Tracking Issue for `string_extend_from_within`

Feature gate: #![feature(string_extend_from_within)]

This is a tracking issue for the String::extend_from_within method.

This method is just like Vec::extend_from_within (which has been stable since 1.53.0).
Note that the original impl PR didn't create a tracking issue.

Public API

// in alloc::string

impl String {
    pub fn extend_from_within<R>(&mut self, src: R)
    where
        R: RangeBounds<usize>;
}

Steps / History

• Implementation: Add String::extend_from_within #85801
• Mention tracking issue in feature gate: Add tracking issue for string_extend_from_within #103807
• Final comment period (FCP)¹
• Stabilization PR

Unresolved Questions

• None yet.

Footnotes

1. https://std-dev-guide.rust-lang.org/feature-lifecycle/stabilization.html ↩

[aDotInTheVoid]

I think this is ready to be stabilised now. CC @rust-lang/libs-api.

Stabilisation report.

This mirrors the now stabilised (#84642) Vec::extend_from_within but on strings. Motivation is the same as that API on vecs, in that it’s hairy and hard to implement correctly, and that a specific API is needed for this for borrow-checker reasons.

[dtolnay]

Is there an example use case you could point out in real-world code? Being hairy to implement is not a motivation by itself.

[aDotInTheVoid]

Sure here you go. This came about while helping with egraphs-good/egglog#496. The code we'd like to use it on is:

https://github.com/egraphs-good/egglog/blob/faeabbc762087fb0648e205f1479c36cbd7ec9a6/src/termdag.rs#L144-L192

We're trying to display a DAG as a string. If a node is a child of multiple parents, we want to use the already rendered string to represent it again. To do this, there's a HashMap<TermId, (usize, usize)> containing where in the output buffer a rendering of that term can be found. Then if we've seen the term before (via a different parent) we can extend_from_within the output, instead of rendering it.

At the moment, this works by using a Vec<u8> to store the output, than converting it to a String at the end. It'd be nicer if this API was also available on String, so the cost of conversion wasn't needed.

(I realize this isn't the simplest of use-cases, but it's the real world one that inspired me to ask to stabilize this)

[dtolnay]

Thanks, that is exactly what I was looking for.

@rfcbot fcp merge

[rfcbot]

Team member @dtolnay has proposed to merge 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!

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

[rfcbot]

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