Tracking issue for RangeBounds

[durka]

This issue tracks the collections_range feature which applies to std::collections::range::RangeArgument. It used to be lumped in with drain (#27711) but that was stabilized while RangeArgument may continue to be affected by experiments with inclusive ranges, so here we are.

[durka]

I'm currently working on inclusive ranges, which is why I noticed this. A PR to change the issue link is incoming.

[withoutboats]

This trait is not actually being exported by std right now, meaning this feature can't be used without importing libcollections. I would submit a PR, but I'm not sure if a new range module in std::collections is the right place to mount it, or if it belongs under a different path.

[sfackler]

The libs team discussed this, and are placing RangeArgument into a final comment period to stabilize. We will need to tweak the API to support inclusive and exclusive bounds on each side to look something like:

pub enum Bound<T> {
    Unbound,
    Inclusive(T),
    Exclusive(T),
}

pub trait RangeArgument<T> {
    fn lower(&self) -> Bound<&T>;

    fn upper(&self) -> Bound<&T>;
}

[Stebalien]

@sfackler shouldn't the new API go though another stabilization period? Also, a way to extract the bounds by-value would be nice (but probably not necessary).