Add cmp::{min_by, min_by_key, max_by, max_by_key}

[timvermeulen]

This adds the following functions to core::cmp:

• min_by
• min_by_key
• max_by
• max_by_key

min_by and max_by are somewhat trivial to implement, but not entirely because min_by returns the first value in case the two are equal (and max_by the second). min and max can be implemented in terms of min_by and max_by, but not as easily the other way around.

To give an example of why I think these functions could be useful: the Iterator::{min_by, min_by_key, max_by, max_by_key} methods all currently hard-code the behavior mentioned above which is an ever so small duplication of logic. If we delegate them to cmp::{min_by, max_by} methods instead, we get the correct behavior for free. (edit: this is now included in the PR)

I added min_by_key / max_by_key for consistency's sake but I wouldn't mind removing them. I don't have a particular use case in mind for them, and min_by / max_by seem to be more useful.

Tracking issue: #64460

[rust-highfive]

r? @withoutboats

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[JohnCSimon]

Ping from triage
@timvermeulen
@withoutboats @cuviper Can you please review this?

Thanks.

[bors]

☔ The latest upstream changes (presumably #64281) made this pull request unmergeable. Please resolve the merge conflicts.

[cuviper]

To give an example of why I think these functions could be useful: the Iterator::{min_by, min_by_key, max_by, max_by_key} methods all currently hard-code the behavior mentioned above which is an ever so small duplication of logic. If we delegate them to cmp::{min_by, max_by} methods instead, we get the correct behavior for free.

Did you try making this change?

[timvermeulen]

@cuviper I did indeed, here's the diff: timvermeulen@aed2e46

It's not a huge win, but it gets rid of the stability logic. And I guess it's also nice to have them in terms of the more general fold1 rather than select_fold1.

[cuviper]

I like it! Care to add that motivation to this PR?

Fill in a tracking issue too, and I think we can land this.

[timvermeulen]

@cuviper Thanks, I added it.

[cuviper]

I meant to actually include the iterator commit here, but it can be a followup if you prefer.

The tracking issue number needs to be set in the #[unstable] attributes.

[timvermeulen]

Ah, I was going to submit a follow-up, but I'm happy to include it here too.

[cuviper]

Looks great, thanks!

@bors r+

[bors]

📌 Commit 7217591 has been approved by cuviper

[bors]

⌛ Testing commit 7217591 with merge 660b5c8cbf99ec24131ddefe0d667303b2bcb15a...

[tmandry]

@bors retry rolled up

[bors]

⌛ Testing commit 7217591 with merge 5349e69...

[bors]

☀️ Test successful - checks-azure
Approved by: cuviper
Pushing 5349e69 to master...