Format derive attributes vertically

[BenLeadbetter]

Hi!

I'd like to format my #[derive(..)] attributes vertically - like this:

#[derive(
  Clone,
  Default,
  PartialEq,
)]

I've tried the following in my rustfmt.toml, but this seemed to have no effect.

attr_fn_like_width = 1 # no effect?
attr_fn_like_width = 0 # nor this

Also adding a trailing comma to the list doesn't work. rustfmt just removes the comma and reformats the list horizontally.

Any help appreciated!

[ytmimi]

Thanks for reaching out!

To the best of my knowledge there isn't a configuration that allows what you're looking for. I think we only format derives vertically if the derive list exceeds the max_width, which defaults to 100. Derives are formatted in format_derive, and we determine how the derives will be laid out here:

rustfmt/src/attr.rs

Lines 150 to 155 in ef91154

	let tactic = definitive_tactic(
	&all_items,
	ListTactic::HorizontalVertical,
	Separator::Comma,
	argument_shape.width,
	);

[BenLeadbetter]

Cool! Thanks for the info.

So derive attributes do not fall into the attr_fn_like category?

If I have a go at implementing this feature, then handling derive statements directly is correct, right?

I just wonder whether there's a broader form that should be handled the same way? #[custom_attr(List, Of, Things)] for instance.

[ytmimi]

So derive attributes do not fall into the attr_fn_like category?

It would seem that derives are special cased. Likely because rustfmt provides the option to merge multiple derives into a single derive. For example this:

#[derive(Debug)]
#[derive(Clone)]
struct s;

could be formatted like:

#[derive(Debug, Clone)]
struct s;

If I have a go at implementing this feature, then handling derive statements directly is correct, right?

PRs are always welcome! Just a hunch, but you'll likely want to make changes to the lines of code I called out above where we determine what the tactic should be. It might be as simple as swapping argument_shape.width with context.config.attr_fn_like_width()

  let tactic = definitive_tactic( 
      &all_items, 
      ListTactic::HorizontalVertical, 
      Separator::Comma, 
-     argument_shape.width,
+     context.config.attr_fn_like_width()
 ); 

[ytmimi]

I just wonder whether there's a broader form that should be handled the same way? #[custom_attr(List, Of, Things)] for instance.

Just so you're aware those are handled here:

rustfmt/src/attr.rs

Lines 296 to 313 in ef91154

	ast::MetaItemKind::List(ref list) => {
	let path = rewrite_path(context, PathContext::Type, None, &self.path, shape)?;
	let has_trailing_comma = crate::expr::span_ends_with_comma(context, self.span);
	overflow::rewrite_with_parens(
	context,
	&path,
	list.iter(),
	// 1 = "]"
	shape.sub_width(1)?,
	self.span,
	context.config.attr_fn_like_width(),
	Some(if has_trailing_comma {
	SeparatorTactic::Always
	} else {
	SeparatorTactic::Never
	}),
	)?
	}

However, derive formatting is handled in format_derive as mentioned above, and it's likely that we can't reuse this code path because of the additional merging we might do when rewriting derives.

Hope all this info helps!