Shared via Rust Playground

playground.rs

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

trait Lookup<K> {
    type V;
    fn get(&self) -> &Self::V;
}

struct Nil;

struct Cons<K, V, T> {
    pd: PhantomData<K>,
    value: V,
    tail: T
}

impl<K, V, T> Cons<K, V, T> {
    fn push<K2, V2>(self, value: V2) -> Cons<K2, V2, Self> {
        Cons {
            pd: PhantomData,
            value: value,
            tail: self
        }
    }
}

impl Nil {
    fn push<K, V>(self, value: V) -> Cons<K, V, Self> {
        Cons {
            pd: PhantomData,
            value: value,
            tail: self
        }
    }
}

trait NotSame {}
impl NotSame for .. {}
impl<T> !NotSame for (T, T) {}

impl<K1, K2, V, T> Lookup<K1> for Cons<K2, V, T> 
where T: Lookup<K1>, (K1, K2): NotSame 
{
    type V = <T as Lookup<K1>>::V;
    fn get(&self) -> &Self::V { self.tail.get() }
}

impl<K, V, T> Lookup<K> for Cons<K, V, T> {
    type V = V;
    fn get(&self) -> &V { &self.value }
}

trait DoLookup {
    fn lookup<K>(&self) -> &<Self as Lookup<K>>::V where Self: Lookup<K>;
}

impl<T> DoLookup for T {
    fn lookup<K>(&self) -> &<Self as Lookup<K>>::V where Self: Lookup<K> {
        <Self as Lookup<K>>::get(&self)
    }
}

fn main() {
    struct Key1; struct Key2; struct Key3;
    let table = Nil.push::<Key1, _>(15).push::<Key2, _>(true);
    let value = table.lookup::<Key1>();
    println!("{:?}", value);
}