type erasure in rust

main.rs

trait AnimalExt {
    fn eat(&self, food: String);
}

struct Dog;

impl AnimalExt for Dog {
    fn eat(&self, food: String) {
        println!("dog: {:?}", food);
    }
}

struct AnyAnimal {
    eat: Box<Fn(String)>,
}

impl AnyAnimal {
    fn new<A>(animal: A) -> Self 
    where
        A: AnimalExt + 'static,
    {
        AnyAnimal {
            eat: Box::new(move |s| animal.eat(s))
        }
    }
}

impl AnimalExt for AnyAnimal {
    fn eat(&self, food: String) {
        (self.eat)(food);   // ok
        // self.eat(food) <- fatal runtime error: stack overflow 
    }
}

fn main() {
    let a = AnyAnimal::new(Dog);
    a.eat("aaa".to_string());
}