Add a new lint: deref coercions

[yegeunyang]

Hi, I added a new lint deref coercions to resolve #7104.
This is my first time contributing to clippy, so please leave me some feedbacks!

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changelog: [deref_coercions]: new lint

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @Jarcho (or someone else) some time within the next two weeks.

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[samueltardieu]

Out of curiosity, do you know of any project or style guide which forbids deref coercions in its codebase?

[yegeunyang]

Out of curiosity, do you know of any project or style guide which forbids deref coercions in its codebase?

I don't know. I love using deref coercions, but I hope this one might be useful to some people... Because, over the years, I've seen some people who dislike it.

[samueltardieu]

I am asking because to me it looks like a lint which enforces a non-existing practice, and for which there is no demand. If I was the one deciding whether to accept it or not, I would seriously weight this against the additional maintenance burden, and probably reject it.