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

[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.