Fmt failure with wrap_comments = true

When having wrap_comments = true, running rustfmt on the following code produces an error:

fn f() {
    g({
        // some trailing spaces on the line below - this needs to be inside the g call
    
        SomeStruct {
            a: b, // // This needs two sets of the slash-slash comments, and this needs to be a very long sentence so bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla
        };
    })
    .x; // the .something needs to be here
}

The produces error is error[internal]: left behind trailing whitespace

Start by reproducing the reported rustfmt failure with wrap_comments = true and the Rust example in the issue body. Trace the formatting path that handles trailing whitespace, wrapped comments, and the chained .x expression; done means the example formats without the internal trailing-whitespace error while preserving the comments and call structure.