Add `is_empty` function to `ExactSizeIterator`

[frewsxcv]

Tracking issue for functionality introduced in #34357

[frewsxcv]

Added reference to this issue in #35429

[SimonSapin]

I wanted to use this today until I realized that it’s unstable. (I could use .len() == 0 in the mean time.) Seems straightforward enough. FCP to stabilize?

[alexcrichton]

@rfcbot fcp merge

Seems good to have consistency!

[rfcbot]

Team member @alexcrichton has proposed to merge this. The next step is review by the rest of the tagged teams:

• @BurntSushi
• @Kimundi
• @alexcrichton
• @aturon
• @brson
• @sfackler

No concerns currently listed.

Once these reviewers reach consensus, 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!

See this document for info about what commands tagged team members can give me.

[bluss]

@SimonSapin I'm curious, when do you use ExactSizeIterator and have the opportunity to use this?

[SimonSapin]

When using a slice or vec iterator as input for a parser, .is_empty() is a way to tell "have I reach the end of the input yet?" https://github.com/servo/html5ever/blob/b5c4552fab/macros/match_token.rs#L175

[bluss]

Aha, that explains it for me: using a specific iterator type, not using ESI generically. This method is good, reinforces existing conventions.

[rfcbot]

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

psst @alexcrichton, I wasn't able to add the final-comment-period label, please do so.