Don't move comments around, or at least move them up instead of down

[RalfJung]

For some reason, rustfmt arbitrarily decides to move the comment around in the following snippet:

fn main() {
    if true { // explanation.
        println!("hello");
    }
}

(I am saying "arbitrarily" as this page has no explanation for why this happens)

This becomes:

fn main() {
    if true {
        // explanation.
        println!("hello");
    }
}

This is bad for 2 reasons:

1. Why even move the comment? Having the explanation for the conditional on the same line as the conditional is perfectly fine, very readable and can help save some vertical space.
2. If rustfmt for some reason must move the comment, it should be moved up. The comment was explaining the if true, and a comment explaining something should not come after the thing already happened! Currently, rustfmt actively degrades the quality of such comments. The reformatted code looks like the comment is explaining the println!.

I am very worried now how many such comments that were in the compiler are wrong / confusing now because of this. :/

[calebcartwright]

Agreed, though I think this is a duplicate of #3255.

I think rustfmt is assuming the same-line comment after the opening { is associated to the first statement in the block, instead of the control flow expression condition, which is why the comment is moved and indented with that first statement

[RalfJung]

Yes, this is a duplicate of #3255 indeed.

[RalfJung]

Closing as duplicate of #3255.