Formatting does not work with match arm having mixed OR clauses and comments

[snprajwal]

Related to #6060 and #6044. For the below snippet, rustfmt does nothing at all:

enum Foo {
    A,
    B,
    C,
    D,
}

fn main() {
    let foo = Foo::A;
    match foo {
        // Here is a comment
Foo::A |
    // And another one
        Foo::B |
        // Hey look! There's another one
    Foo::C |
// That's a lot of comments
        Foo::D => {}
    }
}

If someone more knowledgeable about rustfmt internals can please confirm that this is a solvable bug, I'd be happy to contribute the fix.

[ytmimi]

When you try formatting this with error_on_unformatted=true configured, you'll get the following error message:

error[internal]: not formatted because a comment would be lost
  --> <stdin>:10
   |
10 |     match foo {
   |
   = note: set `error_on_unformatted = false` to suppress the warning against comments or string literals

rustfmt isn't expecting to find comments within the OR pattern and would remove them if the code was formatted.

I'm not sure how simple this would be to solve. We usually use itemize_list to handle formatting lists of things internally, which takes care of rewriting comments as well.

It might be as simple as defining items (line 100) using itemize_list instead of mapping over the pat_strs, but I haven't looked into this.

rustfmt/src/patterns.rs

Lines 90 to 117 in c6c8159

	PatKind::Or(ref pats) => {
	let pat_strs = pats
	.iter()
	.map(|p| p.rewrite_result(context, shape))
	.collect::<Result<Vec<_>, RewriteError>>()?;
	let use_mixed_layout = pats
	.iter()
	.zip(pat_strs.iter())
	.all(|(pat, pat_str)| is_short_pattern(pat, pat_str));
	let items: Vec<_> = pat_strs.into_iter().map(ListItem::from_str).collect();
	let tactic = if use_mixed_layout {
	DefinitiveListTactic::Mixed
	} else {
	definitive_tactic(
	&items,
	ListTactic::HorizontalVertical,
	Separator::VerticalBar,
	shape.width,
	)
	};
	let fmt = ListFormatting::new(shape, context.config)
	.tactic(tactic)
	.separator(" |")
	.separator_place(context.config.binop_separator())
	.ends_with_newline(false);
	write_list(&items, &fmt)
	}

It's also likely that any change here would need to be gated.

If you're interested in working on this one you can comment @rustbot claim to assign yourself.

[snprajwal]

@rustbot claim