Move std::io::Cursor to core::io

[bushrat011899]

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ACP: rust-lang/libs-team#755
Tracking issue: #154046
Subset of: #154684

Description

Moves std::io::Cursor to core::io, leaving the IO trait implementations behind. They will be moved along with the traits themselves. Certain documentation links had to be amended, and a private/unstable into_parts_mut method was added to allow the internals to remain private.

---

Notes

• This can be reviewed independently of the other PRs tracked in Tracking Issue for alloc::io and core::io #154046.
• No AI tooling of any kind was used during the creation of this PR.

[rustbot]

r? @nia-e

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Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: @ChrisDenton, libs
• @ChrisDenton, libs expanded to 8 candidates
• Random selection from Mark-Simulacrum, nia-e

[nia-e]

Looks good modulo a nit, thanks!

View changes since this review

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[bushrat011899]

@rustbot ready

Pending CI passing, but only added a #[inline] so I'm not expecting a failure.

[nia-e]

@bors r+

[rust-bors]

📌 Commit e9125c6 has been approved by nia-e

It is now in the queue for this repository.

🌲 The tree is currently closed for pull requests below priority 1000. This pull request will be tested once the tree is reopened.

[Mark-Simulacrum]

@rust-timer build 4430d4c

cc #156506 (comment)

[rust-timer]

Finished benchmarking commit (4430d4c): 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%	[0.0%, 0.1%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.1%	[-0.1%, -0.1%]	3
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary -1.5%, secondary 0.4%)

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)	1.6%	[0.8%, 2.5%]	2
Improvements ✅ (primary)	-1.5%	[-2.4%, -0.9%]	3
Improvements ✅ (secondary)	-2.1%	[-2.1%, -2.1%]	1
All ❌✅ (primary)	-1.5%	[-2.4%, -0.9%]	3

Cycles

Results (primary -1.9%, secondary 3.9%)

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)	3.9%	[3.3%, 5.4%]	4
Improvements ✅ (primary)	-1.9%	[-1.9%, -1.9%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-1.9%	[-1.9%, -1.9%]	1

Binary size

Results (secondary 0.1%)

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)	0.1%	[0.1%, 0.1%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Bootstrap: 510.527s -> 508.357s (-0.43%)
Artifact size: 400.06 MiB -> 398.05 MiB (-0.50%)