transports/dns/: Don't feature flag std::io import

[mxinden]

Description

Functions like parse_dnsaddr_txt depend on the std::io import. Given that the function is not feature flagged, compilation without features fails.

Links to any relevant issues

Discovered while releasing v0.49.0. Would like to get this in before continuing the v0.49.0 release. #2931

Open Questions

Even though libp2p-dns is useless without either the async-io or tokio feature flag, I do still think that it should compile without. Objections?

Change checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• A changelog entry has been made in the appropriate crates

[mxinden]

I think we can ignore the failing semver checks, given that we are in between releases right now. See #2931.

[mxinden]

Given that this is a very minor change, I will go ahead and merge, thus unblocking in half-published v0.49.0 release.

Please still review this patch. I will take care of any requested follow-ups.

[thomaseizinger]

Why did CI not catch this?

[mxinden]

Why did CI not catch this?

#3044 reverting this is not failing.

My suspicion would be that the tokio or async-std feature is enabled through some other crate and thus e.g. the "Build and test (--no-default-features) (pull_request)" CI step succeeds. One such crate would e.g. be libp2p-mdns:

rust-libp2p/protocols/mdns/Cargo.toml

Lines 40 to 46 in 4d4833f

	[[test]]
	name = "use-async-std"
	required-features = ["async-io"]
	[[test]]
	name = "use-tokio"
	required-features = ["tokio"]

@thomaseizinger does that suspicion make sense?