## Procedural Macros

"Procedural macros" are the second way to implement a macro. For now, the only thing they can be used for is to implement derive on your own types. See the book for a tutorial.

Procedural macros involve a few different parts of the language and its standard libraries. First is the proc_macro crate, included with Rust, that defines an interface for building a procedural macro. The #[proc_macro_derive(Foo)] attribute is used to mark the deriving function. This function must have the type signature:

use proc_macro::TokenStream;

#[proc_macro_derive(Hello)]
pub fn hello_world(input: TokenStream) -> TokenStream


Finally, procedural macros must be in their own crate, with the proc-macro crate type.