The issue here is that rewrite_struct_pat doesn't have any comment recovery code defined for trailing comments after the ... rustfmt errs on the side of caution here and refuses to rewrite the entire match expression because it realizes that if it went ahead with the rewirte it would remove the // this comment here apparently causes trouble comment.

In order to resolve this one we might consider taking a similar approach to rewrite_struct_lit, which is a similar function, for rewriting structs, but doesn't seem to suffer from this trailing comment problem because rewrite_struct_lit considers the .. as part of the struct fields that it needs to rewrite, while rewrite_struct_pat sort of bolts on the .. handling after rewriting the struct fields. @calebcartwright I'd appreciate getting your take on this idea.

intput:

fn main() {
  let x =  MyStruct {
            field_a,
            .. // this comment here apparently causes trouble
        };
}

output:

fn main() {
    let x = MyStruct {
        field_a,
        .. // this comment here apparently causes trouble
    };
}