Disable vec_box when using different allocators

[Jacherr]

Fixes #7114

This PR disables the vec_box lint when the Box and Vec use different allocators (but not when they use the same - custom - allocator).

For example - Vec<Box<i32, DummyAllocator>> will disable the lint, and Vec<Box<i32, DummyAllocator>, DummyAllocator> will not disable the lint.

In addition, the applicability of this lint has been changed to Unspecified due to the automatic fixes potentially breaking code such as the following:

fn foo() -> Vec<Box<i32>> { // -> Vec<i32>
  vec![Box::new(1)]
}

It should be noted that the if_chain->let-chains fix has also been applied to this lint, so the diff does contain many changes.

changelog: disable vec_box lint when using nonstandard allocators

[rustbot]

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

Please see the contribution instructions for more information. Namely, in order to ensure the minimum review times lag, PR authors and assigned reviewers should ensure that the review label (S-waiting-on-review and S-waiting-on-author) stays updated, invoking these commands when appropriate:

• @rustbot author: the review is finished, PR author should check the comments and take action accordingly
• @rustbot review: the author is ready for a review, this PR will be queued again in the reviewer's queue

[xFrednet]

Welcome to Clippy 👋. The changes look good to me, there is one small additional tests I would like to see and then we can merge this :D

[xFrednet]

Looks good to me, thank you for the swift update :D

@bors r+

[bors]

📌 Commit eabc64f has been approved by xFrednet

It is now in the queue for this repository.

[bors]

⌛ Testing commit eabc64f with merge 6be0f74...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: xFrednet
Pushing 6be0f74 to master...