Macro proc_macro_item_impl 

macro_rules! proc_macro_item_impl {
    ($(
        $( #[$attr:meta] )*
        pub fn $func:ident($input:ident: &str) -> String $body:block
    )+) => { ... };
}

Implement a hacky procedural macro that expands to items.

proc_macro_item_impl! {
    /// A function that always returns 2.
    pub fn two_fn_impl(input: &str) -> String {
        format!("fn {}() -> u8 {{ 2 }}", input)
    }
}