error: expected identifier or string literal
  --> $DIR/empty-input.rs:6:14
   |
LL |     () => { ${concat()} }
   |              ^^^^^^^^^^

error: expected expression, found `$`
  --> $DIR/empty-input.rs:6:13
   |
LL |     () => { ${concat()} }
   |             ^ expected expression
...
LL |     let x = empty!();
   |             -------- in this macro invocation
   |
   = note: this error originates in the macro `empty` (in Nightly builds, run with -Z macro-backtrace for more info)

error: aborting due to 2 previous errors