Use cargo clippy instead of the clippy plugin

[oli-obk]

We don't know how long rustc will keep supporting plugins. So we are removing clippy plugin support in favour of the clippy subcommand

[thesentinels]

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[raskchanky]

I'm not sure that anyone actually uses clippy on this project. We might want to just remove it completely.

[oli-obk]

Any particular reason for not using it? Breakage? Nightly? Too many lints? At least the latter has been addressed: you can choose to only use the clippy_correctness lint group, that group only lints unambiguously buggy code and has no stylistic or opinionated lints

[eeyun]

It's not that as much as, we just have never used it. I don't think anyone has it in their workflow, we're using rustfmt extensively and most of us are using RLS in our IDEs + racer and a few other tools. I'd be curious to try clippy out, but its just not part of the common workflow right now. Part of that too is the project follows rust stable, not rust nightly which I think is a requirement for clippy?

[oli-obk]

Indeed, if you are not using it in CI and not in your workflows, removing it entirely and waiting for clippy in rls is the right thing. I'm planning for getting clippy into rls on stable rust by fall, we'll see how that goes.

This PR was mainly meant as a way to prevent the removal of the plugin interface from being a breaking change for you. From you description that's not a problem anyway.

Thanks for the explanations!