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//! A simple macro library for creating phantom enums. Just simple sugar.
//!
//! See the `phantom_enum!` macro for all the details.

/// Create a phantom type enum.
///
/// A phantom type enum is an enum-like arrangement where the enum is a module
/// and trait and the variants are (uninstantiable) types.
///
/// This is very good for the static representation of state machines in which
/// *nothing* can go wrong.
///
/// ```rust
/// #[macro_use] #[no_link]
/// extern crate phantom_enum;
///
/// phantom_enum! {
///     /// Put things here, of course
///     pub enum TableItem {
///         /// A bottle with a paper label reading “DRINK ME”.
///         Potion,
///         /// A cake with the words “EAT ME” marked in currants.
///         Cake,
///     }
/// }
///
/// // Note how this restricts the methods to only meaningful types.
/// struct Person<T: TableItem::Impl> {
///     name: &'static str,
///     phantom: std::marker::PhantomData<T>,
/// }
///
/// impl<T: TableItem::Impl> Person<T> {
///     fn new(name: &'static str) -> Person<T> {
///         Person {
///             name: name,
///             phantom: std::marker::PhantomData,
///         }
///     }
/// }
///
/// impl Person<TableItem::Potion> {
///     fn drink_it(self) -> Person<TableItem::Cake> {
///         println!("Shrinking! Oh look, there’s a box down here!");
///         Person {
///             name: self.name,
///             phantom: std::marker::PhantomData,
///         }
///     }
/// }
///
/// impl Person<TableItem::Cake> {
///     fn eat_it(self) -> () {
///         println!("Growing! OK, that’s enough of the story.");
///         // Who can remember what comes next, anyway?
///     }
/// }
///
/// fn main() {
///     let person = Person::new("Alice");
///     let person = person.drink_it();
///     person.eat_it();
/// }
/// ```
///
/// As you will observe with this example, if you have a `Person<Potion>`, you
/// simply cannot call `.eat_it()`; for that, you must have a `Person<Cake>`.
/// Similarly, once you have drunk that potion, you can’t drink it again.
#[macro_export]
macro_rules! phantom_enum {
    (
        $(#[$enum_attr:meta])*
        pub enum $name:ident {
            $(
                $(#[$variant_attr:meta])+
                $variant:ident
             ),*$(,)*
        }
    ) => {
        $(#[$enum_attr])*
        #[allow(non_snake_case)]
        pub mod $name {
            /// Implemented exclusively by members of this phantom type enum.
            /// This is for use as a generic bound.
            pub trait Impl { }

            $(
                $(#[$variant_attr])+
                #[derive(Copy, Clone)]
                pub enum $variant { }
                impl Impl for $variant { }
            )*
        }
    };

    (
        $(#[$enum_attr:meta])*
        enum $name:ident {
            $(
                $(#[$variant_attr:meta])+
                $variant:ident
             ),*$(,)*
        }
    ) => {
        $(#[$enum_attr])*
        #[allow(non_snake_case)]
        mod $name {
            /// Implemented exclusively by members of this phantom type enum.
            /// This is for use as a generic bound.
            pub trait Impl { }

            $(
                $(#[$variant_attr])+
                #[derive(Copy, Clone)]
                pub enum $variant { }
                impl Impl for $variant { }
            )*
        }
    }
}