consider mutability on useless_vec suggestions

[horacimacias]

fixes #7035

changelog: Now the suggested by useless_vec considers mutability to suggest either &[], as before, or &mut [] if the used reference is mutable.

[rust-highfive]

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[giraffate]

Thanks for your work, and welcome to Clippy!

Overall looks good.
The test looks to fail because of formatting, so can you update on this? This document is helpful: https://github.com/rust-lang/rust-clippy/blob/master/doc/adding_lints.md#running-rustfmt.
Also, can you squash commits?

[horacimacias]

thanks!
I've formatted and squashed.
I think it should be good now.

[giraffate]

LGTM with nits

[giraffate]

@bors r+

Thanks!

[bors]

📌 Commit 8a50923 has been approved by giraffate

[bors]

⌛ Testing commit 8a50923 with merge c18bcff...

[bors]

💔 Test failed - checks-action_test

[giraffate]

@bors retry

[bors]

⌛ Testing commit 8a50923 with merge d2ba777...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: giraffate
Pushing d2ba777 to master...