"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.
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