[xigoi]

But there's the weird limitation that procedural macros have to be in a separate crate.

[efaref]

Why is that weird? Procedural macros are compiler plugins. They get compiled for the platform you're building on, not the one you're building for, and so they need to be a separate compilation unit. In Rust, the crate is the compilation unit.

[xigoi]

Because you can't just throw together a simple procedural macro to use in a specific project, as you can in other languages.