error: no rules expected the token `enum E {}`
  --> $DIR/nonterminal-matching.rs:19:10
   |
LL | macro complex_nonterminal($nt_item: item) {
   |                           --------------
LL |     macro n(a $nt_item b) {
   |     --------------------- when calling this macro
...
LL |     n!(a $nt_item b);
   |          ^^^^^^^^ no rules expected this token in macro call
...
LL | complex_nonterminal!(enum E {});
   | ------------------------------- in this macro invocation
   |
note: while trying to match `enum E {}`
  --> $DIR/nonterminal-matching.rs:15:15
   |
LL |     macro n(a $nt_item b) {
   |               ^^^^^^^^
...
LL | complex_nonterminal!(enum E {});
   | ------------------------------- in this macro invocation
   = note: captured metavariables except for `:tt`, `:ident` and `:lifetime` cannot be compared to other tokens
   = note: see <https://doc.rust-lang.org/nightly/reference/macros-by-example.html#forwarding-a-matched-fragment> for more information
   = help: try using `:tt` instead in the macro definition
   = note: this error originates in the macro `complex_nonterminal` (in Nightly builds, run with -Z macro-backtrace for more info)

error: aborting due to 1 previous error