error: unknown `builtin #` construct `foobar`
  --> $DIR/builtin-syntax.rs:4:5
   |
LL |     builtin # foobar();
   |     ^^^^^^^^^^^^^^^^

error: expected identifier after `builtin #`
  --> $DIR/builtin-syntax.rs:8:15
   |
LL |     builtin # {}();
   |               ^

error: aborting due to 2 previous errors