Procedural macros allow creating syntax extensions as execution of a function. Procedural macros can be used to implement custom 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.