error: unexpected end of macro invocation
 --> tests/ui/missing_colon.rs:4:5
  |
4 |     json!({ "a" });
  |     ^^^^^^^^^^^^^^ missing tokens in macro arguments
  |
  = note: this error originates in the macro `json_internal` which comes from the expansion of the macro `json` (in Nightly builds, run with -Z macro-backtrace for more info)