Derive macros define new inputs for the derive attribute. These macros can create new items given the token stream of a struct, enum, or union. They can also define derive macro helper attributes.
Custom derive macros are defined by a public function with the proc_macro_derive attribute and a signature of (TokenStream) -> TokenStream.
The input TokenStream is the token stream of the item that has the derive attribute on it. The output TokenStream must be a set of items that are then appended to the module or block that the item from the input TokenStream is in.
