The v2 macros generate code that directly uses the crate serde_json. The problem is that the user code might not have this crate as a dependency.

So, they might get confusing errors (about a missing serde_json crate when there is no instance of serde_json string in their project), and in order for the generated code to compile, they have to add it as a dependency which may be picked up as "unused" by linting tools.

From what I gathered writing some proc-macro crates, the good practice is to depend yourself on the crate, re-export the symbols you need and reference it from your own namespace.