Replace archived core2 dependency with no_std_io2

[Benten-Ben]

Summary

core2 was archived upstream on 2026-04-14 with the note "No longer supported. Use core directly." Version 0.4.0 is yanked from crates.io with no replacement, making cid currently unbuildable from a clean checkout against the default registry.

This PR swaps core2 for no_std_io2, an API-compatible drop-in replacement purpose-built for crates that need std::io's trait shape in no_std contexts.

Approach

Mirrors the approach in multiformats/rust-multihash#407 for multiformats org consistency. The diff is the minimum needed:

• Cargo.toml: replace the core2 dependency with no_std_io2 = "0.8.1"; update two feature flags (std, alloc) to reference no_std_io2/alloc instead of core2/alloc.
• src/cid.rs, src/error.rs: replace the two use core2::io; imports with use no_std_io2::io;.

No other changes. no_std_io2's io::Read and io::Error types preserve the exact surface previously consumed from core2::io.

Alternatives considered

• core::io::{Read, Write}: not stable (rust-lang/rust#68315 has been open since 2020, blocked on the std::io::Error OS-coupling problem).
• embedded-io: more idiomatic for new work, but introduces a different trait surface and would be a larger, more opinionated change. Happy to rework as embedded-io if maintainers prefer.
• Vendored internal shim: minimal diff but leaves the org inconsistent with the rust-multihash migration.

Testing

Currently, cid's own dev-dependency chain (multihash-codetable) pulls core2 transitively, so the upstream test suite can't build against a clean registry until #407 merges and a release is cut. Once that lands, the test suite unblocks automatically.

The library itself compiles cleanly against no_std_io2 for --all-features, --no-default-features, and --no-default-features --features alloc.

Refs

Closes #184.

[Benten-Ben]

Note on CI: all 7 checks are failing with error: failed to select a version for the requirement 'core2 = "^0.4.0"' (version 0.4.0 is yanked), which is the pre-existing breakage on master that issue #184 tracks and that this PR begins resolving. The remaining chain is multihash-codetable -> core2, which #407 addresses. Once #407 merges and the multihash crates are released, rust-cid's test suite should go green on this PR (plus possibly a matching dev-dep version bump to multihash-codetable = "0.2").

[hanabi1224]

Could we make no_std_io2 optional and only opt-in when no_std feature is enabled? That is to say,

• when std is enabled, use std traits
• when no_std is enabled, use no_std_io2 traits
• when neither is enabled, skip implementing Write

[LesnyRumcajs]

Could we make no_std_io2 optional and only opt-in when no_std feature is enabled?

That'd be a great improvement, I don't think there's a point in pulling this dependency for every project, especially given that I'd imagine most projects using this crate aren't restricted by no_std.

[vmx]

Thanks @Benten-Ben for the PR. Could you please add a comment to #177. I still hope that license change eventually happens.

[vmx]

Could we make no_std_io2 optional and only opt-in when no_std feature is enabled? That is to say,

@hanabi1224 That sounds good to me. But I first want to get this patch release with minimal changes out quickly. PRs are welcome to improve things in regards to std/no_std.