Trailing whitespace of match arm not stripped when the body expression is very long and follows in the second line

Minimal reproducer:

fn this_is_a_very_long_name_to_lengthen_the_expression_so_that_rustfmt_does_not_join_the_lines() {}

fn main() {
    match () {
        _ => /* (whitespace here) => */ 
            this_is_a_very_long_name_to_lengthen_the_expression_so_that_rustfmt_does_not_join_the_lines(),
    }
}

Output:

error[internal]: left behind trailing whitespace
 --> stdin:5:5:40
  |
5 |         _ => /* (whitespace here) => */ 
  |                                        ^
  |

warning: rustfmt has failed to format. See previous 1 errors.

Start by running the minimal Rust reproducer through rustfmt and confirm the trailing-whitespace diagnostic at stdin:5. Trace the match-arm formatting path responsible for the second-line body expression, then verify that formatting no longer leaves whitespace after the comment marker and that the reproducer completes without the internal error.