rustfmt deletes comment at start of array length expression

[ctz]

I write this code:

struct Abc([u8;
// five
5 +
// ten
10 +
// thirteen
13
]);

rustfmt formats this to:

struct Abc(
    [u8; 5 +
// ten
10 +
// thirteen
13],
);

It has lost the // five comment. The remainder of the formatting is not great.

playground

version:

rustfmt 1.8.0-stable (90b35a6239 2024-11-26)

[ctz]

A workaround is to surround the expression in const { .. }

edit: although a comment before the const is still removed:

struct Abc(
    [u8;
    // blorp
    const {
        // five
        5 +
// ten
10 +
// thirteen
13
    }],
);

is formatted to:

struct Abc(
    [u8; const {
        // five
        5 +
// ten
10 +
// thirteen
13
    }],
);

[ytmimi]

Thanks for the report. This is related to #5297, except the error occurs in expression rewriting:

rustfmt/src/expr.rs

Lines 284 to 291 in a43eef1

	ast::ExprKind::Repeat(ref expr, ref repeats) => rewrite_pair(
	&**expr,
	&*repeats.value,
	PairParts::new("[", "; ", "]"),
	context,
	shape,
	SeparatorPlace::Back,
	),