error: no rules expected the token `_`
  --> $DIR/underscore-ident-matcher.rs:8:19
   |
LL | macro_rules! identity {
   | --------------------- when calling this macro
...
LL |     let identity!(_) = 10;
   |                   ^ no rules expected this token in macro call
   |
note: while trying to match meta-variable `$i:ident`
  --> $DIR/underscore-ident-matcher.rs:2:6
   |
LL |     ($i: ident) => (
   |      ^^^^^^^^^

error: aborting due to previous error