error: no rules expected `"2"`
 --> tests/ui/missing_comma.rs:4:21
  |
4 |     json!({ "1": "" "2": "" });
  |                    -^^^ no rules expected this token in macro call
  |                    |
  |                    help: missing comma here
  |
note: while trying to match `,`
 --> src/macros.rs
  |
  |     ($e:expr , $($tt:tt)*) => {};
  |              ^