error: in expressions, `_` can only be used on the left-hand side of an assignment
  --> $DIR/underscore.rs:6:9
   |
LL |         _
   |         ^ `_` not allowed here
   |
  ::: $DIR/main.rs:5:5
   |
LL |     underscore!();
   |     ------------- in this macro invocation
   |
   = note: this error originates in the macro `underscore` (in Nightly builds, run with -Z macro-backtrace for more info)

error: aborting due to 1 previous error