add SliceIndex wrapper types Last and Clamp<Idx>

[Qelxiros]

Tracking issue: #146179

[rustbot]

r? @jhpratt

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[jhpratt]

@rustbot author

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[Qelxiros]

@rustbot ready

[jhpratt]

Code looks good! Would you mind adding a couple tests (beyond the doctests) that ensure the behavior works as expected? It should afaict, but tests are always nice :)

View changes since this review

[jhpratt]

Suggested change

	impl<T> Sealed for crate::index::Clamp<T> {}
	impl<T> Sealed for crate::index::Clamp<T> where T: Sealed {}

to avoid potentially leaking an implementation detail down the road

[Qelxiros]

looks like https://auth.docker.io/token is 500ing, causing the CI to fail

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.