comment inside of empty enum variant payload with explicit discriminant results in invalid syntax after rustfmt run

[LizzyFleckenstein03]

Example code:

#[repr(u8)]
enum MyEnum {
    MyVariant {
        /* Comment */
    } = 0,
}

This compiles fine.

rustfmt 1.5.2-nightly (50d3ba5 2023-02-04) inserts a line:

#[repr(u8)]
enum MyEnum {
    MyVariant {
        /* Comment */
        } =
    } = 0,
}

This is a syntax error:

error: unexpected closing delimiter: `}`
 --> src/main.rs:7:1
  |
3 |     MyVariant {
  |               - this delimiter might not be properly closed...
4 |         /* Comment */
5 |         } =
  |         - ...as it matches this but it has different indentation
6 |     } = 0,
7 | }
  | ^ unexpected closing delimiter

[calebcartwright]

Thank you for the report!

For anyone interested in working on this, these types of bugs are almost always caused by an incorrect span derivation that only becomes problematic in the presence of comments. In this case it's an empty struct so I'd suggest taking a look in this general vicinity:

rustfmt/src/items.rs

Lines 1328 to 1332 in 5391847

	if fields.is_empty() {
	let inner_span = mk_sp(body_lo, span.hi() - BytePos(1));
	format_empty_struct_or_tuple(context, inner_span, offset, &mut result, "", "}");
	return Some(result);
	}

I suspect that the issue is with the construction of that inner span, and the hi value used (the second arg to the mk_sp call) should instead be the hi bytepos from before the last }

[Swiiz]

Same problem with comments in struct generics:

Example code:

pub trait SystemFn<P /* COMMENT  */> {
    fn run(&self);
}

This compiles fine.
But rustfmt change the code to this:

pub trait SystemFn<P /* COMMENT */> /* COMMENT  */> {
    fn run(&self);
}

which doesn't work.

This might be related

[ytmimi]

@Swiiz thanks for reaching out! That's definitely a bug.

Could you please open up a new issue for that case. Although the result is similar, the error is occurring in a different context (struct generics vs enum explicit discriminant) and therefore is a distinct issue.

[ytmimi]

@Swiiz following up here. The issue that you pointed out is a duplicate of #5358, and the fix was released in rustfmt v1.5.2.