Add next_up and next_down for f32/f64 - take 2

[Urgau]

This is a revival of #88728 which staled due to inactivity of the original author. I've address the last review comment.

---

This is a pull request implementing the features described at rust-lang/rfcs#3173.

@rustbot label +T-libs-api -T-libs
r? @scottmcm
cc @orlp

[rustbot]

Hey! It looks like you've submitted a new PR for the library teams!

If this PR contains changes to any rust-lang/rust public library APIs then please comment with @rustbot label +T-libs-api -T-libs to tag it appropriately. If this PR contains changes to any unstable APIs please edit the PR description to add a link to the relevant API Change Proposal or create one if you haven't already. If you're unsure where your change falls no worries, just leave it as is and the reviewer will take a look and make a decision to forward on if necessary.

Examples of T-libs-api changes:

• Stabilizing library features
• Introducing insta-stable changes such as new implementations of existing stable traits on existing stable types
• Introducing new or changing existing unstable library APIs (excluding permanently unstable features / features without a tracking issue)
• Changing public documentation in ways that create new stability guarantees
• Changing observable runtime behavior of library APIs

[scottmcm]

Thanks for reviving this!

@bors r+ rollup=iffy (probably fine this time, but previously failed in 32-bit i386 so might want its own bors run)

[bors]

📌 Commit 3f10e6c has been approved by scottmcm

It is now in the queue for this repository.

[orlp]

I'd just like to say that my pull request hasn't really stalled due to inactivity, but due to me not knowing what to do about this unresolved question: #91399 (comment)

[scottmcm]

@orlp I guess I'm happy saying that that's something that might block stabilization, but don't need to keep it from going into nightly.

(Unless libs-api previously said otherwise, since its their call.)

[bors]

⌛ Testing commit 3f10e6c with merge 1ea4efd...

[bors]

☀️ Test successful - checks-actions
Approved by: scottmcm
Pushing 1ea4efd to master...

[rust-timer]

Finished benchmarking commit (1ea4efd): comparison URL.

Overall result: ❌✅ regressions and 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.

	mean1	range	count2
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	1.2%	[1.2%, 1.2%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.5%	[-0.5%, -0.5%]	1
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results

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.

	mean1	range	count2
Regressions ❌ (primary)	2.1%	[2.1%, 2.1%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-2.7%	[-2.7%, -2.7%]	1
Improvements ✅ (secondary)	-6.0%	[-6.0%, -6.0%]	1
All ❌✅ (primary)	-0.3%	[-2.7%, 2.1%]	2

Cycles

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

Footnotes

1. the arithmetic mean of the percent change ↩ ↩²

2. number of relevant changes ↩ ↩²