Move std::io::buffered to alloc::io

[bushrat011899]

ACP: rust-lang/libs-team#755
Tracking issue: #154046
Split From: #156527
Blocked On: #158546

Description

Moves std::io::BufReader/BufWriter/LineWriter/etc. to alloc::io. This is effectively a direct cut and paste. Blocked on #158546.

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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.

[clarfonthey]

Just adding a reminder for review here: I think there was one of the public+unstable methods that would no longer need to be public with this change, but I might just be misremembering and should look into it when I get to reviewing this.

Also, r? clarfonthey

[bushrat011899]

Just adding a reminder for review here: I think there was one of the public+unstable methods that would no longer need to be public with this change, but I might just be misremembering and should look into it when I get to reviewing this.

No you're remembering correctly! I've updated this PR to ensure anything exported from alloc::io privately is actually used within std, and likewise for core::io and alloc. I'll note here that DEFAULT_BUF_SIZE, default_read_buf, default_read_vectored, and default_write_vectored are appropriately marked as allow(unused_imports) as they are only used within std::sys for certain targets.