Move std::io::copy to alloc::io

[bushrat011899]

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ACP: rust-lang/libs-team#755
Tracking issue: #154046
Split From: #156527
Blocked On: #158547

Description

Moves std::io::copy into alloc::io. Blocked on #158547.

This relies on specialization to allow std to provide optimised copy implementations for its types where appropriate. The exact technique involves defining a new trait, alloc::io::SpecCopy:

#[doc(hidden)]
#[unstable(feature = "core_io_internals", reason = "exposed only for libstd", issue = "none")]
#[rustc_specialization_trait]
pub trait SpecCopy: Read {
    /// Attempt to copy from this reader to the provided writer using a specialized
    /// process.
    fn copy<R: Read + ?Sized, W: Write + ?Sized>(
        _reader: &mut R,
        _writer: &mut W,
    ) -> Result<CopyState>;
}

Since optimised copying requires both the reader and writer to support the operation between each other, we can choose one of them to be the implementer of the copy algorithm, and delegate specialization to it. In this case, I've chosen the reader to be the provider of the specialized copy implementation arbitrarily. Note that the SpecCopy::copy function is generic over the reader specifically to allow wrappers like Take<R> to be visible to the implementation of copy.

Because this introduces a new layer of specialization to io::copy, I think this PR should be benchmarked to make sure performance characteristics aren't too different. I am expecting compilation time to be slightly worse, since there's just more specialization happening, but the actual code run should be the same.

---

Notes

• No AI tooling of any kind was used during the creation of this PR.
• Please see Tracking Issue for alloc::io and core::io #154046 (comment) for a review order and broader context for this PR.

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[bushrat011899]

@rustbot blocked

[clarfonthey]

LGTM minus the extra comment about SpecCopyInner: #158548 (comment)

[clarfonthey]

@bors try
@rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: a168732 (a16873222e0640d7d31675ec6e963944dba0c3a6)
Base parent: cb9d1b0 (cb9d1b0640549f1b041aae430dc413ce93f8c204)

[rust-timer]

Finished benchmarking commit (a168732): comparison URL.

Overall result: ✅ 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
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.2%	[-0.3%, -0.2%]	5
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary 0.5%, secondary 0.9%)

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

	mean	range	count
Regressions ❌ (primary)	0.5%	[0.4%, 0.6%]	4
Regressions ❌ (secondary)	0.9%	[0.4%, 4.8%]	18
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.5%	[0.4%, 0.6%]	4

Cycles

Results (primary -0.2%, 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)	1.0%	[0.7%, 1.6%]	3
Regressions ❌ (secondary)	2.3%	[0.6%, 4.7%]	7
Improvements ✅ (primary)	-0.7%	[-1.0%, -0.6%]	7
Improvements ✅ (secondary)	-3.0%	[-13.2%, -0.4%]	5
All ❌✅ (primary)	-0.2%	[-1.0%, 1.6%]	10

Binary size

Results (primary 0.0%)

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

	mean	range	count
Regressions ❌ (primary)	0.0%	[0.0%, 0.1%]	7
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.0%	[0.0%, 0.1%]	7

Bootstrap: 490.618s -> 492.892s (0.46%)
Artifact size: 390.50 MiB -> 390.52 MiB (0.01%)

[bushrat011899]

Happy to see no downsides! The "improvement" is only the include-blob bench, just across 5 different compilation modes, so I'm inclined to just call that noise.

[clarfonthey]

@bors r+ rollup

Hooray!

[rust-bors]

📌 Commit a8ef638 has been approved by clarfonthey

It is now in the queue for this repository.