collections" feature... see Add missing stability attributes to VecDeque. by eefriedman · Pull Request #28146 · rust-lang/rust · GitHub .
Some of the methods are newer features than the others. They simply need some time to stabilize.