Move std::io::Read to alloc::io

[bushrat011899]

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ACP: rust-lang/libs-team#755
Tracking issue: #154046
Split From: #156527
Blocked On: #158541

Description

Moves std::io::Read to alloc::io. This is mostly a direct cut and paste, but with a large diff due to how many implementations need to be moved into alloc. Blocked on #158541.

There is some new code written for some of the Read implementations to account for the no_global_oom_handling configuration, since Vec::extend_from_slice and String::push_str are unavailable. I've highlighted all of these such instances with review comments for clarity.

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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.

[rustbot]

r? @Mark-Simulacrum

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• Owners of files modified in this PR: @ChrisDenton, libs
• @ChrisDenton, libs expanded to 13 candidates
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[bushrat011899]

@rustbot blocked

[clarfonthey]

r? clarfonthey since this is the next one in the sequence and Mark seemed fine with me doing this.

[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]

@rustbot ready
From this point on, std::io items can only be moved to alloc, as Read depends on Vec and String, and everything left either directly or indirectly relies on Read.

[bushrat011899]

@rustbot label -T-compiler -T-rustdoc
I am pretty sure these labels are incorrectly applied because of some earlier rebase. T-rustdoc-frontend I believe is appropriate since the hardcoded URLs in one of their tests is now different.

[clarfonthey]

Doesn't actually matter, but I guess this should be alloc_io_internals since Read can't be moved to core yet.

View changes since the review

[bushrat011899]

Yeah I think I'll leave this one as-is if that's ok? Just makes things a tiny bit cleaner, even if the label isn't completely accurate.