Return the delimiter from slice::split_once

[olivia-fl]

This is a breaking change to the unstable slice_split_once feature. It was originally proposed in #112811 (comment).

This would be a departure from the str::split_once API that the slice_split_once methods were based on. The justification is that for arbitrary T, the split element and be complex and the relationship between the split element and the predicate can be more indirect than with typical str patterns.

[rustbot]

r? @cuviper

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[olivia-fl]

Marking this as a draft, since there was another good proposal for the api here: #112811 (comment).

[cuviper]

I think we'll need an API member to weigh in on this.

@rustbot label -T-libc +T-libs-api
r? libs-api

[joshtriplett]

r? libs-api

[Amanieu]

We discussed this in a previous libs-api meeting.

Finding the delimiter in slices is relatively easy to do using slice.iter().position(...). As such, we view that changing the signature of slice::split_once is not worth the cost of losing API consistency with str::split_once.

On the other hand, str::split_once would gain value by returning the delimiter due to the flexibility in the kinds of patterns that are accepted by the API. It unfortunately can't be done since that API is already stable.

Perhaps a better avenue would be to explore a new API on str and then later port that API back to slice.

[Amanieu]

@rfcbot fcp close

[rfcbot]

Team member @Amanieu has proposed to close this. The next step is review by the rest of the tagged team members:

• @Amanieu
• @BurntSushi
• @dtolnay
• @joshtriplett
• @m-ou-se

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

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[rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔

[rfcbot]

The final comment period, with a disposition to close, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.