Unable to format chained method call with long generic argument list

[MingweiSamuel]

This does not get formatted:

fn main() {
    x.f::<
        A, A, A, A, A, A, A, A, A, A, A, A, A, A, A, A, A, A,
                        A, A, A, A, A, A, A, A, A, A, A, A,
    >();
}

It also happens for fewer number of longer type arguments:

fn main() {
    x.f::<
        AAAA<BBBB::CCCC, (DDDD, EEEE), FFFF>,
            AAAA<BBBB::CCCC, (DDDD, EEEE), FFFF>,
        AAAA<BBBB::CCCC, (DDDD, EEEE), FFFF>
        >();
}

[ytmimi]

Thank you for the report!

Right now I'm not sure what the underlying issue is, but I'll be sure to share my thoughts when I've had a chance to investigate further.

[ytmimi]

@rustbot claim

[ytmimi]

Alright, I did some digging last night and have the PR to fix this mostly ready (just need to add some additional tests).

When you get a chance could you update the title of the issue to be Unable to format chained method call with long generic argument list or something along those lines. Just want to note that this issue is hitting a code path for chains and not function calls specifically.

The main issue we're running up against in the examples you've provided is that rustfmt is always trying to format the generic arguments horizontally. The issue can be tracked down to line 253 of ChainItem::rewrite_method_call

rustfmt/src/chains.rs

Lines 237 to 258 in 272fb42

	fn rewrite_method_call(
	method_name: symbol::Ident,
	types: &[ast::GenericArg],
	args: &[ptr::P<ast::Expr>],
	span: Span,
	context: &RewriteContext<'_>,
	shape: Shape,
	) -> Option<String> {
	let type_str = if types.is_empty() {
	String::new()
	} else {
	let type_list = types
	.iter()
	.map(|ty| ty.rewrite(context, shape))
	.collect::<Option<Vec<_>>>()?;
	format!("::<{}>", type_list.join(", "))
	};
	let callee_str = format!(".{}{}", rewrite_ident(context, method_name), type_str);
	rewrite_call(context, &callee_str, &args[1..], span, shape)
	}
	}

The problem we run into is that we exceed the max_width when trying to write the generic arguments list horizontally and that's why rustfmt gives up on formatting the code.