Add unnecessary_ref_mut lint

[not-elm]

Related Issues: #12361

Added lint to detect whether a mutable reference is actually needed.

changelog: Add(unnecessary_ref_mut)

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[samueltardieu]

As-is, this will erroneously flag https://github.com/smol-rs/async-io/blob/master/src/lib.rs#L481 (*when = next; later requires that ref mut is used).

[Manishearth]

Yeah I think we should be careful about this lint.

I don't think this makes sense as a restriction lint. I'm also a bit worried it's low-value.

[not-elm]

Yeah I think we should be careful about this lint.

I don't think this makes sense as a restriction lint. I'm also a bit worried it's low-value.

Sorry, I'm still learning English, so I couldn't read the text accurately.
Do you mean that I should correct the category of lint?
In that case, would 'nursery' or 'pedantic' ?

[Manishearth]

Yes, probably pedantic but as I mentioned I'm not sure if we should be adding this in the first place.

[bors]

☔ The latest upstream changes (presumably #12077) made this pull request unmergeable. Please resolve the merge conflicts.

[not-elm]

Oh... I understand. Thank you.
Sorry, ignore this PR.