[feat]: make unnecessary_mut_passed lint auto applicable

[MCJOHN974]

Description

Make [unnecessary_mut_passed] auto applicable and automatically fixable via cargo clippy --fix

changelog: [`unnecessary_mut_passed`]: is auto applicable now

Why it is necessary

There is no case where auto-applying this lint can harm, and it is always nice to have lint applied automatically.

fixes #14617

Checklist

• Followed [lint naming conventions][lint_naming]
• Edited UI tests (including committed .stderr file) and make them passing
• cargo test passes locally
• Executed cargo dev update_lints
• Added lint documentation
• Run cargo dev fmt

Disclaimer

It's my first PR in clippy, so sorry if my PR deviates from any guidelines or traditions, and thanks you for the feedback!

[rustbot]

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[samueltardieu]

Cursory review:

It would probably be best if you use span_lint_and_then(), with .span_suggestion() (or .span_suggestion_verbose(), whether this gives a prettier result or not), and propose to rewrite &mut as &, instead of reconstructing the snippet. You can proceed as follows:

• The span to replace is from the start of argument to the start of the AddrOf argument.
• You should also ensure that the context of argument is the same one as the context of the AddrOf argument, to ensure that the &mut has not been built by a macro.
• Add tests where &mut comes from a macro for example (build a macro refmut and call refmut!(variable)), and where the expression comes from a macro (build a macro identity and call &mut identity!(var)).

Also, in the PR message (changelog) and title, you should use the lint name in lower case.

[MCJOHN974]

Also, are the tests I edited enough, or clippy have some tests to check if lint actually is machine-applicable and is applied correctly ?

[Alexendoo]

You can remove this

rust-clippy/tests/ui/mut_reference.rs

Line 2 in 6ab0b14

//@no-rustfix

from the existing test file, that will create .fixed file when you cargo uibless it