style-guide: Document newline rules for assignment operators

[joshtriplett]

The style guide gives general rules for binary operators including
assignment, and one of those rules says to put the operator on the
subsequent line; the style guide needs to explicitly state the exception
of breaking after assignment operators rather than before.

This is already what rustfmt does and what users do; this fixes the
style guide to match the expected default style.

[rustbot]

r? @compiler-errors

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[rustbot]

Some changes occurred in src/doc/style-guide

cc @rust-lang/style

[calebcartwright]

While I'm personally in agreement with this change, I wonder if it will need an FCP?

I could certainly see both sides of "this is a bug in the Style Guide" and "this is a bug in Rustfmt"

[joshtriplett]

@calebcartwright Given the stability guarantees rustfmt and the style-edition RFC provide, I don't think changing rustfmt (for the 2015 style edition) would be an option here even if we wanted to. If we wanted to change rustfmt to do what the current style guide claims is the default style, we'd have to do so in the 2024 style edition and that would need an FCP.

[joshtriplett]

I'm going to go ahead and start an FCP, but I also think we should talk in the next meeting about whether we think "fix style guide to match rustfmt" needs an FCP in general.

@rfcbot merge

[rfcbot]

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

• @calebcartwright
• @compiler-errors
• @joshtriplett
• @yaahc

Concerns:

• all-boxes-should-be-checked (style-guide: Document newline rules for assignment operators #113145 (comment))

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.

[calebcartwright]

Apologies, I was trying to quickly and briefly share a thought and failed to include relevant context.

To be clear, I don't know if this needs an FCP, and I'm not actively pushing for one. It's more that, to the best of my recollection, we hadn't yet decided how we wanted to handle this specific case outside the 2024 style edition boundary.

I opened rust-lang/style-team#179 so that we could keep track of it, and we had some informal discussion in zulip, but I wasn't sure if we ever reached a consensus.

I'm in favor of making this change (certainly as part of 2024 style edition, and I'm open to doing it earlier), though I feel we should either discuss first, or, get everyone's buy in here (whether that's reactions, GitHub PR approvals, FCP, etc. doesn't matter to me)

[joshtriplett]

Some further info that may be helpful, as evidence that this is likely a textual bug in the style guide:

The style guide, in statements.md under "Let statements", does specifically say to break let statements after =. It seems unlikely to me that the style guide would want let statements and assignments to have different, inconsistent line-break styles.

[joshtriplett]

I don't know if this needs an FCP either; I just figured we could make the decision for if things like this need one in parallel with kicking one off in case the answer is "yes".

[rfcbot]

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

[calebcartwright]

@rfcbot concern all-boxes-should-be-checked

[joshtriplett]

Since 1) we have checkboxes from everyone anyway, and 2) we agreed in today's @rust-lang/style meeting that bugfixes to make the style guide match rustfmt don't require an FCP, I'm going to go ahead and merge this based on team member reviews rather than waiting for 10 days.

@bors r+

[bors]

📌 Commit 03e64f4 has been approved by joshtriplett

It is now in the queue for this repository.

[joshtriplett]

@bors rollup