std::writeln!

( $ dst : expr , $ fmt : expr ) => (
write ! ( $ dst , concat ! ( $ fmt , "\n" ) ) ) ; (
$ dst : expr , $ fmt : expr , $ ( $ arg : tt ) * ) => (
write ! ( $ dst , concat ! ( $ fmt , "\n" ) , $ ( $ arg ) * ) ) ;

Equivalent to the write! macro, except that a newline is appended after the message is written.