new multi_assignment lint

[roynrishingha]

This fixes #6576

If you added a new lint, here's a checklist for things that will be
checked during review or continuous integration.

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

---

changelog: add [multi_assignments] lint

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @llogiq (or someone else) soon.

Please see the contribution instructions for more information.

[roynrishingha]

r?@llogiq

[llogiq]

Thank you for making clippy better! I have left suggestions to fix the failing test. Otherwise this looks good to merge. 👍

[roynrishingha]

@llogiq all tests passed now!

[llogiq]

Thank you!

@bors r+

[bors]

📌 Commit fb7dffe has been approved by llogiq

It is now in the queue for this repository.

[bors]

⌛ Testing commit fb7dffe with merge 21f103a...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: llogiq
Pushing 21f103a to master...