Rustfmt ate a single line comment in a long list of trait bounds

[Lokathor]

I ran a rustfmt cleanup as a commit on a repo, found this in the diff.

Lokathor/randomize@103992e#diff-b316dc9cf67a100913b39ef102078235L48

Particularly:

-    + Into<u128> // Note(Evrey): Because Rust is drunk.
+    + Into<u128>

Now, in this particular case the comment was just a joke, but it could have been very important, and it might have even applied to that particular line in the chain of bounds, so the comment should stay in, and it should stay with that exact bound in the bound list.

[nagisa]

It will eat as many comments as you have, only leaving one if it appeared after, I think, the last bound.

[ytmimi]

I believe the issue is that we don't handle comments between binary operators

[danjl1100]

I ran into this issue, and saw the link in the original post isn't working, so adding a playground link of what I'm seeing.

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=c43f042fc662356658bc4a662a1dc566

 trait Foo {}

-trait Bar<T> //: Foo
+trait Bar<T>
 where
     T: Copy,
 {
 }

The comment is deleted by rustfmt (1.5.2-nightly (2023-06-09 43062c4)) on the playground.

[calebcartwright]

@fee1-dead - flagging this as another one for you in case it's something you'd be interested in, if/when you have bandwidth

I will also link a few issues/PRs that likely bare some relevance/overlap: #4666, #5059 and #2055

[connortsui20]

Is this a good first issue to try?

I'd like to try and take a stab at this. It seems like a lot of people have attempted this in the past, but none of them have been merged? I'll have to get more familiar with the code base, but I'm willing to learn, and I can base things off of previous attempts.

[ytmimi]

@connortsui20 Thanks for showing interest in this one. No, I wouldn't classify this as a good first issue, but if you're feeling motivated to tackle this and run into issue you can reach out on Zulip