Fix mut_mutex_lock when Mutex is behind immutable deref

[lukaslueg]

I think the problem here is the if let ty::Ref(_, _, Mutability::Mut) = cx.typeck_results().expr_ty(recv).kind() line tries to check if the Mutex can be mutably borrowed (there already is a test for Arc<Mutex<_>>), but gets bamboozled by the &mut Arc indirection. And I think checking the deref-adjustment to filter immutable-adjust (the deref through the Arc, starting from &mut Arc) is the correct fix.

Fixes #9415

changelog: Fix mut_mutex_lock when Mutex is behind immutable deref

[rust-highfive]

r? @llogiq

(rust-highfive has picked a reviewer for you, use r? to override)

[llogiq]

Thank you!

@bors r+

[bors]

📌 Commit ffc75af has been approved by llogiq

It is now in the queue for this repository.

[bors]

⌛ Testing commit ffc75af with merge c36696a...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: llogiq
Pushing c36696a to master...