Add style guide for default field values

[agraven]

Tracking issue: #132162

See in particular rust-lang/style-team#205.

[rustbot]

Some changes occurred in src/doc/style-guide

cc @rust-lang/style

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[clarfonthey]

Shouldn't this line have an extra indent?

[agraven]

I was following the example of the style guide for let statements. I do think the extra indent would be arguably slightly nicer but consistency feels more important to me.

let pattern:
    Type =
    expr;

[clarfonthey]

Oh, that makes a lot of sense, but I also didn't know this was the current precedent. I also definitely was not fully awake when I was reading this, so, keeping in mind that it's both a type and a value, I think this is very reasonable.

[traviscross]

On the style team, we've been wanting to move the = to the start of the next line rather than the end of the preceding one in the next style edition. Since this is a new thing we're formatting, perhaps we should consider doing that here too, in the spirit of prioritizing inter-edition consistency over intra-edition consistency.

cc @rust-lang/style

[agraven]

I am fine with doing that. Is this something that requires consensus from the team first, or should I just go ahead and do it?

[joshtriplett]

@agraven Please do go ahead and make the change. We talked about this in the style team meeting, and we're reasonably confident we'd like to 1) change this to have = at the start of the line with the expr and 2) indent an extra time if breaking for both the type and the expr, per @estebank's example in the issue that led to this.

Once that change is made, we're happy to start an FCP on this.

[agraven]

@joshtriplett Done

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.