rustfmt removes a block comment in an argument position

[ozkriff]

Input:

fn a(_ /*comment*/: u8) {}
fn b(/*comment*/ _: u8) {}

fn main() {}

Expected output (no changes):

fn a(_ /*comment*/: u8) {}
fn b(/*comment*/ _: u8) {}

fn main() {}

Real output (on of the comments is removed):

fn a(_: u8) {}
fn b(/*comment*/ _: u8) {}

fn main() {}

Replacing _ with normal identifier doesn't help.

I guess it's related to #1096.