[Feature request] Chained closure formatting

[wyfo]

Today, chained closure are formatted like this:

let closure = |a| {
    |b| {
        println!("{a}{b}");
    }
};

I personally find it less readable than:

let closure = |a| |b| {
    println!("{a}{b}");
};

Could it be an option for this formatting?

[ytmimi]

Thanks for reaching out. I think this is a change that could be made, and it seems that the proposed formatting aligns with the style guide rules for closure expression. Specifically:

Use closures without the enclosing {}, if possible.

Any change to the existing formatting would need to be version gated.

The reason why we wrap this in an additional block is because it doesn't fit on a single line, however we allow other block-like expressions to be written without the extra blocks, so the proposed formatting seems to align with what we're already doing for match, loop, structs, etc.

For anyone interested in working on this you'll likely want to make adjustments to allow_multi_line here:

rustfmt/src/closures.rs

Lines 180 to 219 in 5391847

	// Rewrite closure with a single expression without wrapping its body with block.
	fn rewrite_closure_expr(
	expr: &ast::Expr,
	prefix: &str,
	context: &RewriteContext<'_>,
	shape: Shape,
	) -> Option<String> {
	fn allow_multi_line(expr: &ast::Expr) -> bool {
	match expr.kind {
	ast::ExprKind::Match(..)
	| ast::ExprKind::Async(..)
	| ast::ExprKind::Block(..)
	| ast::ExprKind::TryBlock(..)
	| ast::ExprKind::Loop(..)
	| ast::ExprKind::Struct(..) => true,
	ast::ExprKind::AddrOf(_, _, ref expr)
	| ast::ExprKind::Box(ref expr)
	| ast::ExprKind::Try(ref expr)
	| ast::ExprKind::Unary(_, ref expr)
	| ast::ExprKind::Cast(ref expr, _) => allow_multi_line(expr),
	_ => false,
	}
	}
	// When rewriting closure's body without block, we require it to fit in a single line
	// unless it is a block-like expression or we are inside macro call.
	let veto_multiline = (!allow_multi_line(expr) && !context.inside_macro())
	|| context.config.force_multiline_blocks();
	expr.rewrite(context, shape)
	.and_then(|rw| {
	if veto_multiline && rw.contains('\n') {
	None
	} else {
	Some(rw)
	}
	})
	.map(|rw| format!("{} {}", prefix, rw))
	}

[calebcartwright]

I wouldn't recommend anyone attempt to work on this just yet. This seems like a rather bespoke formatting preference and I'm not sure if it warrants long term maintenance and support from us. Additionally, it's the type of thing we'd like to see if the Style Team would have any vehement objections to rustfmt enabling

[ytmimi]

@calebcartwright I was also thinking the style team would want to weigh in on this. I just wasn't sure if I should tag them in this issue.