add box-default lint

[llogiq]

This adds a box-default lint to suggest using Box::default() instead of Box::new(Default::default()), which offers less moving parts and potentially better performance according to the perf book.

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changelog: add [box_default] lint

[rust-highfive]

r? @giraffate

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

[llogiq]

r? @Alexendoo (as giraffate seems to be busy at the moment)

[Alexendoo]

Could add a check for expr.span.ctxt() == arg.span.ctxt() to avoid linting across macro boundaries while still catching it in local macro definitions

[llogiq]

Good idea. Done.

[Alexendoo]

Can remove the macro note from Known problems with that unless there's a case I'm missing

[Alexendoo]

Oh maybe things like Box::new(cfg_if! ... { String::new() } else { ... }) where the expressions aren't coming from the macro

[llogiq]

Exactly. Though if you want, I can remove the note until I find a clearer wording.

[Alexendoo]

I'd say remove it, apparently you can't use cfg_if in an expression position, nor #[cfg] as attributes on expressions are unstable

[llogiq]

Got it! r?

[Alexendoo]

Ahh right the parent of Box::new would probably be the impl Box {} not Box itself

@bors r+

[bors]

📌 Commit 63f441e has been approved by Alexendoo

It is now in the queue for this repository.

[bors]

⌛ Testing commit 63f441e with merge 9aa85dc...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: Alexendoo
Pushing 9aa85dc to master...