Shared via Rust Playground

playground.rs

#![feature(specialization)]
use std::marker::PhantomData;

#[derive(Clone, Copy)]
struct Key<K, V> {
    _k: PhantomData<K>,
    _v: PhantomData<V>,
}

trait KeyType<N> {
    fn is_value() -> bool {
        false
    }
}

trait SameType<A, B> {}

impl <T> SameType<T, T> for T {}

impl Key<(), ()> {
    pub fn new() -> Key<(), ()> {
        Key {
            _k: PhantomData,
            _v: PhantomData,
        }
    }
}

impl <T, A, B> KeyType<T> for Key<A, B> where A: KeyType<T> {
    default fn is_value() -> bool {
        A::is_value()
    }
}

impl <T, A, B> KeyType<T> for Key<A, B> where A: KeyType<T>, T: SameType<T, B> {
    fn is_value() -> bool {
        true
    }
}

impl <T> KeyType<T> for () {
    fn is_value() -> bool {
        false
    }
}

impl <K, V> Key<K, V> {
    pub fn with<N>(self) -> Key<Key<K, V>, N> {
        Key {
            _k: PhantomData,
            _v: PhantomData,
        }
    }
    pub fn has<N>(self) -> bool where K: KeyType<N>  {
        <Self as KeyType<N>>::is_value()
    }
}


fn main() {
    let key = Key::new()
        .with::<i32>()
        .with::<u8>();
        
    println!("{}", key.has::<bool>());
    println!("{}", key.has::<i32>());
    println!("{}", key.has::<i8>());
    println!("{}", key.has::<u8>());
}