Inner comments in binop expressions prevents formatting

[wooster0]

fn                                                      main() {
    let   x    =    512_u16;
    let   byte    =  (  x  % 10   )    as  u8  /* make it printable */   +    b'0'        ;
    dbg!(      char::from(              
          byte));
}

is formatted to

fn main() {
    let x = 512_u16;
    let byte = (  x  % 10   )    as  u8  /* make it printable */   +    b'0';
    dbg!(char::from(byte));
}

but I expected

fn main() {
    let x = 512_u16;
    let byte = (x % 10) as u8 /* make it printable */ + b'0';
    dbg!(char::from(byte));
}

[ytmimi]

Thanks for the report!

Could you please include the rustfmt version as well as any non default configuration options you might be using?
Although, I don't think the issue has to do with configuration options it would still be nice to know.

If I had to guess, I'd say that rustfmt isn't explicitly checking to see if a comment exists in that location and for that reason it bails on formatting that line because doing so would drop the comment.

[wooster0]

This is rustfmt 1.4.37-stable (59eed8a 2021-11-01) with no rustfmt configurations changed.

it bails on formatting that line because doing so would drop the comment

Good point, for that I've opened another issue: #5100

[calebcartwright]

rustfmt's inability to handle nested comments contained within a binary expression is a known, and long standing issue. It's also one that's unlikely to be resolved any time soon due the inherent complexity, especially given the tendency for chaining. See #4417 for a previous attempt at support this.

I'd suggest that in general with bin expressions that readability/clarity will be improved with inline comments leading/placed above the expression as opposed to trying to embed comments within, but for those that find themselves absolutely needing embedded comments, just be aware/prepared for no reformatting support of those expressions.

[ytmimi]

Closing as this is a duplicate of #3167 and #3591