Style Guide prescriptions on ABI explicitness

[calebcartwright]

rust-lang/rfcs#3722 seems likely to land albeit for some future edition beyond 2024, and when it does I would propose we remove the existing ABI prescriptions from the Style Guide text

Formatters (at least rustfmt) do not necessarily have the relevant context to know whether formatting changes that involved the ABI would be semantics preserving, and the behavior that matched the "always be explicit" rule in the Style Guide did cause a couple of issues over the years (e.g. rust-lang/rustfmt#2908)

Once rust-lang/rfcs#3722 is enacted I think rustfmt should fallback to just maintaining ABI as it exists within the AST, and leave the consideration of whether an ABI has a default and/or is required as a lang item

[traviscross]

I propose we adopt @calebcartwright's proposal that:

rustfmt should fallback to just maintaining ABI as it exists within the AST, and leave the consideration of whether an ABI has a default and/or is required as a lang item

Included in this is that if there's not an ABI at all in the AST, the formatter should not add one.

On the matter of what prescriptions we should make in older editions about whether to add an ABI, I'd propose that the style guide can simply be silent about this since we now have a lang side lint in all editions against extern without an ABI, and we don't need to speak on the style side to things that are already handled via warning-level lang lints.

@rfcbot fcp merge

[rfcbot]

Team member @traviscross has proposed to merge this. The next step is review by the rest of the tagged team members:

• @calebcartwright
• @joshtriplett
• @traviscross

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

[rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔

psst @traviscross, I wasn't able to add the final-comment-period label, please do so.

[calebcartwright]

Remind me, will the current process require 2 checkboxes or will it enter the final comment period as soon as any one person has checked their box?

Edit: never mind, I remember what we talked about on this topic when the team first changed to three members