new lint null_pointer_optimization

[Centri3]

Although this doesn't close #9983, I think this is good enough and is more general than such a change to large_enum_variant :)

changelog: New lint [null_pointer_optimization]

[rustbot]

r? @Alexendoo

(rustbot has picked a reviewer for you, use r? to override)

[bors]

☔ The latest upstream changes (presumably #10951) made this pull request unmergeable. Please resolve the merge conflicts.

[Alexendoo]

Sometimes you do need a &/&mut Option<T> which you can't get from the Option<Box<T>>, it would be worth mentioning that somewhere, in a note or the description at the very least

[Centri3]

IIUC, into_inner would let you but it's unstable, so, but yes I will add it to the description

[Alexendoo]

Box::into_inner(b) is just *b

So technically yeah you can move the T out of the box and place it into an Option and take a reference to that, but this consumes the box and can't be done if you have a &/&mut Option<Box<T>>

[Centri3]

We could probably add an additional check for #[repr(transparent)] types around Box or NonNull, they're likely uncommon but it's still guaranteed to be optimized

Description should probably be updated (again!) to mention *b over into_inner

[bors]

☔ The latest upstream changes (presumably #11316) made this pull request unmergeable. Please resolve the merge conflicts.

[xFrednet]

Hey this is triage, I'm closing this due to inactivity. Currently, @Centri3 sadly doesn't have the time to continue this implementation. If anyone is interested in continuing this PR, you're more than welcome to create a new PR and push it over the finish line. :D

Thank you to @Centri3 and the reviewers for the time, that you already put into this!

@rustbot label +S-inactive-closed -S-waiting-on-author -S-waiting-on-review