Move IoSlice and IoSliceMut to core::io

[bushrat011899]

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ACP: rust-lang/libs-team#755
Tracking issue: #154046
Related: #152918
Related: #155625

Description

Moves std::io::IoSlice and std::io::IoSliceMut into core::io. This is required for the Read and Write traits to be moved into alloc and/or core, as they contain stable methods which work with IO slices. Similar to #155574, this PR inlines the std::sys types required to create an ABI compatible type for the platforms where such compatibility is guaranteed.

Additionally, I've moved the relevant tests out of std::io::tests into coretests::io::io_slice.

---

Notes

• This PR overlaps with Move OS-dependent parts of std::io to alloc #152918, but goes further than moving the IO slice types to alloc and instead moves them straight to core. Since these types have no interaction with allocation, and doing so will allow Write to move to core::io, I consider this a better home for these types.
• Some discussion around the decision to not use a core::sys module can be found here.
• I've renamed unsupported to generic to better reflect that IoSlice(Mut) is supported by all platforms, it just doesn't have a special ABI-compatible type. I don't want to imply that parts of core "don't work" depending on the target; IoSlice(Mut) works exactly as expected on all targets.
• I've made pub items within each platform-specific representation pub(super) to highlight that everything within core::io::io_slice is an internal implementation detail not meant for any other part of the crate to be aware of.
• No AI tooling of any kind was used during the creation of this PR.

[rustbot]

r? @jhpratt

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Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: @scottmcm, libs
• @scottmcm, libs expanded to 7 candidates
• Random selection from Mark-Simulacrum, jhpratt, scottmcm

[lygstate]

a long awating staff, thanks.

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[bushrat011899]

I've split this PR into 3 commits to better highlight how little actually changes in order to facilitate this move. For reviewers, I recommend going through each commit individually.

[jhpratt]

As a heads up, I am currently exploring Europe and won't be home for a couple more weeks. If you'd like a review before then, feel free to reroll.

[bushrat011899]

As a heads up, I am currently exploring Europe and won't be home for a couple more weeks. If you'd like a review before then, feel free to reroll.

Thanks for the heads up, hope you have a great time!

r? libs

[nia-e]

I will probably also take a day or two before I can get to this, but given the stuff it's touching it's likely worth a perf run:

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: 3b58d69 (3b58d695c0cd85a0de0390442cb7bbad84c6476e, parent: eab3d9776409dfb564130c3cd5f6ecacef3d3279)

[rust-timer]

Finished benchmarking commit (3b58d69): comparison URL.

Overall result: ❌✅ regressions and improvements - no action needed

Benchmarking means the PR may be perf-sensitive. Consider adding rollup=never if this change is not fit for rolling up.

@rustbot label: -S-waiting-on-perf -perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.1%	[0.1%, 0.1%]	1
Improvements ✅ (primary)	-0.3%	[-0.3%, -0.3%]	2
Improvements ✅ (secondary)	-0.1%	[-0.1%, -0.1%]	1
All ❌✅ (primary)	-0.3%	[-0.3%, -0.3%]	2

Max RSS (memory usage)

Results (secondary -2.2%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.2%	[-2.2%, -2.2%]	1
All ❌✅ (primary)	-	-	0

Cycles

Results (primary 2.5%, secondary 0.5%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	3.4%	[2.8%, 4.2%]	5
Regressions ❌ (secondary)	3.4%	[2.6%, 4.5%]	3
Improvements ✅ (primary)	-2.2%	[-2.2%, -2.2%]	1
Improvements ✅ (secondary)	-3.7%	[-4.7%, -2.7%]	2
All ❌✅ (primary)	2.5%	[-2.2%, 4.2%]	6

Binary size

This perf run didn't have relevant results for this metric.

Bootstrap: 509.956s -> 530.998s (4.13%)
Artifact size: 398.07 MiB -> 400.08 MiB (0.50%)