Macro proc_macro_item_decl 

macro_rules! proc_macro_item_decl {
    (#[$attr:meta] $($rest:tt)+) => { ... };
    ($name:ident ! => $name_impl:ident) => { ... };
}

Declare a hacky procedural macro that expands to items.

proc_macro_item_decl! {
    /// A function that always returns 2.
    two_fn! => two_fn_impl
}