error: macro expansion ends with an incomplete expression: expected expression
  --> $DIR/macro-in-expression-context-2.rs:5:16
   |
LL | macro_rules! empty { () => () }
   |                            -- in this macro arm
...
LL |         _ => { empty!() }
   |                ^^^^^^^^ expected expression
   |
   = note: the macro call doesn't expand to an expression, but it can expand to a statement
help: add `;` to interpret the expansion as a statement
   |
LL |         _ => { empty!(); }
   |                        +

error: aborting due to previous error