Move significant_drop_in_scrutinee into nursey

[Jarcho]

The current suggestion of extending the lifetime of every sub-expression is not great and doesn't fix the error given in the lint's example, though it does make the potential deadlock easier to see, but it can also cause it's own issues by delaying the drop of the lock guard.

e.g.

match x.lock().foo {
    ..
}
// some stuff
let y = x.lock();

The suggestion would create a deadlock at the second x.lock() call.

This also lints even when a significant drop type isn't created as a temporary. (#9072)

I agree @kpreid (#8987 (comment)) that this should be back-ported before the lint hits stable.

changelog: Move significant_drop_in_scrutinee into nursey

[rust-highfive]

r? @llogiq

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[flip1995]

The Vec::drain bug was already fixed AFAIK. The for loop message was improved.

But yeah the suggestion of just moving it in front of the loop/match has other deadlock potential. This should probably be addressed by scoping the match block.

That it warns even if the mutex is not used inside the loop/match was intentional, but maybe should also be revisited.

But moving it back to nursery SGTM. Beta was already branched though. I try to backport it anyway.

@bors r+

[bors]

📌 Commit aa0b0af has been approved by flip1995

It is now in the queue for this repository.

[bors]

⌛ Testing commit aa0b0af with merge 97a0cf2...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: flip1995
Pushing 97a0cf2 to master...