Chain comments after `.` prevent chain from being formatted because comment would be lost

[ytmimi]

If a comment follows the . in a chain, then the chain is not formatted because a comment would be lost. This behavior can be seen when running rustfmt with error_on_unformatted=true. However the chain is rewritten if the comment comes before the .

tested using rustfmt 1.5.1-nightly (ef911542 2022-09-19).

Input

fn not_formatted() {
    let a = b./*cc*/       ddd;
    let e = f.// gg
hhh;
}

fn formated() {
    let i = j/*kk*/       .lll;
    let m = n// oo
.ppp;
}

Output

fn not_formatted() {
    let a = b./*cc*/       ddd;
    let e = f.// gg
hhh;
}

fn formated() {
    let i = j /*kk*/
        .lll;
    let m = n // oo
        .ppp;
}

Error Output

error[internal]: not formatted because a comment would be lost
 --> <stdin>:2
  |
2 |     let a = b./*cc*/       ddd;
  |
  = note: set `error_on_unformatted = false` to suppress the warning against comments or string literals

error[internal]: not formatted because a comment would be lost
 --> <stdin>:3
  |
3 |     let e = f.// gg
  |
  = note: set `error_on_unformatted = false` to suppress the warning against comments or string literals

I think the issue is that we don't expect to find comments after the . when rewriting ChainItem, and the examples above specifically highlight the ChainItemKind::StructField match arm:

rustfmt/src/chains.rs

Lines 246 to 271 in ef91154

	impl Rewrite for ChainItem {
	fn rewrite(&self, context: &RewriteContext<'_>, shape: Shape) -> Option<String> {
	let shape = shape.sub_width(self.tries)?;
	let rewrite = match self.kind {
	ChainItemKind::Parent(ref expr) => expr.rewrite(context, shape)?,
	ChainItemKind::MethodCall(ref segment, ref types, ref exprs) => {
	Self::rewrite_method_call(segment.ident, types, exprs, self.span, context, shape)?
	}
	ChainItemKind::StructField(ident) => format!(".{}", rewrite_ident(context, ident)),
	ChainItemKind::TupleField(ident, nested) => format!(
	"{}.{}",
	if nested && context.config.version() == Version::One {
	" "
	} else {
	""
	},
	rewrite_ident(context, ident)
	),
	ChainItemKind::Await => ".await".to_owned(),
	ChainItemKind::Comment(ref comment, _) => {
	rewrite_comment(comment, false, shape, context.config)?
	}
	};
	Some(format!("{}{}", rewrite, "?".repeat(self.tries)))
	}
	}