feat(allocator): add HashMap::from_iter_in

[sapphi-red]

This PR adds HashMap::from_iter_in to the arena allocated HashMap.

[sapphi-red]

• feat(allocator): add HashSet #14212 : 2 dependent PRs (#14079 , #14314 )
• feat(allocator): add HashMap::from_iter_in #14211 👈 (View in Graphite)
• main

---

How to use the Graphite Merge Queue

Add either label to this PR to merge it via the merge queue:

• 0-merge - adds this PR to the back of the merge queue
• hotfix - for urgent hot fixes, skip the queue and merge this PR next

You must have a Graphite account in order to use the merge queue. Sign up using this link.

An organization admin has enabled the Graphite Merge Queue in this repository.

Please do not merge from GitHub as this will restart CI on PRs being processed by the merge queue.

This stack of pull requests is managed by Graphite. Learn more about stacking.

[codspeed-hq]

CodSpeed Instrumentation Performance Report

Merging #14211 will not alter performance

_{Comparing 09-28-feat-allocator-add-hashmap-from-iter-in (7a1c339) with main (3656908)}

Summary

✅ 37 untouched

[overlookmotel]

I think it's good to add this method. It's a basic functionality, and weird we didn't have it already.

One thing: hashbrown::HashMap uses the lower bound of the size hint:

https://github.com/rust-lang/hashbrown/blob/9e812fdd350b58e9d60888c1f9dc9c5eb953f716/src/map.rs#L4538-L4554

Whereas your impl here uses the upper bound.

I don't know which is best - it depends on the iterator! Upper bound could be a large over-estimate in the case of a filter iterator.

Unless you have some reason to think upper bound is better in our use cases, I would tend to copy hashbrown. It's the underlying implementation of Rust std lib's HashMap, so I'd assume there's some rationale for how it is.

[sapphi-red]

I didn't have any reason to use the upper bound. Sorry, I had not taken a look at the code after it was generated by AI. I've now taken a look and aligned the implementation with hashbrown.

[Copilot]

Pull Request Overview

Adds a constructor to create an arena-allocated HashMap from an iterator.

• Introduces HashMap::from_iter_in to build a map using a provided allocator
• Documents the new API

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}

[overlookmotel]

I'll follow up with a PR to add some comments.

[overlookmotel]

Merge activity

• Oct 4, 8:48 AM UTC: The merge label '0-merge' was detected. This PR will be added to the Graphite merge queue once it meets the requirements.
• Oct 4, 8:48 AM UTC: overlookmotel added this pull request to the Graphite merge queue.
• Oct 4, 8:55 AM UTC: Merged by the Graphite merge queue.