//! Higher-ranked traits and types.
//!
//! A higher-ranked type is a type that needs a lifetime to be "complete". Consider the following
//! code.
//! ```rs
//! type IntBorrow<'a> = &'a i32;
//! ```
//! What if we could pass just `IntBorrow` around as a type; without it's lifetime.
//! In Rust syntax, this type would be `for<'a> IntBorrow<'a>`. `'a` can be filled in with any
//! lifetime. Sadly, Rust only allows `for<'a>` in trait objects. However, trait object provide
//! enough for us to implement arbitrary hifher-ranked types with some extra effort.
//!
//! In this module the [`bijective_higher_ranked_type!`] macro generates a type that needs a lifetime to
//! be complete. Before a higher-ranked type can be generated we need a higher-ranked trait to
//! describe what behavior the resulting "true" type will have.
//!
//! The [`bijective_higher_ranked_trait!`] macro cgenerates a trait that can be used with
//! [`bijective_higher_ranked_type!`] and as a bound by code wanting to accept a higher ranked type.
//! Bounding a generic by a higher-ranked trait simultaniously allows any higher-ranked type with
//! a "true" type with the required traits and any lifetime to be given to that "true" type.

use core::marker::PhantomData;

#[derive(Debug, Default, Copy, Clone)]
#[repr(transparent)]
pub struct Invariant<'a>(PhantomData<fn(&'a ()) -> &'a ()>);

#[repr(transparent)]
pub struct Marker<T: ?Sized>(PhantomData<fn() -> *const T>);

impl<T: ?Sized> Copy for Marker<T> {}
impl<T: ?Sized> Clone for Marker<T> {
    fn clone(&self) -> Self {
        *self
    }
}

impl<T: ?Sized> core::fmt::Debug for Marker<T> {
    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
        f.debug_tuple("Marker").finish()
    }
}

impl<T: ?Sized> Default for Marker<T> {
    fn default() -> Self {
        Self(PhantomData)
    }
}

#[doc(hidden)]
#[macro_export]
macro_rules! higher_ranked_trait {
    {
        $vis:vis type class $name:ident$([$($generic:tt)*])? for<$($lt:lifetime),+> <$($for_lt:lifetime),+> {
            type Bound = $bound:ty;

            type T: {$($lower:tt)*} $(where {$($lower_where:tt)*})?;

            type HigherRanked: {$($higher:tt)*} $(where {$($higher_where:tt)*})?;
        }
    } => {
        $vis mod $name {
            #![allow(non_snake_case)]

            #[allow(unused)]
            use super::*;

            pub trait LowerForLt<$($lt,)+ $($($generic)*,)? B>
            where
                $($($lower_where)*)?
                $($($higher_where)*)?
            {
                type T:
                    RaiseForLt<$($lt,)+ $($($generic)*,)? B, HigherRanked = Self>
                    + for<$($for_lt),+> RaiseForLt<$($for_lt,)+ $($($generic)*,)? Bound<$($for_lt,)+ $($($generic)*)?>>
                    + ?Sized + $($lower)*;
            }

            pub trait RaiseForLt<$($lt,)+ $($($generic)*,)? B>
            where
                $($($lower_where)*)?
                $($($higher_where)*)?
            {
                type HigherRanked:
                    LowerForLt<$($lt,)+ $($($generic)*,)? B, T = Self>
                    + for<$($for_lt),+> LowerForLt<$($for_lt,)+ $($($generic)*,)? Bound<$($for_lt,)+ $($($generic)*)?>>
                    + ?Sized + $($higher)*;
            }

            pub type Bound<$($lt,)+ $($($generic)*)?> = $bound;

            pub trait MemberType$(<$($generic)*>)?:
                for<$($lt,)+> LowerForLt<$($lt,)+ $($($generic)*,)? Bound<$($lt,)+ $($($generic)*)?>> + $($higher)*
            $(where $($higher_where)*)?
            {}

            impl<$($($generic)*,)? __: ?Sized> MemberType$(<$($generic)*>)? for __
            where
                __: for<$($lt,)+> LowerForLt<$($lt,)+ $($($generic)*,)? Bound<$($lt,)+ $($($generic)*)?>> + $($higher)?,
                $($($higher_where)*)?
            {}

            pub trait LowerType<$($lt,)+ $($($generic)*)?>:
                RaiseForLt<$($lt,)+ $($($generic)*,)? Bound<$($lt,)+ $($($generic)*)?>> + $($lower)*
            where
                $($($lower_where)*)?
                $($($higher_where)*)?
            {}

            impl<$($lt,)+ $($($generic)*,)? __: ?Sized> LowerType<$($lt,)+ $($($generic)*)?> for __
            where
                __: RaiseForLt<$($lt,)+ $($($generic)*,)? Bound<$($lt,)+ $($($generic)*)?>> + $($lower)*,
                $($($lower_where)*)?
                $($($higher_where)*)?
            {}

            pub type T<$($lt,)+ $($($generic)*,)? __> =
                <__ as LowerForLt<$($lt,)+ $($($generic)*,)? Bound<$($lt,)+ $($($generic)*)?>>>::T;

            pub type HigherRanked<$($lt,)+ $($($generic)*,)? __> =
                <__ as RaiseForLt<$($lt,)+ $($($generic)*,)? Bound<$($lt,)+ $($($generic)*)?>>>::HigherRanked;
            //
            // pub type HigherRankedAll<$($lt,)+ $($($generic)*,)? __> =
            //     <__ as RaiseForLtAll<$($($generic)*,)? Bound<$($lt,)+ $($($generic)*)?>>>::HigherRankedAll;
        }
    }
}
pub use higher_ranked_trait;

#[doc(hidden)]
#[macro_export]
macro_rules! higher_ranked_type {
    {
        impl $higher_trait:ident {
            impl$([$($lower_generic:tt)*])? type T[$($lower_forward:tt)*] for $lower_higher:ty =
                $lower:ty
            $(where {$($lower_where:tt)*})?;

            impl$([$($higher_generic:tt)*])? type HigherRanked[$($higher_lt:lifetime),+]$([$($higher_forward:tt)*])? for $higher_lower:ty =
                $higher:ty
            $(where {$($higher_where:tt)*})?;
            //
            // impl$([$($higher_all_generic:tt)*])? type HigherRankedAll[$($higher_all_lt:lifetime),+]$([$($higher_all_forward:tt)*])? for $higher_all_lower:ty =
            //     $higher_all:ty
            // $(where {$($higher_all_where:tt)*})?;
        }
    } => {
        impl$(<$($lower_generic)*>)* $higher_trait::LowerForLt<$($lower_forward)*, $higher_trait::Bound<$($lower_forward)*>> for $lower_higher
        $(where $($lower_where)*)?
        {
            type T = $lower;
        }

        impl$(<$($higher_generic)*>)* $higher_trait::RaiseForLt<$($higher_lt,)+ $($($higher_forward)*,)? $higher_trait::Bound<$($higher_lt,)+ $($($higher_forward)*)?>> for $higher_lower
        $(where $($higher_where)*)?
        {
            type HigherRanked = $higher;
        }
        //
        // impl$(<$($higher_all_generic)*>)* $higher_trait::RaiseForLtAll<$($($higher_all_forward)*,)? $higher_trait::Bound<$($higher_all_lt,)+ $($($higher_all_forward)*)?>> for $higher_all_lower
        // $(where $($higher_all_where)*)?
        // {
        //     type HigherRankedAll = $higher_all;
        // }
    }
}
pub use higher_ranked_type;

// #[cfg(test)]
// mod test {
//     use super::*;
//
//     /// Some trait with two lifetimes and a generic.
//     trait Demo<'lt, 'ctx, T: Send + 'lt> {
//         fn add(&self, x: &'lt &'ctx i32) -> i32;
//     }
//
//     impl<'lt, 'ctx, T: Send + 'lt> Demo<'lt, 'ctx, T> for i32 {
//         fn add(&self, x: &'lt &'ctx i32) -> i32 {
//             self + **x
//         }
//     }
//
//     impl<'lt, 'ctx, T: Send + 'lt> Demo<'lt, 'ctx, T> for &'lt (dyn Demo<'lt, 'ctx, T> + 'lt) {
//         fn add(&self, x: &'lt &'ctx i32) -> i32 {
//             (**self).add(x)
//         }
//     }
//
//     // Higher-ranked trait that requires the "true" type to implement Demo.
//     bijective_higher_ranked_trait! {
//         type class Any['ctx, T]: [for<'lt> Demo<'lt, 'ctx, T>]
//         where {
//             T: Send,
//         }
//         for where {
//             T: 'lt
//         }
//     }
//
//     // Higher-ranked type with a "true" type of a borrow of a trait object.
//     // The complex part to support here is the `+ 'lt`.
//     // This entire module is specialized to support this usecase because that is what treaty needs
//     // for it's API.
//     higher_ranked_type! {
//         type X['ctx, T]: (Any)[T]
//         where {
//             T: Send,
//         } = for<'lt> &'lt (dyn Demo<'lt, 'ctx, T> + 'lt)
//     }
//
//     // We bound the generic T by the higher-ranked trait Any.
//     //
//     // We then inject a 'lt into the higher-ranked type to get a "true" type.
//     fn demo<'lt, 'ctx, T: Any::Trait<'ctx, i32>>(x: Any::T<'lt, 'ctx, T, i32>, y: &'lt &'ctx i32) {
//         assert_eq!(x.add(y), 10);
//     }
//
//     #[test]
//     fn can_use_hkt() {
//         let ctx = 2;
//         let ctx = &ctx;
//
//         {
//             // The lifetimes here can't be 'static because local isn't static even though z could be.
//             let local = &ctx;
//             let z: &dyn Demo<'_, '_, i32> = &8i32;
//             demo::<X<'_, i32>>(z, local);
//         }
//     }
// }