Disallow *references* to `static mut` [Edition Idea]

[scottmcm]

Now that we have ptr::addr_of_mut!, the 2024 edition would be a great time to do @RalfJung's idea from #53639 (comment):

Disallow &MY_STATIC_MUT and &mut MY_STATIC_MUT entirely.

ptr::addr_of_mut!(MY_STATIC_MUT) could even be safe -- unlike taking a reference which is unsafe and often insta-UB -- and helps encourage avoiding races in the accesses, which is more practical than trying to avoid the UB from concurrent existence of references.

(Maybe people will end up just doing &mut *ptr::addr_of_mut!(MY_STATIC_MUT) without thinking through all the safety requirements -- in particular, making sure that the reference stops being used before a second reference is created -- but we can't force them to drink.)

[Lokathor]

I think that this would be much better if methods on a type could take *mut self, instead of only allowing &mut self, but that's not a hard blocker.

[scottmcm]

We discussed this in the lang team meeting today, and the consensus was in favour of going in this direction.

(With https://doc.rust-lang.org/std/cell/struct.SyncUnsafeCell.html and https://doc.rust-lang.org/std/ptr/macro.addr_of_mut.html we have options that we didn't back in 2018.)

Steps needed, I think:

• Add a lint that warns on using a & or &mut to a static mut, with some help suggesting other options.
• Make doing so a hard error in 2024 (in addition to the lint that affects the other editions)

[obeis]

@rustbot claim

[traviscross]

@rustbot labels -I-lang-nominated

I'm going to unnominate this, since there's nothing more to discuss until there is a PR to review. Please nominate such a PR as soon as it is posted.

(This is correctly labeled so we can track it with other ideas for the next edition without the nomination.)