Fixed Issue 'Trailing comma is duplicated when obstructed by comments'

[whizsid]

Fixes #3876 .

[calebcartwright]

This is a slight improvement in that the commas are not duplicated, though the formatting doesn't feel quite right. In particular I'm confused by the indentation/alignment in the first snippet, as well as the position of the , in the second.

I think a big question is whether there's a valid use case for comments before the comma, or whether that's just unintentional i.e.:

fn main() {
    foo(
        x, // comment 1
        // comment 2
    )
}

// or
fn main() {
    foo(
        x,
        // comment 1
        // comment 2
    )
}

although at the moment I suspect rustfmt may want to do this, though that's largely due to a bug in default visual alignment for comments (refs #4108)

fn main() {
    foo(
        x, // comment 1
           // comment 2
    )
}

[whizsid]

Sorry I was away from open source in last months. @calebcartwright Can you review again?

[calebcartwright]

Thanks for this! It's in pretty good shape though think we can optimize it a bit and have also requested some additional tests

[calebcartwright]

Excellent thank you!