trailing comma after function call chains may need a comment barrier

[ratmice]

When given a function call chain, and a comment rustfmt will move the comma above the comment,
which cause

fn main() {
    ().foo(
        ().foo
            .foo()
            .foo()
            .foo()
            .foo()
            .foo()
            .foo()
            .foo()
            .foo()
            .foo()
            .foo()
            //.foo()
            ,
    )
}

afterwords the last line looks like:
.foo(), //.foo()

Which moves the comment from the first parameter, to the second (nonexistent!) parameter if one were to uncomment it,
This makes the addition of a comment, followed by rustfmt, followed by the removal of a comment syntactically invalid which can be inconvenient.

[ytmimi]

Thanks for submitting the report!

I can definitely see why this would be inconvenient.

Comments aren't explicitly represented in the AST, which rustfmt uses to rewrite your code, so rustfmt isn't aware of the trailing comment when it goes to rewrite the chain.

In most cases the current behavior is probably what you want (where the comma comes before the comment). Without taking a deeper dive into the code it's hard for me to say whether this would be easy to address.