If you ran into trouble while running this recipe, you can use David Tolney's cargo-expand (https://github.com/dtolnay/cargo-expand) to show you how the compiler expanded your proc_macros. It's a really useful tool to debug your macros, so be sure to check it out.
The reason behind the two-crate restriction is historical and only temporary. In the beginning, there was only one way to define macros, macro_rules!. People with exotic needs, who were ready to put in the effort, were (and still are) able to extend their programs by directly hooking into the Rust compiler itself. Crates written this way are called compiler plugins. Of course, this is incredibly unstable because every minor Rust release can break your plugin, but people kept on doing it because it gave them one big advantage, custom derives. The core team reacted to the increased demand for language extensibility by deciding to launch macros2.0 at some point in the future, bringing an overhaul...