## 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.