Shared via Rust Playground

playground.rs

struct Red;
struct Green;

// Any trait allows dynamic type checks
use std::any::{Any, TypeId};
use std::collections::HashMap;

// Every Feature also implements Any trait
trait Feature : Any {
}

impl Feature for Red {}
impl Feature for Green {}

type FeatureHash = HashMap<TypeId, Box<Any>>;

trait FeatureFinder {
    fn add<T: Feature>(&mut self, feature: T);
    fn find<T: Feature>(&self) -> Option<&T>;
}

impl FeatureFinder for FeatureHash {
    fn add<T: Feature>(&mut self, feature: T) {
        self.insert(TypeId::of::<T>(), Box::new(feature));
    }
    fn find<T: Feature>(&self) -> Option<&T> {
        self.get(&TypeId::of::<T>()).and_then(|f| f.downcast_ref())
    }
}

fn main() {
    let mut features = FeatureHash::new();

    features.add(Red);
    features.add(Green);

    // Note that find() magically works! It picks the type from the variable.
    let red: &Red = features.find().expect("red should be in there");
    let green: &Green = features.find().expect("green should be in there, too");
}