`empty_item_single_line` should apply to trait definitions as well

[nebulaeandstars]

1.4.37-nightly

Using this rustfmt.toml

brace_style = "AlwaysNextLine"
empty_item_single_line = true

rustfmt will format the following code like this:

fn function() {}

struct Struct {}

enum Enum {}

trait Trait
{
}

impl<T> Trait for T {}

This is inconsistent, and empty traits end up sticking out like a sore thumb. Intended behaviour is probably this:

fn function() {}
struct Struct {}
enum Enum {}
trait Trait {}
impl<T> Trait for T {}

This is important, as empty traits are often quite useful. For instance, you might want something like this:

trait Trait: Send + Clone {}
impl<T: Send + Clone> Trait for T {}

which will look quite ugly at the moment.

[nebulaeandstars]

Update: This seems to be a problem with AlwaysNextLine rather than empty_item_single_line. You can remove the latter from rustfmt.toml for identical results.

[calebcartwright]

Thanks for the report. Seems the handling of brace_style when AlwaysNextLine isn't accounting for the configured empty_item_single_line value (which is true by default)

For anyone interested in working on this, suspect you'll probably want to start somewhere around here:

rustfmt/src/items.rs

Lines 1125 to 1172 in bc46af9

	match context.config.brace_style() {
	_ if last_line_contains_single_line_comment(&result)
	|| last_line_width(&result) + 2 > context.budget(offset.width()) =>
	{
	result.push_str(&offset.to_string_with_newline(context.config));
	}
	BraceStyle::AlwaysNextLine => {
	result.push_str(&offset.to_string_with_newline(context.config));
	}
	BraceStyle::PreferSameLine => result.push(' '),
	BraceStyle::SameLineWhere => {
	if result.contains('\n')
	|| (!generics.where_clause.predicates.is_empty() && !trait_items.is_empty())
	{
	result.push_str(&offset.to_string_with_newline(context.config));
	} else {
	result.push(' ');
	}
	}
	}
	result.push('{');
	let block_span = mk_sp(generics.where_clause.span.hi(), item.span.hi());
	let snippet = context.snippet(block_span);
	let open_pos = snippet.find_uncommented("{")? + 1;
	let outer_indent_str = offset.block_only().to_string_with_newline(context.config);
	if !trait_items.is_empty() || contains_comment(&snippet[open_pos..]) {
	let mut visitor = FmtVisitor::from_context(context);
	visitor.block_indent = offset.block_only().block_indent(context.config);
	visitor.last_pos = block_span.lo() + BytePos(open_pos as u32);
	for item in trait_items {
	visitor.visit_trait_item(item);
	}
	visitor.format_missing(item.span.hi() - BytePos(1));
	let inner_indent_str = visitor.block_indent.to_string_with_newline(context.config);
	result.push_str(&inner_indent_str);
	result.push_str(visitor.buffer.trim());
	result.push_str(&outer_indent_str);
	} else if result.contains('\n') {
	result.push_str(&outer_indent_str);
	}
	result.push('}');

The format_impl function in that same file may serve as a useful reference for how to account for those. When adding tests, please be sure to include a combination of cases that include where empty_item_single_line is true with brace_style = AlwaysNextLine as well as one where empty_item_single_line is false

[mujpao]

Hi. Can I try this one?

[calebcartwright]

Hi. Can I try this one?

Absolutely!