[WIP]: Move std::io::Error into core

[bushrat011899]

ACP: rust-lang/libs-team#755
Tracking issue: #154046
Related: #155574
Related: #152918

Description

Moves std::io::Error into core, deferring Box-adjacent methods to incoherent implementations in alloc, and RawOsError methods to std. This requires some substantial changes to the internals of Error, but none of them are breaking changes or externally visible.

Notably, I've replaced usage of Box with a wrapper around a pointer and an appropriate drop function. This requires the addition of quite a few lines of unsafe, but is required to work around Box only being accessible from alloc. Additionally, an atomic pointer to a VTable is used for working with RawOsError in core, since we cannot know the required implementations without std.

To-Do

• Move std::io::Error (and friends) into core, leaving incoherent implementations behind in alloc::io and std::io.
• Compile and pass tests locally
• Pass CI
• Refactor commit history for easier review

Created as a WIP as I would like to split the various steps of this effort into smaller, less controversial PRs, such as #155574.

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Notes

• This PR adopts the VTable technique from Move OS-dependent parts of std::io to alloc #152918
• This PR builds on my previous PR Move std::io::RawOsError to core::io #155574
• No AI tooling of any kind was used during the creation of this PR.

[programmerjake]

example of how to link from core's docs to std: https://doc.rust-lang.org/1.95.0/src/core/str/mod.rs.html#200

[bushrat011899]

example of how to link from core's docs to std: https://doc.rust-lang.org/1.95.0/src/core/str/mod.rs.html#200

That's a clever trick I never knew about! Looks like it's also required for linking to incoherent implementations even within the file that creates them too.

[programmerjake]

label -O-unix
See

maybe just leave it until you get the PR to work, that way there's less comment spam.