Macro proc_macro_expr_decl 

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

Declare a hacky procedural macro that expands to an expression.

proc_macro_expr_decl! {
    /// Add one to an expression.
    add_one! => add_one_impl
}