impl Default for HashMap/HashSet iterators that don't already have it

[clarfonthey]

This is a follow-up to #128261 that isn't included in that PR because it depends on:

• Implement Default for iterators hashbrown#542 (Default)
• hashbrown release containing above

It also wasn't included in #128261 initially and should have its own FCP, since these are also insta-stable.

Changes added:

• Default for hash_map::{Iter, IterMut, IntoIter, IntoKeys, IntoValues, Keys, Values, ValuesMut}
• Default for hash_set::{Iter, IntoIter}

Changes that were added before FCP, but are being deferred to later:

• Clone for hash_map::{IntoIter, IntoKeys, IntoValues} where K: Clone, V: Clone
• Clone for hash_set::IntoIter where K: Clone

[rustbot]

r? @tgross35

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[tgross35]

This seems reasonable but will need an FCP like #128261.

r? libs-api

[dtolnay]

@rust-lang/libs-api:
@rfcbot fcp merge

• Default for hash_map::{Iter, IterMut, IntoIter, IntoKeys, IntoValues, Keys, Values, ValuesMut}
• Clone for hash_map::{IntoIter, IntoKeys, IntoValues} where K: Clone, V: Clone
• Default for hash_set::{Iter, IntoIter}
• Clone for hash_set::IntoIter where T: Clone

These are impls of a stable trait for stable types, and therefore instantly stable.

[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. 🔔

[clarfonthey]

May be unprecedented with the FCP still going on, but I'm dropping the Clone impls from this particular PR and deferring them to a later change.

Main issue is that the Clone impls are turning out to be substantially more complicated than I thought they'd be, and I think it'd be better to do a bit of refactoring in hashbrown to have safer internal parts rather than just hacking together a solution to get these impls to work. It doesn't really have any internal sense of ownership, and so IntoIter and Iter just reuse the same code, plus an extra pointer stored in the struct to make sure it gets freed.

This also means that we wouldn't depend on the second PR to hashbrown, and only on a new release being pushed out with the Default impls. (Which still is necessary, but doesn't require additional review.)

[clarfonthey]

Since I'm working on the other API changes from hashbrown 0.15 and this already passed FCP anyway, I'm going to just include this change in the PR that does that. So, closing this, but will link to clarify that FCP passed.

[clarfonthey]

Just kidding, there was another PR that added these changes that's already in the queue. So, I'll rebase after that.

[clarfonthey]

@rustbot ready

[dtolnay]

Thanks!

[dtolnay]

@bors r+

[bors]

📌 Commit 11f738f has been approved by dtolnay

It is now in the queue for this repository.

[bors]

⌛ Testing commit 11f738f with merge f7c8928...

[bors]

☀️ Test successful - checks-actions
Approved by: dtolnay
Pushing f7c8928 to master...

[rust-timer]

Finished benchmarking commit (f7c8928): comparison URL.

Overall result: ✅ improvements - no action needed

@rustbot label: -perf-regression

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-1.5%	[-3.1%, -1.1%]	7
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary -0.7%, secondary 1.7%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	1.4%	[1.4%, 1.4%]	1
Regressions ❌ (secondary)	1.7%	[1.4%, 1.9%]	2
Improvements ✅ (primary)	-1.7%	[-2.4%, -1.0%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.7%	[-2.4%, 1.4%]	3

Cycles

Results (primary 0.7%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.7%	[0.7%, 0.7%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.7%	[0.7%, 0.7%]	1

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 768.983s -> 772.929s (0.51%)
Artifact size: 341.97 MiB -> 341.95 MiB (-0.01%)