fix: replace archived core2 dependency with no_std_io2 (#185)

core2 was archived upstream on 2026-04-14 ("No longer supported. Use
core directly."). core::io::{Read, Write} are not yet stable in Rust
(tracking issue rust-lang/rust#68315), so a direct migration is not
currently possible.

no_std_io2 0.8.1 is an API-compatible drop-in replacement for core2's
io module, purpose-built for crates that need the std::io trait shape
in no_std contexts.

This mirrors the approach taken in multiformats/rust-multihash#407
for multiformats org consistency.

The dependencies that also had this `no_std_io2` change were also
explicitly updated to the corresponding versions.

Closes #184.

Author: Benten-Ben

Cargo.toml

@@ -12,15 +12,15 @@ rust-version = "1.81"
 
 [features]
 default = ["std"]
-std = ["alloc", "core2/alloc", "multihash/std", "unsigned-varint/std"]
-alloc = ["dep:multibase", "core2/alloc", "multihash/alloc"]
+std = ["alloc", "no_std_io2/alloc", "multihash/std", "unsigned-varint/std"]
+alloc = ["dep:multibase", "no_std_io2/alloc", "multihash/alloc"]
 arb = ["dep:arbitrary", "dep:quickcheck", "dep:rand", "multihash/arb"]
 scale-codec = ["dep:parity-scale-codec", "multihash/scale-codec"]
 serde-codec = ["serde"] # Deprecated, don't use.
 serde = ["alloc", "dep:serde", "dep:serde_bytes", "multihash/serde"]
 
 [dependencies]
-multihash = { version = "0.19.0", default-features = false }
+multihash = { version = "0.19.4", default-features = false }
 unsigned-varint = { version = "0.8.0", default-features = false }
 
 multibase = { version = "0.9.1", optional = true, default-features = false }
@@ -31,9 +31,9 @@ serde = { version = "1.0.116", default-features = false, optional = true }
 serde_bytes = { version = "0.11.5", default-features = false, features = ["alloc"], optional = true }
 arbitrary = { version = "1.1.0", optional = true }
 
-core2 = { version = "0.4", default-features = false }
+no_std_io2 = { version = "0.8.1", default-features = false }
 
 [dev-dependencies]
-multihash-derive = { version = "0.9.0", default-features = false }
+multihash-derive = { version = "0.9.2", default-features = false }
 serde_json = { version = "1.0.59", default-features = false, features = ["alloc"]}
-multihash-codetable = { version = "0.1.0", default-features = false, features = ["sha2"] }
+multihash-codetable = { version = "0.2.1", default-features = false, features = ["sha2"] }

src/cid.rs

@@ -51,7 +51,7 @@ pub(crate) fn varint_read_u64<R: io::Read>(mut r: R) -> Result<u64> {
 use std::io;
 
 #[cfg(not(feature = "std"))]
-use core2::io;
+use no_std_io2::io;
 
 use crate::error::{Error, Result};
 use crate::version::Version;

src/error.rs

@@ -4,7 +4,7 @@ use core::fmt;
 use std::io;
 
 #[cfg(not(feature = "std"))]
-use core2::io;
+use no_std_io2::io;
 
 /// Type alias to use this library's [`Error`] type in a `Result`.
 pub type Result<T> = core::result::Result<T, Error>;